Web Server SRG

Web server management includes the ability to control the number of users and user sessions that utilize a web server. Limiting the number of allowed users and sessions per user is helpful in limiting risks related to several types of Denial of Service attacks. Although there is some latitude concerning the settings themselves, the settings should follow DoD-recommended values, but the settings should be configurable to allow for future DoD direction. While the DoD will specify recommended values, the values can be adjusted to accommodate the operational requirement of a given system.

Session management is the practice of protecting the bulk of the user authorization and identity information. Storing of this data can occur on the client system or on the server. When the session information is stored on the client, the session ID, along with the user authorization and identity information, is sent along with each client request and is stored in either a cookie, embedded in the uniform resource locator (URL), or placed in a hidden field on the displayed form. Each of these offers advantages and disadvantages. The biggest disadvantage to all three is the hijacking of a session along with all of the user's credentials. When the user authorization and identity information is stored on the server in a protected and encrypted database, the communication between the client and web server will only send the session identifier, and the server can then retrieve user credentials for the session when needed. If, during transmission, the session were to be hijacked, the user's credentials would not be compromised.

The web server has several remote communications channels. Examples are user requests via http/https, communication to a backend database, or communication to authenticate users. The encryption used to communicate must match the data that is being retrieved or presented. Methods of communication are http for publicly displayed information, https to encrypt when user data is being transmitted, VPN tunneling, or other encryption methods to a database.

Data exchanged between the user and the web server can range from static display data to credentials used to log into the hosted application. Even when data appears to be static, the non-displayed logic in a web page may expose business logic or trusted system relationships. The integrity of all the data being exchanged between the user and web server must always be trusted. To protect the integrity and trust, encryption methods should be used to protect the complete communication session.

Remote access to the web server is any access that communicates through an external, non-organization-controlled network. Remote access can be used to access hosted applications or to perform management functions. By providing remote access information to an external monitoring system, the organization can monitor for cyber attacks and monitor compliance with remote access policies. The organization can also look at data organization wide and determine an attack or anomaly is occurring on the organization which might not be noticed if the data were kept local to the web server. Examples of external applications used to monitor or control access would be audit log monitoring systems, dynamic firewalls, or infrastructure monitoring systems.

To control access to sensitive information and hosted applications by entities that have been issued certificates by DOD-approved PKIs, the web server must be properly configured to incorporate a means of authorization that does not simply rely on the possession of a valid certificate for access. Access decisions must include a verification that the authenticated entity is permitted to access the information or application. Authorization decisions must leverage a variety of methods, such as mapping the validated PKI certificate to an account with an associated set of permissions on the system. If the web server relied only on the possession of the certificate and did not map to system roles and privileges, each user would have the same abilities and roles to make changes to the production system. This requirement also applies to Zero Trust initiatives.

Log records can be generated from various components within the web server (e.g., httpd, plug-ins to external backends, etc.). From a web server perspective, certain specific web server functionalities may be logged as well. The web server must allow the definition of what events are to be logged. As conditions change, the number and types of events to be logged may change, and the web server must be able to facilitate these changes. The minimum list of logged events should be those pertaining to system startup and shutdown, system access, and system authentication events. If these events are not logged at a minimum, any type of forensic investigation would be missing pertinent information needed to replay what occurred.

An attacker can compromise a web server during the startup process. If logging is not initiated until all the web server processes are started, key information may be missed and not available during a forensic investigation. To assure all logable events are captured, the web server must begin logging once the first web server process is initiated.

Web server logging capability is critical for accurate forensic analysis. Without sufficient and accurate information, a correct replay of the events cannot be determined. For web servers, events logging includes, but is not limited to, the detection of the following: • XSS attacks (detect in server, mproxy, and WAF types logs). • Cross Site Request Forgery attacks. • Web Cache Poisoning. • Instances of Session Hijacking. • Instances of Server Side Request Forgery. Ascertaining the correct type of event that occurred is important during forensic analysis. The correct determination of the event and when it occurred is important in relation to other events that happened at that same time. Without sufficient information establishing what type of log event occurred, investigation into the cause of event is severely hindered. Log record content that may be necessary to satisfy the requirement of this control includes, but is not limited to, time stamps, source and destination IP addresses, user/process identifiers, event descriptions, application-specific events, success/fail indications, file names involved, access control, or flow control rules invoked.

Web server logging capability is critical for accurate forensic analysis. Without sufficient and accurate information, a correct replay of the events cannot be determined. Ascertaining the correct order of the events that occurred is important during forensic analysis. Events that appear harmless by themselves might be flagged as a potential threat when properly viewed in sequence. By also establishing the event date and time, an event can be properly viewed with an enterprise tool to fully see a possible threat in its entirety. Without sufficient information establishing when the log event occurred, investigation into the cause of event is severely hindered. Log record content that may be necessary to satisfy the requirement of this control includes, but is not limited to, time stamps, source and destination IP addresses, user/process identifiers, event descriptions, application-specific events, success/fail indications, file names involved, access control, or flow control rules invoked.

Web server logging capability is critical for accurate forensic analysis. Without sufficient and accurate information, a correct replay of the events cannot be determined. Ascertaining the correct location or process within the web server where the events occurred is important during forensic analysis. Correctly determining the web service, plug-in, or module will add information to the overall reconstruction of the logged event. For example, an event that occurred during communication to a cgi module might be handled differently than an event that occurred during a communication session to a user. Without sufficient information establishing where the log event occurred within the web server, investigation into the cause of event is severely hindered. Log record content that may be necessary to satisfy the requirement of this control includes, but is not limited to, time stamps, source and destination IP addresses, user/process identifiers, event descriptions, application-specific events, success/fail indications, file names involved, access control, or flow control rules invoked.

Web server logging capability is critical for accurate forensic analysis. Without sufficient and accurate information, a correct replay of the events cannot be determined. Ascertaining the correct source, e.g. source IP, of the events is important during forensic analysis. Correctly determining the source will add information to the overall reconstruction of the logable event. By determining the source of the event correctly, analysis of the enterprise can be undertaken to determine if the event compromised other assets within the enterprise. Without sufficient information establishing the source of the logged event, investigation into the cause of event is severely hindered. Log record content that may be necessary to satisfy the requirement of this control includes, but is not limited to, time stamps, source and destination IP addresses, user/process identifiers, event descriptions, application-specific events, success/fail indications, file names involved, access control, or flow control rules invoked.

Web server logging capability is critical for accurate forensic analysis. Without sufficient and accurate information, a correct replay of the events cannot be determined. Ascertaining the correct source, e.g. source IP, of the events is important during forensic analysis. Correctly determining the source of events will add information to the overall reconstruction of the logable event. By determining the source of the event correctly, analysis of the enterprise can be undertaken to determine if events tied to the source occurred in other areas within the enterprise. A web server behind a load balancer or proxy server, when not configured correctly, will record the load balancer or proxy server as the source of every logable event. When looking at the information forensically, this information is not helpful in the investigation of events. The web server must record with each event the client source of the event.

Web server logging capability is critical for accurate forensic analysis. Without sufficient and accurate information, a correct replay of the events cannot be determined. Ascertaining the success or failure of an event is important during forensic analysis. Correctly determining the outcome will add information to the overall reconstruction of the logable event. By determining the success or failure of the event correctly, analysis of the enterprise can be undertaken to determine if events tied to the event occurred in other areas within the enterprise. Without sufficient information establishing the success or failure of the logged event, investigation into the cause of event is severely hindered. The success or failure also provides a means to measure the impact of an event and help authorized personnel to determine the appropriate response. Log record content that may be necessary to satisfy the requirement of this control includes, but is not limited to, time stamps, source and destination IP addresses, user/process identifiers, event descriptions, application-specific events, success/fail indications, file names involved, access control, or flow control rules invoked.

Web server logging capability is critical for accurate forensic analysis. Without sufficient and accurate information, a correct replay of the events cannot be determined. Determining user accounts, processes running on behalf of the user, and running process identifiers also enable a better understanding of the overall event. User tool identification is also helpful to determine if events are related to overall user access or specific client tools. Log record content that may be necessary to satisfy the requirement of this control includes: time stamps, source and destination addresses, user/process identifiers, event descriptions, success/fail indications, file names involved, and access control or flow control rules invoked.

Reviewing log data allows an investigator to recreate the path of an attacker and to capture forensic data for later use. Log data is also essential to system administrators in their daily administrative duties on the hosted system or within the hosted applications. If the logging system begins to fail, events will not be recorded. Organizations shall define logging failure events, at which time the application or the logging mechanism the application utilizes will provide a warning to the ISSO and SA at a minimum.

Without an internal clock used as the reference for the time stored on each event to provide a trusted common reference for the time, forensic analysis would be impeded. Determining the correct time a particular event occurred on the web server is critical when conducting forensic analysis and investigating system events. If the internal clock is not used, the web server may not be able to provide time stamps for log messages. The web server can use the capability of an operating system or purpose-built module for this purpose. Time stamps generated by the web server shall include both date and time. The time may be expressed in Coordinated Universal Time (UTC), a modern continuation of Greenwich Mean Time (GMT), or local time with an offset from UTC.

Log data is essential in the investigation of events. If log data were to become compromised, then competent forensic analysis and discovery of the true source of potentially malicious system activity would be difficult, if not impossible, to achieve. In addition, access to log records provides information an attacker could potentially use to their advantage since each event record might contain communication ports, protocols, services, trust relationships, user names, etc. The web server must protect the log data from unauthorized read, write, copy, etc. This can be done by the web server if the web server is also doing the logging function. The web server may also use an external log system. In either case, the logs must be protected from access by non-privileged users.

Log data is essential in the investigation of events. The accuracy of the information is always pertinent. Information that is not accurate does not help in the revealing of potential security risks and may hinder the early discovery of a system compromise. One of the first steps an attacker will undertake is the modification or deletion of log records to cover his tracks and prolong discovery. The web server must protect the log data from unauthorized modification. This can be done by the web server if the web server is also doing the logging function. The web server may also use an external log system. In either case, the logs must be protected from modification by non-privileged users.

Log data is essential in the investigation of events. The accuracy of the information is always pertinent. Information that is not accurate does not help in the revealing of potential security risks and may hinder the early discovery of a system compromise. One of the first steps an attacker will undertake is the modification or deletion of audit records to cover his tracks and prolong discovery. The web server must protect the log data from unauthorized deletion. This can be done by the web server if the web server is also doing the logging function. The web server may also use an external log system. In either case, the logs must be protected from deletion by non-privileged users.

Protection of log data includes assuring log data is not accidentally lost or deleted. Backing up log records to an unrelated system or onto separate media than the system the web server is actually running on helps to assure that, in the event of a catastrophic system failure, the log records will be retained.

Being able to verify that a patch, upgrade, certificate, etc., being added to the web server is unchanged from the producer of the file is essential for file validation and nonrepudiation of the information. The web server or hosting system must have a mechanism to verify that files, before installation, are valid. Examples of validation methods are sha1 and md5 hashes and checksums.

In the case of a production web server, areas for content development and testing will not exist, as this type of content is only permissible on a development website. The process of developing on a functional production website entails a degree of trial and error and repeated testing. This process is often accomplished in an environment where debugging, sequencing, and formatting of content are the main goals. The opportunity for a malicious user to obtain files that reveal business logic and login schemes is high in this situation. The existence of such immature content on a web server represents a significant security risk that is totally avoidable. The web server must enforce, internally or through an external utility, the signing of modules before they are implemented into a production environment. By signing modules, the author guarantees that the module has been reviewed and tested before production implementation.

User management and authentication can be an essential part of any application hosted by the web server. Along with authenticating users, the user management function must perform several other tasks like password complexity, locking users after a configurable number of failed logins, and management of temporary and emergency accounts; and all of this must be done enterprise-wide. The web server contains a minimal user management function, but the web server user management function does not offer enterprise-wide user management, and user management is not the primary function of the web server. User management for the hosted applications should be done through a facility that is built for enterprise-wide user management, like LDAP and Active Directory.

A web server can provide many features, services, and processes. Some of these may be deemed unnecessary or too unsecure to run on a production DoD system. The web server must provide the capability to disable, uninstall, or deactivate functionality and services that are deemed to be non-essential to the web server mission or can adversely impact server performance.

A web server should be primarily a web server or a proxy server but not both, for the same reasons that other multi-use servers are not recommended. Scanning for web servers that will also proxy requests into an otherwise protected network is a very common attack making the attack anonymous.

Web server documentation, sample code, example applications, and tutorials may be an exploitable threat to a web server because this type of code has not been evaluated and approved. A production web server must only contain components that are operationally necessary (e.g., compiled code, scripts, web-content, etc.). Any documentation, sample code, example applications, and tutorials must be removed from a production web server. To make certain that the documentation and code are not installed or uninstalled completely; the web server must offer an option as part of the installation process to exclude these packages or to uninstall the packages if necessary.

When accounts used for web server features such as documentation, sample code, example applications, tutorials, utilities, and services are created even though the feature is not installed, they become an exploitable threat to a web server. These accounts become inactive, are not monitored through regular use, and passwords for the accounts are not created or updated. An attacker, through very little effort, can use these accounts to gain access to the web server and begin investigating ways to elevate the account privileges. The accounts used for web server features not installed must not be created and must be deleted when these features are uninstalled.

Just as running unneeded services and protocols is a danger to the web server at the lower levels of the OSI model, running unneeded utilities and programs is also a danger at the application layer of the OSI model. Office suites, development tools, and graphical editors are examples of such programs that are troublesome. Individual productivity tools have no legitimate place or use on an enterprise, production web server and they are also prone to their own security risks. The web server installation process must provide options allowing the installer to choose which utility programs, services, and modules are to be installed or removed. By having a process for installation and removal, the web server is guaranteed to be in a more stable and secure state than if these services and programs were installed and removed manually.

Controlling what a user of a hosted application can access is part of the security posture of the web server. Any time a user can access more functionality than is needed for the operation of the hosted application poses a security issue. A user with too much access can view information that is not needed for the user's job role, or the user could use the function in an unintentional manner. A MIME tells the web server what type of program various file types and extensions are and what external utilities or programs are needed to execute the file type. A shell is a program that serves as the basic interface between the user and the operating system, so hosted application users must not have access to these programs. Shell programs may execute shell escapes and can then perform unauthorized activities that could damage the security posture of the web server.

Scripts allow server side processing on behalf of the hosted application user or as processes needed in the implementation of hosted applications. Removing scripts not needed for application operation or deemed vulnerable helps to secure the web server. To assure scripts are not added to the web server and run maliciously, those script mappings that are not needed or used by the web server for hosted application operation must be removed.

Resource mapping is the process of tying a particular file type to a process in the web server that can serve that type of file to a requesting client and to identify which file types are not to be delivered to a client. By not specifying which files can and which files cannot be served to a user, the web server could deliver to a user web server configuration files, log files, password files, etc. The web server must only allow hosted application file types to be served to a user and all other types must be disabled.

A web server can be installed with functionality that, just by its nature, is not secure. Web Distributed Authoring (WebDAV) is an extension to the HTTP protocol that, when developed, was meant to allow users to create, change, and move documents on a server, typically a web server or web share. Allowing this functionality, development, and deployment is much easier for web authors. WebDAV is not widely used and has serious security concerns because it may allow clients to modify unauthorized files on the web server.

A web server may host one too many applications. Each application will need certain system resources and privileged operations to operate correctly. The web server must be configured to contain and control the applications and protect the system resources and privileged operations from those not needed by the application for operation. Limiting the application will confine the potential harm a compromised application could cause to a system.

A web server is designed to deliver content and execute scripts or applications on the request of a client or user. Containing user requests to files in the directory tree of the hosted web application and limiting the execution of scripts and applications guarantees that the user is not accessing information protected outside the application's realm. The web server must also prohibit users from jumping outside the hosted application directory tree through access to the user's home directory, symbolic links or shortcuts, or through search paths for missing files.

The web server must be configured to listen on a specified IP address and port. Without specifying an IP address and port for the web server to utilize, the web server will listen on all IP addresses available to the hosting server. If the web server has multiple IP addresses, i.e., a management IP address, the web server will also accept connections on the management IP address. Accessing the hosted application through an IP address normally used for non-application functions opens the possibility of user access to resources, utilities, files, ports, and protocols that are protected on the desired application IP address.

Data used to authenticate, especially passwords, needs to be protected at all times, and encryption is the standard method for protecting authentication data during transmission. Data used to authenticate can be passed to and from the web server for many reasons. Examples include data passed from a user to the web server through an HTTPS connection for authentication, the web server authenticating to a backend database for data retrieval and posting, and the web server authenticating to a clustered web server manager for an update.

A certificate's certification path is the path from the end entity certificate to a trusted root certification authority (CA). Certification path validation is necessary for a relying party to make an informed decision regarding acceptance of an end entity certificate. Certification path validation includes checks such as certificate issuer trust, time validity and revocation status for each certificate in the certification path. Revocation status information for CA and subject certificates in a certification path is commonly provided via certificate revocation lists (CRLs) or online certificate status protocol (OCSP) responses.

The web server's private key is used to prove the identity of the server to clients and securely exchange the shared secret key used to encrypt communications between the web server and clients. By gaining access to the private key, an attacker can pretend to be an authorized server and decrypt the SSL traffic between a client and the web server.

Encryption is only as good as the encryption modules utilized. Unapproved cryptographic module algorithms cannot be verified, and cannot be relied upon to provide confidentiality or integrity, and DoD data may be compromised due to weak algorithms. FIPS 140-2 is the current standard for validating cryptographic modules and NSA Type-X (where X=1, 2, 3, 4) products are NSA-certified, hardware-based encryption modules. The web server must provide FIPS-compliant encryption modules when storing encrypted data and configuration settings.

Encryption is only as good as the encryption modules utilized. Unapproved cryptographic module algorithms cannot be verified and cannot be relied upon to provide confidentiality or integrity, and DoD data may be compromised due to weak algorithms. FIPS 140-2 is the current standard for validating cryptographic modules and NSA Type-X (where X=1, 2, 3, 4) products are NSA-certified, hardware-based encryption modules. The web server must provide FIPS-compliant encryption modules when authenticating users and processes.

Mobile code in hosted applications allows the developer to add functionality and displays to hosted applications that are fluid, as opposed to a static web page. The data presentation becomes more appealing to the user, is easier to analyze, and navigation through the hosted application and data is much less complicated. Some mobile code technologies in use in today's applications are: Java, JavaScript, ActiveX, PDF, Postscript, Shockwave movies, Flash animations, and VBScript. The DoD has created policies that define the usage of mobile code on DoD systems. The usage restrictions and implementation guidance apply to both the selection and use of mobile code installed on organizational servers and mobile code downloaded and executed on individual workstations. The web server may host applications that contain mobile code and therefore, must meet the DoD-defined requirements regarding the deployment and/or use of mobile code. This includes digitally signing applets in order to provide a means for the client to establish application authenticity.

As a rule, accounts on a web server are to be kept to a minimum. Only administrators, web managers, developers, auditors, and web authors require accounts on the machine hosting the web server. The resources to which these accounts have access must also be closely monitored and controlled. Only the system administrator needs access to all the system's capabilities, while the web administrator and associated staff require access and control of the web content and web server configuration files. This requirement also applies to Zero Trust initiatives.

To properly monitor the changes to the web server and the hosted applications, logging must be enabled. Additionally, each record must properly contain the changes made and the names of those who made the changes. Allowing anonymous users the capability to change the web server or the hosted application will not generate proper log information that can then be used for forensic reporting in the case of a security issue. Allowing anonymous users to make changes will also grant change capabilities to anybody without forcing a user to authenticate before the changes can be made. This requirement also applies to Zero Trust initiatives.

The separation of user functionality from web server management can be accomplished by moving management functions to a separate IP address or port. To further separate the management functions, separate authentication methods and certificates should be used. By moving the management functionality, the possibility of accidental discovery of the management functions by nonprivileged users during hosted application use is minimized. This requirement also applies to Zero Trust initiatives.

Captured sessions can be reused in "replay" attacks. This requirement limits the ability of adversaries from capturing and continuing to employ previously valid session IDs. Session IDs are tokens generated by web applications to uniquely identify an application user's session. Unique session IDs help to reduce predictability of said identifiers. When a user logs out, or when any other session termination event occurs, the web server must terminate the user session to minimize the potential for an attacker to hijack that particular user session.

Cookies are used to exchange data between the web server and the client. Cookies, such as a session cookie, may contain session information and user credentials used to maintain a persistent connection between the user and the hosted application since HTTP/HTTPS is a stateless protocol. When the cookie parameters are not set properly (i.e., domain and path parameters), cookies can be shared within hosted applications residing on the same web server or to applications hosted on different web servers residing on the same domain.

Communication between a client and the web server is done using the HTTP protocol, but HTTP is a stateless protocol. In order to maintain a connection or session, a web server will generate a session identifier (ID) for each client session when the session is initiated. The session ID allows the web server to track a user session and, in many cases, the user, if the user previously logged into a hosted application. When a web server accepts session identifiers that are not generated by the web server, the web server creates an environment where session hijacking, such as session fixation, could be used to access hosted applications through session IDs that have already been authenticated. Forcing the web server to only accept web server-generated session IDs and to create new session IDs once a user is authenticated will limit session hijacking.

Communication between a client and the web server is done using the HTTP protocol, but HTTP is a stateless protocol. In order to maintain a connection or session, a web server will generate a session identifier (ID) for each client session when the session is initiated. The session ID allows the web server to track a user session and, in many cases, the user, if the user previously logged into a hosted application. Unique session IDs are the opposite of sequentially generated session IDs, which can be easily guessed by an attacker. Unique session identifiers help to reduce predictability of generated identifiers. Unique session IDs address man-in-the-middle attacks, including session hijacking or insertion of false information into a session. If the attacker is unable to identify or guess the session information related to pending application traffic, the attacker will have more difficulty in hijacking the session or otherwise manipulating valid sessions.

Communication between a client and the web server is done using the HTTP protocol, but HTTP is a stateless protocol. In order to maintain a connection or session, a web server will generate a session identifier (ID) for each client session when the session is initiated. The session ID allows the web server to track a user session and, in many cases, the user, if the user previously logged into a hosted application. By being able to guess session IDs, an attacker can easily perform a man-in-the-middle attack. To truly generate random session identifiers that cannot be reproduced, the web server session ID generator, when used twice with the same input criteria, must generate an unrelated random ID. The session ID generator also needs to be a FIPS 140-2 approved generator.

Generating a session identifier (ID) that is not easily guessed through brute force is essential to deter several types of session attacks. By knowing the session ID, an attacker can hijack a user session that has already been user authenticated by the hosted application. The attacker does not need to guess user identifiers and passwords or have a secure token since the user session has already been authenticated. Generating session IDs that are at least 128 bits (16 bytes) in length will cause an attacker to take a large amount of time and resources to guess, reducing the likelihood of an attacker guessing a session ID.

Generating a session identifier (ID) that is not easily guessed through brute force is essential to deter several types of session attacks. By knowing the session ID, an attacker can hijack a user session that has already been user-authenticated by the hosted application. The attacker does not need to guess user identifiers and passwords or have a secure token since the user session has already been authenticated. By generating session IDs that contain as much of the character set as possible, i.e., A-Z, a-z, and 0-9, the session ID becomes exponentially harder to guess.

Generating a session identifier (ID) that is not easily guessed through brute force is essential to deter several types of session attacks. By knowing the session ID, an attacker can hijack a user session that has already been user authenticated by the hosted application. The attacker does not need to guess user identifiers and passwords or have a secure token since the user session has already been authenticated. Random and unique session IDs are the opposite of sequentially generated session IDs, which can be easily guessed by an attacker. Random session identifiers help to reduce predictability of said identifiers. The session ID must be unpredictable (random enough) to prevent guessing attacks, where an attacker is able to guess or predict the ID of a valid session through statistical analysis techniques. For this purpose, a good PRNG (Pseudo Random Number Generator) must be used. Unique session IDs address man-in-the-middle attacks, including session hijacking or insertion of false information into a session. If the attacker is unable to identify or guess the session information related to pending application traffic, they will have more difficulty in hijacking the session or otherwise manipulating valid sessions. At least half of a session ID must be created using a definable source of entropy (PRNG).

Making certain that the web server has not been updated by an unauthorized user is always a concern. Adding patches, functions, and modules that are untested and not part of the baseline opens the possibility for security risks. The web server must offer, and not hinder, a method that allows for the quick and easy reinstallation of a verified and patched baseline to guarantee the production web server is up-to-date and has not been modified to add functionality or expose security risks. When the web server does not offer a method to roll back to a clean baseline, external methods, such as a baseline snapshot or virtualizing the web server, can be used.

Determining a safe state for failure and weighing that against a potential DoS for users depends on what type of application the web server is hosting. For an application presenting publicly available information that is not critical, a safe state for failure might be to shut down for any type of failure; but for an application that presents critical and timely information, a shutdown might not be the best state for all failures. Performing a proper risk analysis of the hosted applications and configuring the web server according to what actions to take for each failure condition will provide a known fail safe state for the web server.

The web server may host applications that display information that cannot be disrupted, such as information that is time-critical or life-threatening. In these cases, a web server that shuts down or ceases to be accessible when there is a failure is not acceptable. In these types of cases, clustering of web servers is used. Clustering of multiple web servers is a common approach to providing fail-safe application availability. To assure application availability, the web server must provide clustering or some form of failover functionality.

Data at rest is inactive data stored physically in any digital form (e.g., databases, data warehouses, spreadsheets, archives, tapes, off-site backups, mobile devices, etc.). Data at rest includes, but is not limited to, archived data, data that is not accessed or changed frequently, files stored on hard drives, USB thumb drives, files stored on backup tape and disks, and files stored off-site or on a storage area network. While data at rest can reside in many places, data at rest for a web server is data on the hosting system storage devices. Data stored as a backup on tape or stored off-site is no longer under the protection measures covered by the web server. There are several pieces of data the web server uses during operation. The web server must use an accepted encryption method, such as AES-256, to protect the confidentiality and integrity of the information.

A web server is used to deliver content on the request of a client. The content delivered to a client must be controlled, allowing only hosted application files to be accessed and delivered. To allow a client access to system files of any type is a major security risk that is entirely avoidable. Obtaining such access is the goal of directory traversal and URL manipulation vulnerabilities. To facilitate such access by misconfiguring the web document (home) directory is a serious error. In addition, having the path on the same drive as the system folder compounds potential attacks such as drive space exhaustion.

A web server can limit the ability of the web server being used in a DoS attack through several methods. The methods employed will depend upon the hosted applications and their resource needs for proper operation. An example setting that could be used to limit the ability of the web server being used in a DoS attack is bandwidth throttling.

Invalid user input occurs when a user inserts data or characters into a hosted application's data entry field and the hosted application is unprepared to process that data. This results in unanticipated application behavior, potentially leading to an application compromise. Invalid user input is one of the primary methods employed when attempting to compromise an application. An attacker can also enter Unicode into hosted applications in an effort to break out of the document home or root home directory or to bypass security checks. The web server, by defining the character set available for data entry, can trap efforts to bypass security checks or to compromise an application.

The goal is to completely control the web user's experience in navigating any portion of the web document root directories. Ensuring all web content directories have at least the equivalent of an index.html file is a significant factor to accomplish this end. Enumeration techniques, such as URL parameter manipulation, rely upon being able to obtain information about the web server's directory structure by locating directories without default pages. In the scenario, the web server will display to the user a listing of the files in the directory being accessed. By having a default hosted application web page, the anonymous web user will not obtain directory browsing information or an error message that reveals the server type and version.

Information needed by an attacker to begin looking for possible vulnerabilities in a web server includes any information about the web server, backend systems being accessed, and plug-ins or modules being used. Web servers will often display error messages to client users displaying enough information to aid in the debugging of the error. The information given back in error messages may display the web server type, version, patches installed, plug-ins and modules installed, type of code being used by the hosted application, and any backends being used for data storage. This information could be used by an attacker to blueprint what type of attacks might be successful. The information given to users must be minimized to not aid in the blueprinting of the web server.

Information needed by an attacker to begin looking for possible vulnerabilities in a web server includes any information about the web server and plug-ins or modules being used. When debugging or trace information is enabled in a production web server, information about the web server, such as web server type, version, patches installed, plug-ins and modules installed, type of code being used by the hosted application, and any backends being used for data storage may be displayed. Since this information may be placed in logs and general messages during normal operation of the web server, an attacker does not need to cause an error condition to gain this information.

Leaving sessions open indefinitely is a major security risk. An attacker can easily use an already authenticated session to access the hosted application as the previously authenticated user. By closing sessions after an absolute period of time, the user is forced to re-authenticate guaranteeing the session is still in use. Enabling an absolute timeout for sessions closes sessions that are still active. Examples would be a runaway process accessing the web server or an attacker using a hijacked session to slowly probe the web server.

Leaving sessions open indefinitely is a major security risk. An attacker can easily use an already authenticated session to access the hosted application as the previously authenticated user. By closing sessions after a set period of inactivity, the web server can make certain that those sessions that are not closed through the user logging out of an application are eventually closed. Acceptable values are 5 minutes for high-value applications, 10 minutes for medium-value applications, and 20 minutes for low-value applications.

Remote access to the web server is any access that communicates through an external, non-organization-controlled network. Remote access can be used to access hosted applications or to perform management functions. A web server can be accessed remotely and must be able to enforce remote access policy requirements or work in conjunction with enterprise tools designed to enforce policy requirements. Examples of the web server enforcing a remote access policy are implementing IP filtering rules, using https instead of http for communication, implementing secure tokens, and validating users.

Remote access to the web server is any access that communicates through an external, non-organization-controlled network. Remote access can be used to access hosted applications or to perform management functions. A web server can be accessed remotely and must be capable of restricting access from what the DoD defines as nonsecure zones. Nonsecure zones are defined as any IP, subnet, or region that is defined as a threat to the organization. The nonsecure zones must be defined for public web servers logically located in a DMZ, as well as private web servers with perimeter protection devices. By restricting access from nonsecure zones, through internal web server access list, the web server can stop or slow denial of service (DoS) attacks on the web server.

During an attack on the web server or any of the hosted applications, the system administrator may need to disconnect or disable access by users to stop the attack. The web server must provide a capability to disconnect users to a hosted application without compromising other hosted applications unless deemed necessary to stop the attack. Methods to disconnect or disable connections are to stop the application service for a specified hosted application, stop the web server, or block all connections through web server access list. The web server capabilities used to disconnect or disable users from connecting to hosted applications and the web server must be documented to make certain that, during an attack, the proper action is taken to conserve connectivity to any other hosted application if possible and to make certain log data is conserved for later forensic analysis.

By separating web server security functions from non-privileged users, roles can be developed that can then be used to administer the web server. Forcing users to change from a non-privileged account to a privileged account when operating on the web server or on security-relevant information forces users to only operate as a web server administrator when necessary. Operating in this manner allows for better logging of changes and better forensic information and limits accidental changes to the web server.

In order to make certain that the logging mechanism used by the web server has sufficient storage capacity in which to write the logs, the logging mechanism needs to be able to allocate log record storage capacity. The task of allocating log record storage capacity is usually performed during initial installation of the logging mechanism. The system administrator will usually coordinate the allocation of physical drive space with the web server administrator along with the physical location of the partition and disk. Refer to NIST SP 800-92 for specific requirements on log rotation and storage dependent on the impact of the web server.

Writing events to a centralized management audit system offers many benefits to the enterprise over having dispersed logs. Centralized management of audit records and logs provides for efficiency in maintenance and management of records, enterprise analysis of events, and backup and archiving of event records enterprise-wide. The web server and related components are required to be capable of writing logs to centralized audit log servers.

A web server will typically utilize logging mechanisms for maintaining a historical log of activity that occurs within a hosted application. This information can then be used for diagnostic purposes, forensics purposes, or other purposes relevant to ensuring the availability and integrity of the hosted application. While it is important to log events identified as being critical and relevant to security, it is equally important to notify the appropriate personnel in a timely manner so they are able to respond to events as they occur. Manual review of the web server logs may not occur in a timely manner, and each event logged is open to interpretation by a reviewer. By integrating the web server into an overall or organization-wide log review, a larger picture of events can be viewed, and analysis can be done in a timely and reliable manner.

It is critical for the appropriate personnel to be aware if a system is at risk of failing to process logs as required. Log processing failures include: software/hardware errors, failures in the log capturing mechanisms, and log storage capacity being reached or exceeded. If log capacity were to be exceeded, then events subsequently occurring would not be recorded. Organizations shall define a maximum allowable percentage of storage capacity serving as an alarming threshold (e.g., web server has exceeded 75% of log storage capacity allocated), at which time the web server or the logging mechanism the web server utilizes will provide a warning to the ISSO and SA at a minimum. This requirement can be met by configuring the web server to utilize a dedicated log tool that meets this requirement.

If time stamps are not consistently applied and there is no common time reference, it is difficult to perform forensic analysis across multiple devices and log records. Time stamps generated by the web server include date and time. Time is commonly expressed in Coordinated Universal Time (UTC), a modern continuation of Greenwich Mean Time (GMT), or local time with an offset from UTC.

Without sufficient granularity of time stamps, it is not possible to adequately determine the chronological order of records. Time stamps generated by the web server include date and time and must be to a granularity of one second.

A web server can be modified through parameter modification, patch installation, upgrades to the web server or modules, and security parameter changes. With each of these changes, there is the potential for an adverse effect such as a DoS, web server instability, or hosted application instability. To limit changes to the web server and limit exposure to any adverse effects from the changes, files such as the web server application files, libraries, and configuration files must have permissions and ownership set properly to only allow privileged users access.

Web servers provide numerous processes, features, and functionalities that utilize TCP/IP ports. Some of these processes may be deemed unnecessary or too unsecure to run on a production system. The web server must provide the capability to disable or deactivate network-related services that are deemed to be non-essential to the server mission, are too unsecure, or are prohibited by the PPSM CAL and vulnerability assessments.

Non-DOD approved PKIs have not been evaluated to ensure that they have security controls and identity vetting procedures in place which are sufficient for DOD systems to rely on the identity asserted in the certificate. PKIs lacking sufficient security controls and identity vetting procedures risk being compromised and issuing certificates that enable adversaries to impersonate legitimate users.

When data is written to digital media, such as hard drives, mobile computers, external/removable hard drives, personal digital assistants, flash/thumb drives, etc., there is risk of data loss and data compromise. User identities and passwords stored on the hard drive of the hosting hardware must be encrypted to protect the data from easily being discovered and used by an unauthorized user to access the hosted applications. The cryptographic libraries and functionality used to store and retrieve the user identifiers and passwords must be part of the web server.

An attacker has at least two reasons to stop a web server. The first is to cause a DoS, and the second is to put in place changes the attacker made to the web server configuration. To prohibit an attacker from stopping the web server, the process ID (pid) of the web server and the utilities used to start/stop the web server must be protected from access by non-privileged users. By knowing the pid and having access to the web server utilities, a non-privileged user has a greater capability of stopping the server, whether intentionally or unintentionally.

A Denial of Service (DoS) can occur when the web server is so overwhelmed that it can no longer respond to additional requests. A web server not properly tuned may become overwhelmed and cause a DoS condition even with expected traffic from users. To avoid a DoS, the web server must be tuned to handle the expected traffic for the hosted applications.

Preventing the disclosure of transmitted information requires that the web server take measures to employ some form of cryptographic mechanism in order to protect the information during transmission. This is usually achieved through the use of Transport Layer Security (TLS). Transmission of data can take place between the web server and a large number of devices/applications external to the web server. Examples are a web client used by a user, a backend database, an audit server, or other web servers in a web cluster. If data is transmitted unencrypted, the data then becomes vulnerable to disclosure. The disclosure may reveal user identifier/password combinations, website code revealing business logic, or other user personal information.

The HTTP protocol is a stateless protocol. To maintain a session, a session identifier is used. The session identifier is a piece of data that is used to identify a session and a user. If the session identifier is compromised by an attacker, the session can be hijacked. By encrypting the session identifier, the identifier becomes more difficult for an attacker to hijack, decrypt, and use before the session has expired.

A cookie is used when a web server needs to share data with the client's browser. The data is often used to remember the client when the client returns to the hosted application at a later date. A session cookie is a special type of cookie used to remember the client during the session. The cookie will contain the session identifier (ID) and may contain authentication data to the hosted application. To protect this data from easily being compromised, the cookie can be encrypted. When a cookie is sent encrypted via SSL/TLS, an attacker must spend a great deal of time and resources to decrypt the cookie. If, along with encryption, the cookie is compressed, the attacker can now use a combination of plaintext injection and inadvertent information leakage through data compression to reduce the time needed to decrypt the cookie. This attack is called Compression Ratio Info-leak Made Easy (CRIME). Cookies shared between the web server and the client when encrypted should not also be compressed.

A cookie can be read by client-side scripts easily if cookie properties are not set properly. By allowing cookies to be read by the client-side scripts, information such as session identifiers could be compromised and used by an attacker who intercepts the cookie. Setting cookie properties (i.e. HttpOnly property) to disallow client-side scripts from reading cookies better protects the information inside the cookie.

Cookies can be sent to a client using TLS/SSL to encrypt the cookies, but TLS/SSL is not used by every hosted application since the data being displayed does not require the encryption of the transmission. To safeguard against cookies, especially session cookies, being sent in plaintext, a cookie can be encrypted before transmission. To force a cookie to be encrypted before transmission, the cookie Secure property can be set.

Transport Layer Security (TLS) is a required transmission protocol for a web server hosting controlled information. The use of TLS provides confidentiality of data in transit between the web server and client. FIPS 140-2 approved TLS versions must be enabled and non-FIPS-approved SSL versions must be disabled. NIST SP 800-52 defines the approved TLS versions for government applications.

During the initial setup of a Transport Layer Security (TLS) connection to the web server, the client sends a list of supported cipher suites in order of preference. The web server will reply with the cipher suite it will use for communication from the client list. If an attacker can intercept the submission of cipher suites to the web server and place, as the preferred cipher suite, a weak export suite, the encryption used for the session becomes easy for the attacker to break, often within minutes to hours.

Information can be either unintentionally or maliciously disclosed or modified during preparation for transmission, including, for example, during aggregation, at protocol transformation points, and during packing/unpacking. These unauthorized disclosures or modifications compromise the confidentiality or integrity of the information. An example of this would be an SMTP queue. This queue may be added to a web server through an SMTP module to enhance error reporting or to allow developers to add SMTP functionality to their applications. Any modules used by the web server that queue data before transmission must maintain the confidentiality and integrity of the information before the data is transmitted.

Information can be either unintentionally or maliciously disclosed or modified during reception, including, for example, during aggregation, at protocol transformation points, and during packing/unpacking. These unauthorized disclosures or modifications compromise the confidentiality or integrity of the information. Protecting the confidentiality and integrity of received information requires that application servers take measures to employ approved cryptography in order to protect the information during transmission over the network. This is usually achieved through the use of Transport Layer Security (TLS), SSL VPN, or IPsec tunnel. The web server must utilize approved encryption when receiving transmitted data.

Security flaws with software applications are discovered daily. Vendors are constantly updating and patching their products to address newly discovered security vulnerabilities. Organizations (including any contractor to the organization) are required to promptly install security-relevant software updates (e.g., patches, service packs, and hot fixes). Flaws discovered during security assessments, continuous monitoring, incident response activities, or information system error handling must also be addressed expeditiously. The web server will be configured to check for and install security-relevant software updates from an authoritative source within an identified time period from the availability of the update. By default, this time period will be every 24 hours.

During installation of the web server software, accounts are created for the web server to operate properly. The accounts installed can have either no password installed or a default password, which will be known and documented by the vendor and the user community. The first things an attacker will try when presented with a login screen are the default user identifiers with default passwords. Installed applications may also install accounts with no password, making the login even easier. Once the web server is installed, the passwords for any created accounts should be changed and documented. The new passwords must meet the requirements for all passwords, i.e., upper/lower characters, numbers, special characters, time until change, reuse policy, etc. Service accounts or system accounts that have no login capability do not need to have passwords set or changed.

Configuring the web server to implement organization-wide security implementation guides and security checklists guarantees compliance with federal standards and establishes a common security baseline across the DoD that reflects the most restrictive security posture consistent with operational requirements. Configuration settings are the set of parameters that can be changed that affect the security posture and/or functionality of the system. Security-related parameters are those parameters impacting the security state of the web server, including the parameters required to satisfy other security control requirements.

Cryptography is only as strong as the encryption modules/algorithms employed to encrypt the data. Use of weak or untested encryption algorithms undermines the purposes of utilizing encryption to protect data. NSA has developed Type 1 algorithms for protecting classified information. The Committee on National Security Systems (CNSS) National Information Assurance Glossary (CNSS Instruction No. 4009) defines Type 1 products as: "Cryptographic equipment, assembly or component classified or certified by NSA for encrypting and decrypting classified and sensitive national security information when appropriately keyed. Developed using established NSA business processes and containing NSA-approved algorithms are used to protect systems requiring the most stringent protection mechanisms." Although persons may have a security clearance, they may not have a "need-to-know" and are required to be separated from the information in question. The web server must employ NSA-approved cryptography to protect classified information from those individuals who have no "need-to-know" or when encryption of compartmentalized data is required by data classification.

Disabling expired, inactive, or otherwise anomalous accounts supports the concepts of least privilege and least functionality which reduce the attack surface of the system.

Disabling expired, inactive, or otherwise anomalous accounts supports the concepts of least privilege and least functionality which reduce the attack surface of the system.

Automated mechanisms for centralized reviews and analyses include security information and event management products.

Audit information includes all information needed to successfully audit system activity, such as audit records, audit log settings, audit reports, and personally identifiable information. Audit logging tools are those programs and devices used to conduct system audit and logging activities. Protection of audit information focuses on technical protection and limits the ability to access and execute audit logging tools to authorized individuals. Physical protection of audit information is addressed by both media protection controls and physical and environmental protection controls.

Organizations log system accesses associated with applying configuration changes to ensure that configuration change control is implemented and to support after-the-fact actions should organizations discover any unauthorized changes.

Individual authentication prior to shared group authentication mitigates the risk of using group accounts or authenticators.

The purpose of requiring a device separate from the system to which the user is attempting to gain access for one of the factors during multifactor authentication is to reduce the likelihood of compromising authenticators or credentials stored on the system. Adversaries may be able to compromise such authenticators or credentials and subsequently impersonate authorized users. Implementing one of the factors on a separate device (e.g., a hardware token), provides a greater strength of mechanism and an increased level of assurance in the authentication process.

The purpose of requiring a device that is separate from the system to which the user is attempting to gain access for one of the factors during multifactor authentication is to reduce the likelihood of compromising authenticators or credentials stored on the system. Adversaries may be able to compromise such authenticators or credentials and subsequently impersonate authorized users. Implementing one of the factors on a separate device (e.g., a hardware token), provides a greater strength of mechanism and an increased level of assurance in the authentication process.

Password-based authentication applies to passwords regardless of whether they are used in single-factor or multifactor authentication. Long passwords or passphrases are preferable over shorter passwords. Enforced composition rules provide marginal security benefits while decreasing usability. However, organizations may choose to establish certain rules for password generation (e.g., minimum character length for long passwords) under certain circumstances and can enforce this requirement in IA-5(1)(h). Account recovery can occur, for example, in situations when a password is forgotten. Cryptographically protected passwords include salted one-way cryptographic hashes of passwords. The list of commonly used, compromised, or expected passwords includes passwords obtained from previous breach corpuses, dictionary words, and repetitive or sequential characters. The list includes context-specific words, such as the name of the service, username, and derivatives thereof.

Password-based authentication applies to passwords regardless of whether they are used in single-factor or multifactor authentication. Long passwords or passphrases are preferable over shorter passwords. Enforced composition rules provide marginal security benefits while decreasing usability. However, organizations may choose to establish certain rules for password generation (e.g., minimum character length for long passwords) under certain circumstances and can enforce this requirement in IA-5(1)(h). Account recovery can occur, for example, in situations when a password is forgotten. Cryptographically protected passwords include salted one-way cryptographic hashes of passwords. The list of commonly used, compromised, or expected passwords includes passwords obtained from previous breach corpuses, dictionary words, and repetitive or sequential characters. The list includes context-specific words, such as the name of the service, username, and derivatives thereof.

Password-based authentication applies to passwords regardless of whether they are used in single-factor or multifactor authentication. Long passwords or passphrases are preferable over shorter passwords. Enforced composition rules provide marginal security benefits while decreasing usability. However, organizations may choose to establish certain rules for password generation (e.g., minimum character length for long passwords) under certain circumstances and can enforce this requirement in IA-5(1)(h). Account recovery can occur, for example, in situations when a password is forgotten. Cryptographically protected passwords include salted one-way cryptographic hashes of passwords. The list of commonly used, compromised, or expected passwords includes passwords obtained from previous breach corpuses, dictionary words, and repetitive or sequential characters. The list includes context-specific words, such as the name of the service, username, and derivatives thereof.

Password-based authentication applies to passwords regardless of whether they are used in single-factor or multifactor authentication. Long passwords or passphrases are preferable over shorter passwords. Enforced composition rules provide marginal security benefits while decreasing usability. However, organizations may choose to establish certain rules for password generation (e.g., minimum character length for long passwords) under certain circumstances and can enforce this requirement in IA-5(1)(h). Account recovery can occur, for example, in situations when a password is forgotten. Cryptographically protected passwords include salted one-way cryptographic hashes of passwords. The list of commonly used, compromised, or expected passwords includes passwords obtained from previous breach corpuses, dictionary words, and repetitive or sequential characters. The list includes context-specific words, such as the name of the service, username, and derivatives thereof.

Password-based authentication applies to passwords regardless of whether they are used in single-factor or multifactor authentication. Long passwords or passphrases are preferable over shorter passwords. Enforced composition rules provide marginal security benefits while decreasing usability. However, organizations may choose to establish certain rules for password generation (e.g., minimum character length for long passwords) under certain circumstances and can enforce this requirement in IA-5(1)(h). Account recovery can occur, for example, in situations when a password is forgotten. Cryptographically protected passwords include salted one-way cryptographic hashes of passwords. The list of commonly used, compromised, or expected passwords includes passwords obtained from previous breach corpuses, dictionary words, and repetitive or sequential characters. The list includes context-specific words, such as the name of the service, username, and derivatives thereof.

Password-based authentication applies to passwords regardless of whether they are used in single-factor or multifactor authentication. Long passwords or passphrases are preferable over shorter passwords. Enforced composition rules provide marginal security benefits while decreasing usability. However, organizations may choose to establish certain rules for password generation (e.g., minimum character length for long passwords) under certain circumstances and can enforce this requirement in IA-5(1)(h). Account recovery can occur, for example, in situations when a password is forgotten. Cryptographically protected passwords include salted one-way cryptographic hashes of passwords. The list of commonly used, compromised, or expected passwords includes passwords obtained from previous breach corpuses, dictionary words, and repetitive or sequential characters. The list includes context-specific words, such as the name of the service, username, and derivatives thereof.

Password-based authentication applies to passwords regardless of whether they are used in single-factor or multifactor authentication. Long passwords or passphrases are preferable over shorter passwords. Enforced composition rules provide marginal security benefits while decreasing usability. However, organizations may choose to establish certain rules for password generation (e.g., minimum character length for long passwords) under certain circumstances and can enforce this requirement in IA-5(1)(h). Account recovery can occur, for example, in situations when a password is forgotten. Cryptographically protected passwords include salted one-way cryptographic hashes of passwords. The list of commonly used, compromised, or expected passwords includes passwords obtained from previous breach corpuses, dictionary words, and repetitive or sequential characters. The list includes context-specific words, such as the name of the service, username, and derivatives thereof.

Password-based authentication applies to passwords regardless of whether they are used in single-factor or multifactor authentication. Long passwords or passphrases are preferable over shorter passwords. Enforced composition rules provide marginal security benefits while decreasing usability. However, organizations may choose to establish certain rules for password generation (e.g., minimum character length for long passwords) under certain circumstances and can enforce this requirement in IA-5(1)(h). Account recovery can occur, for example, in situations when a password is forgotten. Cryptographically protected passwords include salted one-way cryptographic hashes of passwords. The list of commonly used, compromised, or expected passwords includes passwords obtained from previous breach corpuses, dictionary words, and repetitive or sequential characters. The list includes context-specific words, such as the name of the service, username, and derivatives thereof.

Password-based authentication applies to passwords regardless of whether they are used in single-factor or multifactor authentication. Long passwords or passphrases are preferable over shorter passwords. Enforced composition rules provide marginal security benefits while decreasing usability. However, organizations may choose to establish certain rules for password generation (e.g., minimum character length for long passwords) under certain circumstances and can enforce this requirement in IA-5(1)(h). Account recovery can occur, for example, in situations when a password is forgotten. Cryptographically protected passwords include salted one-way cryptographic hashes of passwords. The list of commonly used, compromised, or expected passwords includes passwords obtained from previous breach corpuses, dictionary words, and repetitive or sequential characters. The list includes context-specific words, such as the name of the service, username, and derivatives thereof.

Public key cryptography is a valid authentication mechanism for individuals, machines, and devices. For PKI solutions, status information for certification paths includes certificate revocation lists or certificate status protocol responses. For PIV cards, certificate validation involves the construction and verification of a certification path to the Common Policy Root trust anchor, which includes certificate policy processing. Implementing a local cache of revocation data to support path discovery and validation also supports system availability in situations where organizations are unable to access revocation information via the network.

Nonlocal maintenance and diagnostic activities are conducted by individuals who communicate through either an external or internal network. Communications paths can be logically separated using encryption.

Public key infrastructure (PKI) certificates are certificates with visibility external to organizational systems and certificates related to the internal operations of systems, such as application-specific time services. In cryptographic systems with a hierarchical structure, a trust anchor is an authoritative source (i.e., a certificate authority) for which trust is assumed and not derived. A root certificate for a PKI system is an example of a trust anchor. A trust store or certificate store maintains a list of trusted root certificates.

A Trusted Platform Module (TPM) is an example of a hardware-protected data store that can be used to protect cryptographic keys.

Time synchronization of system clocks is essential for the correct execution of many system services, including identification and authentication processes that involve certificates and time-of-day restrictions as part of access control. Denial of service or failure to deny expired credentials may result without properly synchronized clocks within and between systems and system components. Time is commonly expressed in Coordinated Universal Time (UTC), a modern continuation of Greenwich Mean Time (GMT), or local time with an offset from UTC. The granularity of time measurements refers to the degree of synchronization between system clocks and reference clocks, such as clocks synchronizing within hundreds of milliseconds or tens of milliseconds. Organizations may define different time granularities for system components. Time service can be critical to other security capabilities such as access control and identification and authentication depending on the nature of the mechanisms used to support the capabilities.

Synchronization of internal system clocks with an authoritative source provides uniformity of time stamps for systems with multiple system clocks and systems connected over a network.

Authenticity protection provides protection against man-in-the-middle attacks/session hijacking and the insertion of false information into sessions. Application communication sessions are protected utilizing transport encryption protocols, such as TLS. TLS provides web applications with a means to be able to authenticate user sessions and encrypt application traffic. Session authentication can be single (one way) or mutual (two way) in nature. Single authentication authenticates the server for the client, whereas mutual authentication provides a means for both the client and the server to authenticate each other. This requirement addresses communications protection at the application session, versus the network packet, and establishes grounds for confidence at both ends of communications sessions in ongoing identities of other parties and in the validity of information transmitted. Depending on the required degree of confidentiality and integrity, web services will require the use of TLS mutual authentication (two-way/bidirectional).

Authenticity protection provides protection against man-in-the-middle attacks/session hijacking and the insertion of false information into sessions. Application communication sessions are protected utilizing transport encryption protocols, such as TLS. TLS provides web applications with a means to be able to authenticate user sessions and encrypt application traffic. Session authentication can be single (one way) or mutual (two way) in nature. Single authentication authenticates the server for the client, whereas mutual authentication provides a means for both the client and the server to authenticate each other. This requirement addresses communications protection at the application session, versus the network packet, and establishes grounds for confidence at both ends of communications sessions in ongoing identities of other parties and in the validity of information transmitted. Depending on the required degree of confidentiality and integrity, web services will require the use of TLS mutual authentication (two-way/bidirectional).

HTTP/2, like HTTPS, enhances security compared to HTTP/1.x by minimizing the risk of header-based attacks (e.g., header injection and manipulation). Websites that fully utilize HTTP/2 are inherently protected and defend against smuggling attacks. HTTP/2 provides the method for specifying the length of a request, which removes any potential for ambiguity that can be leveraged by an attacker. This is applicable to all web architectures such as load balancing/proxy use cases. - The front-end and back-end servers should both be configured to use HTTP/2. - HTTP/2 must be used for communications between web servers. - Browser vendors have agreed to only support HTTP/2 only in HTTPS mode, thus TLS must be configured to meet this requirement. TLS configuration is out of scope for this requirement.

HTTP/2 is backward compatible with HTTP/1.x, so it is possible to configure the architecture to implement a front-end server for HTTP/2 while communicating with one or more back-end servers that support only HTTP/1.x. Thus, the front end effectively has to translate or downgrade the requests it receives into the less secure protocol. HTTP downgrading negates the benefits of HTTP/2. If HTTP downgrading cannot be avoided, validate the rewritten/downgraded request against the HTTP/1.1 specification. For example, reject requests that contain newlines in the headers, colons in header names, and spaces in the request method.

Request smuggling attacks involve placing both the Content-Length header and the Transfer-Encoding header into a single HTTP/1 request and manipulating it so that web servers (i.e., back-end, front-end, load balancers) process the request differently. There are a number of variants of this type of attack with different names. However, all variants are addressed by configuring the front-end server to exclusively use HTTP/2 when communicating with other web servers. Specific instances of this vulnerability can be resolved by reconfiguring the front-end server to normalize ambiguous requests before routing them onward. However, if the request cannot be made unambiguous or normalized, configure both the front-end and back-end servers to reject the message and close the connection. It is important to not assume requests do not have a body. For all web servers, examine requests that report message body length as zero in the HTTP header and drop the request. For load balancing or reverse proxying implementation: -The front-end web server must interpret and forward HTTP requests, such that the back-end server receives a consistent interpretation of the request, or terminate the TCP connection. -The back-end web server must drop ambiguous requests that cannot be normalized and terminate the TCP connection.

The web server defines a set of exceptions for every HTTP status code. Each exception class has a status code according to RFC 2068: Codes with 100-300 are not really errors; 400s are client errors, and 500s are server errors. If not directly specified, headers will be added to the default response headers. In the event of an anomaly or exception during the processing of requests, it is safer to terminate the connection to prevent malformed requests from exploiting potential protocol vulnerabilities.

Without protection of the transmitted information, confidentiality and integrity may be compromised since unprotected communications can be intercepted and read or altered. Communication paths outside the physical protection of a controlled boundary are exposed to the possibility of interception and modification. It is crucial that the web server and forward proxy agree about the boundaries between requests. Otherwise, an attacker may be able to send ambiguous and other smuggling attacks to the web server.

Attribution is a critical component of a security concept of operations. The ability to identify source and destination points for information flowing in information systems, allows the forensic reconstruction of events when required, and encourages policy compliance by attributing policy violations to specific organizations/individuals. Successful domain authentication requires that information system labels distinguish among systems, organizations, and individuals involved in preparing, sending, receiving, or disseminating information. This requirement applies to applications that transfer information between different security domains (e.g., multilevel security). This requirement also applies to Zero Trust initiatives.

Unsupported software and systems should not be used because fixes to newly identified bugs will not be implemented by the vendor. The lack of support can result in potential vulnerabilities. Software and systems at unsupported servicing levels or releases will not receive security updates for new vulnerabilities, which leaves them subject to exploitation. When maintenance updates and patches are no longer available, software is no longer considered supported and should be upgraded or decommissioned.