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XCCDF
Microsoft Exchange 2019 Mailbox Server Security Technical Implementation Guide
Microsoft Exchange 2019 Mailbox Server Security Technical Implementation Guide
An XCCDF Benchmark
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File Metadata
67 rules organized in 67 groups
SRG-APP-000014
1 Rule
Exchange must use encryption for RPC client access.
Medium Severity
This setting controls whether client machines are forced to use secure channels to communicate with the server. If this feature is enabled, clients will only be able to communicate with the server over secure communication channels. Failure to require secure connections to the client access server increases the potential for unintended eavesdropping or data loss.
SRG-APP-000014
1 Rule
Exchange must use encryption for Outlook Web App (OWA) access.
Medium Severity
This setting controls whether client machines should be forced to use secure channels to communicate with this virtual directory. If this feature is enabled, clients will only be able to communicate with the directory if they are capable of supporting secure communication with the server. The use of secure communication prevents eavesdroppers from reading or modifying communications between servers and clients. The network and DMZ STIG identify criteria for OWA and Public Folder configuration in the network, including Common Access Card (CAC)-enabled preauthentication through an application firewall proxy. Failure to require secure connections on a website increases the potential for unintended eavesdropping or data loss.
SRG-APP-000014
1 Rule
Exchange must have forms-based authentication enabled.
Medium Severity
Identification and Authentication provide the foundation for access control. Access to email services applications in the DOD requires authentication using DOD Public Key Infrastructure (PKI) certificates. Authentication for Outlook Web App (OWA) is used to enable web access to user email mailboxes and should assume that certificate-based authentication has been configured. This setting controls whether forms-based logon should be used by the OWA website. Because the DOD requires Common Access Card (CAC)-based authentication to applications, OWA access must be brokered through an application proxy or other preauthenticator, which performs CAC authentication prior to arrival at the CA server. The authenticated request is then forwarded directly to OWA, where authentication is repeated without requiring the user to repeat authentication steps. For this scenario to work, the Application Proxy server must have forms-based authentication enabled, and Exchange must have forms-based Authentication disabled. If forms-based Authentication is enabled on the Exchange CA server, it is evidence that the application proxy server is either not correctly configured, or it may be missing.
SRG-APP-000027
1 Rule
Exchange must have administrator audit logging enabled.
Medium Severity
Unauthorized or malicious data changes can compromise the integrity and usefulness of the data. Automated attacks or malicious users with elevated privileges have the ability to effect change using the same mechanisms as email administrators. Auditing any changes to access mechanisms not only supports accountability and nonrepudiation for those authorized to define the environment but also enables investigation of changes made by others who may not be authorized. Note: This administrator auditing feature audits all exchange changes regardless of the user's assigned role or permissions.
SRG-APP-000033
1 Rule
Exchange servers must use approved DOD certificates.
Medium Severity
Server certificates are required for many security features in Exchange; without them, the server cannot engage in many forms of secure communication. Failure to implement valid certificates makes it virtually impossible to secure Exchange's communications.
SRG-APP-000033
1 Rule
Exchange must have authenticated access set to integrated Windows authentication only.
Medium Severity
To mitigate the risk of unauthorized access to sensitive information by entities that have been issued certificates by DOD-approved PKIs, all DOD systems (e.g., networks, web servers, and web portals) must be properly configured to incorporate access control methods that do not rely solely on the possession of a certificate for access. Successful authentication must not automatically give an entity access to an asset or security boundary. Authorization procedures and controls must be implemented to ensure each authenticated entity also has a validated and current authorization. Authorization is the process of determining whether an entity, once authenticated, is permitted to access a specific asset. Information systems use access control policies and enforcement mechanisms to implement this requirement. Access control policies include identity-based policies, role-based policies, and attribute-based policies. Access enforcement mechanisms include access control lists, access control matrices, and cryptography. These policies and mechanisms must be employed by the application to control access between users (or processes acting on behalf of users) and objects (e.g., devices, files, records, processes, programs, and domains) in the information system. This requirement is applicable to access control enforcement applications (e.g., authentication servers) and other applications that perform information and system access control functions.
SRG-APP-000038
1 Rule
Exchange auto-forwarding email to remote domains must be disabled or restricted.
Medium Severity
Attackers can use automated messages to determine whether a user account is active, in the office, traveling, and so on. An attacker might use this information to conduct future attacks. Verify Automatic Forwards to remote domains are disabled, except for enterprise mail that must be restricted to forward only to .mil and .gov. domains. Before enabling this setting, configure a remote domain.
SRG-APP-000089
1 Rule
Exchange connectivity logging must be enabled.
Medium Severity
A connectivity log is a record of the SMTP connection activity of the outbound message delivery queues to the destination Mailbox server, smart host, or domain. Connectivity logging is available on Mailbox servers in Exchange 2019 as it holds Mailbox, Client Access, and Hub Transport roles. This must also be completed on Edge Transport servers, as that is a separate role. By default, connectivity logging is disabled. If events are not recorded, it may be difficult or impossible to determine the root cause of system problems or the unauthorized activities of malicious users. Note: Transport configuration settings apply to the organization/global level of the Exchange SMTP path. By checking and setting them on the Mailbox server, the setting will apply to both Hub and Edge server roles.
SRG-APP-000089
1 Rule
The Exchange email diagnostic log level must be set to the lowest level.
Medium Severity
Log files help establish a history of activities and can be useful in detecting attack attempts or determining tuning adjustments to improve availability. Diagnostic logging, however, characteristically produces large volumes of data and requires care in managing the logs to prevent risk of disk capacity denial-of-service conditions. Exchange diagnostic logging is divided into 29 main "services", each of which has anywhere from two to 26 "categories" of events to be monitored. Each category may be set to one of four levels of logging: Lowest, Low, Medium, and High, depending on how much detail is required. Higher levels of detail require more disk space to store the audit material. Diagnostic logging is intended to help administrators debug problems with their systems, not as a general-purpose auditing tool. Because the diagnostic logs collect a great amount of information, the log files may grow large very quickly. Diagnostic log levels may be raised for limited periods of time when attempting to debug relevant pieces of Exchange functionality. Once debugging has finished, diagnostic log levels should be reduced again.
SRG-APP-000089
1 Rule
Exchange audit record parameters must be set.
Low Severity
Log files help establish a history of activities and can be useful in detecting attack attempts. This item declares the fields that must be available in the audit log file to adequately research events that are logged. Audit records should include the following fields to supply useful event accounting: Object modified, Cmdlet name, Cmdlet parameters, Modified parameters, Caller, Succeeded, and Originating server.
SRG-APP-000090
1 Rule
The RBAC role for audit log management must be defined and restricted.
Medium Severity
The RBAC role for the audit log management "Audit Log Role" should be defined in the Organizational or Enterprise Domain Security Plan (EDSP) to define the necessary personnel that are required to handle audit logs for the Microsoft Exchange application. Group membership should be audited regularly by checking the EDSP regularly and determine who should and should not have group membership. There are three built-in groups that automatically have membership: Organization Management, Compliance Management, and Records Management.
SRG-APP-000098
1 Rule
Exchange email subject line logging must be disabled.
Medium Severity
Log files help establish a history of activities and can be useful in detecting attack attempts or determining tuning adjustments to improve availability. When "message tracking" is enabled, only the sender, recipients, time, and other delivery information is included by default. Information such as the subject and message body is not included. However, the absence of the message subject line can make it difficult to locate a specific message in the log unless one knows roughly what time the message was sent. To simplify searches through these logs, Exchange offers the ability to include the message "subject line" in the log files and in the Message Tracking Center display. This can make it significantly easier to locate a specific message. However, this feature creates larger log files and will contain information that may raise privacy and legal concerns. Enterprise policy should be consulted before this feature is enabled. Also, because the log files may contain sensitive information in the form of the subject line, the log files will need to be protected, commensurate with the sensitivity level, as the content may be of interest to an attacker. For these reasons, it is recommended that subject logging not be enabled during regular production operations. Instead, treat this feature as a diagnostic that can be used if needed. The tradeoff is that finding the correct message in the message tracking logs will become more difficult because the administrator will need to search using only the time the message was sent and the message's sender. This control will have no effect unless Message Tracking is enabled. However, the setting should be disabled in case message tracking is enabled in the future.
SRG-APP-000098
1 Rule
Exchange message tracking logging must be enabled.
Medium Severity
A message tracking log provides a detailed log of all message activity as messages are transferred to and from a computer running Exchange. If events are not recorded, it may be difficult or impossible to determine the root cause of system problems or the unauthorized activities of malicious users.
SRG-APP-000098
1 Rule
Exchange circular logging must be disabled.
Low Severity
Logging provides a history of events performed and can also provide evidence of tampering or attack. Failure to create and preserve logs adds to the risk that suspicious events may go unnoticed and raises the potential that insufficient history will be available to investigate them. This setting controls how log files are written. If circular logging is enabled, one log file is stored with a default size of 1024 KB. Once the size limit has been reached, additional log entries overwrite the oldest log entries. If circular logging is disabled, once a log file reaches the size limit, a new log file is created. Mailbox should not use circular logging. Logs should be written to a partition separate from the operating system, with log protection and backups being incorporated into the overall System Security Plan.
SRG-APP-000111
1 Rule
Exchange queue monitoring must be configured with threshold and action.
Medium Severity
Monitors are automated "process watchers" that respond to performance changes and can be useful in detecting outages and alerting administrators where attention is needed. Exchange has built-in monitors that enable the administrator to generate alerts if thresholds are reached, better enabling them to react in a timely fashion. This field offers choices of alerts when a "warning" or "critical" threshold is reached on the SMTP queue. A good rule of thumb (default) is to issue warnings when SMTP queue growth exceeds 10 minutes and critical messages when it exceeds 20 minutes, which should only happen occasionally. Frequent alerts against this counter may indicate a network or other issue (such as inbound ExchangeMER traffic) that directly impacts email delivery. Notification choices include email alert to an email-enabled account (for example, an email administrator) or invoke a script to take other action (for example, to add an event to the Microsoft Application Event Log, where external monitors might detect it).
SRG-APP-000118
1 Rule
Exchange must protect audit data against unauthorized read access.
Medium Severity
Log files help establish a history of activities and can be useful in detecting attack attempts or determining tuning adjustments to improve availability. Audit log content must always be considered sensitive and in need of protection. Audit data available for modification by a malicious user can be altered to conceal malicious activity. Audit data might also provide a means for the malicious user to plan unauthorized activities that exploit weaknesses. The contents of audit logs are protected against unauthorized access, modification, or deletion. Only authorized auditors and the audit functions should be granted "Read" and "Write" access to audit log data.
SRG-APP-000119
1 Rule
Exchange must protect audit data against unauthorized access.
Medium Severity
Log files help establish a history of activities and can be useful in detecting attack attempts or determining tuning adjustments to improve availability. Audit log content must always be considered sensitive and in need of protection. Audit data available for modification by a malicious user can be altered to conceal malicious activity. Audit data might also provide a means for the malicious user to plan unauthorized activities that exploit weaknesses. The contents of audit logs are protected against unauthorized access, modification, or deletion. Only authorized auditors and the audit functions should be granted "Read" and "Write" access to audit log data.
SRG-APP-000120
1 Rule
Exchange must protect audit data against unauthorized deletion.
Medium Severity
Log files help establish a history of activities and can be useful in detecting attack attempts or determining tuning adjustments to improve availability. Audit log content must always be considered sensitive and in need of protection. Audit data available for modification by a malicious user can be altered to conceal malicious activity. Audit data might also provide a means for the malicious user to plan unauthorized activities that exploit weaknesses. The contents of audit logs are protected against unauthorized access, modification, or deletion. Only authorized auditors and the audit functions should be granted "Read" and "Write" access to audit log data.
SRG-APP-000125
1 Rule
Exchange audit data must be on separate partitions.
Medium Severity
Log files help establish a history of activities and can be useful in detecting attack attempts or determining tuning adjustments to improve availability. Audit log content must always be considered sensitive and in need of protection. Successful exploit of an application server vulnerability may well be logged by monitoring or audit processes when it occurs. Writing log and audit data to a separate partition where separate security contexts protect them may offer the ability to protect this information from being modified or removed by the exploit mechanism.
SRG-APP-000131
1 Rule
Exchange local machine policy must require signed scripts.
Medium Severity
Scripts often provide a way for attackers to infiltrate a system, especially scripts downloaded from untrusted locations. By setting machine policy to prevent unauthorized script executions, unanticipated system impacts can be avoided. Failure to allow only signed remote scripts reduces the attack vector vulnerabilities from unsigned remote scripts.
SRG-APP-000141
1 Rule
Exchange Send Fatal Errors to Microsoft must be disabled.
Medium Severity
It is detrimental for applications to provide, or install by default, functionality exceeding requirements or mission objectives. These unnecessary capabilities or services are often overlooked and therefore may remain unsecured. They increase the risk to the platform by providing additional attack vectors. Applications are capable of providing a wide variety of functions and services. Some of the functions and services, provided by default, may not be necessary to support essential organizational operations (e.g., key missions, functions). Examples of nonessential capabilities include but are not limited to advertising software or browser plug-ins not related to requirements or providing a wide array of functionality not required for every mission but that cannot be disabled. All system errors in Exchange will result in outbound traffic that may be identified by an eavesdropper. For this reason, the "Report Fatal Errors to Microsoft" feature must be disabled.
SRG-APP-000141
1 Rule
Exchange must not send customer experience reports to Microsoft.
Medium Severity
It is detrimental for applications to provide, or install by default, functionality exceeding requirements or mission objectives. These unnecessary capabilities or services are often overlooked and therefore may remain unsecured. They increase the risk to the platform by providing additional attack vectors. Applications are capable of providing a wide variety of functions and services. Some of the functions and services, provided by default, may not be necessary to support essential organizational operations (e.g., key missions, functions). Examples of nonessential capabilities include, but are not limited to, advertising software or browser plug-ins not related to requirements or providing a wide array of functionality not required for every mission but that cannot be disabled. Customer Experience reports in Exchange will result in outbound traffic that may be identified by an eavesdropper. For this reason, the Customer Experience reports must not be sent to Microsoft.
SRG-APP-000141
1 Rule
The Exchange Internet Message Access Protocol 4 (IMAP4) service must be disabled.
Medium Severity
IMAP4 is not approved for use within the DOD. It uses a clear-text-based user name and password and does not support the DOD standard for PKI for email access. User name and password could easily be captured from the network, allowing a malicious user to access other system features. Uninstalling or disabling the service will prevent the use of the IMAP4 protocol.
SRG-APP-000141
1 Rule
The Exchange Post Office Protocol 3 (POP3) service must be disabled.
Medium Severity
POP3 is not approved for use within the DOD. It uses a clear-text-based user name and password and does not support the DOD standard for PKI for email access. User name and password could easily be captured from the network, allowing a malicious user to access other system features. Uninstalling or disabling the service will prevent the use of POP3.
SRG-APP-000211
1 Rule
Exchange Mailbox databases must reside on a dedicated partition.
Medium Severity
In the same way that added security layers can provide a cumulative positive effect on security posture, multiple applications can provide a cumulative negative effect. A vulnerability and subsequent exploit to one application can lead to an exploit of other applications sharing the same security context. For example, an exploit to a web server process that leads to unauthorized administrative access to the host system can most likely lead to a compromise of all applications hosted by the same system. Email services should be installed to a discrete set of directories on a partition that does not host other applications. Email services should never be installed on a Domain Controller/Directory Services server.
SRG-APP-000213
1 Rule
Exchange internet-facing send connectors must specify a smart host.
Medium Severity
When identifying a "Smart Host" for the email environment, a logical Send connector is the preferred method. A Smart Host acts as an internet-facing concentrator for other email servers. Appropriate hardening can be applied to the Smart Host, rather than at multiple locations throughout the enterprise. Failure to identify a Smart Host could default to each email server performing its own lookups (potentially through protective firewalls). Exchange servers should not be internet facing and should therefore not perform any Smart Host functions. When the Exchange servers are internet facing, they must be configured to identify the internet-facing server that is performing the Smart Host function.
SRG-APP-000231
1 Rule
Exchange mailboxes must be retained until backups are complete.
Medium Severity
Backup and recovery procedures are an important part of overall system availability and integrity. Complete backups reduce the chance of accidental deletion of important information and make it possible to have complete recoveries. It is not uncommon for users to receive and delete messages in the scope of a single backup cycle. This setting ensures at least one backup has been run on the mailbox store before the message physically disappears. By enabling this setting, all messages written to recipients who have accounts on this store will reside in backups even if they have been deleted by the user before the backup has run.
SRG-APP-000231
1 Rule
Exchange email forwarding must be restricted.
Medium Severity
Auto-forwarded email accounts do not meet the requirement for digital signature and encryption of Controlled Unclassified Information (CUI) and Personally Identifiable Information (PII) in accordance with DODI 8520.2 (reference ee) and DOD Director for Administration and Management memorandum, "Safeguarding Against and Responding to the Breach of Personally Identifiable Information". Use of forwarding set by an administrator interferes with nonrepudiation requirements that each end user be responsible for creation and destination of email data.
SRG-APP-000231
1 Rule
Exchange email-forwarding SMTP domains must be restricted.
Medium Severity
Auto-forwarded email accounts do not meet the requirement for digital signature and encryption of Controlled Unclassified Information (CUI) and Personally Identifiable Information (PII) in accordance with DODI 8520.2 (reference ee) and DOD Director for Administration and Management memorandum, "Safeguarding Against and Responding to the Breach of Personally Identifiable Information". Use of forwarding set by an administrator interferes with nonrepudiation requirements that each end user be responsible for creation and destination of email data.
SRG-APP-000246
1 Rule
Exchange mailbox stores must mount at startup.
Low Severity
Administrator responsibilities include the ability to react to unplanned maintenance tasks or emergency situations that may require Mailbox data manipulation. Occasionally, there may be a need to start the server with "unmounted" data stores if manual maintenance is being performed on them. Failure to uncheck the "do not mount on startup" condition will result in unavailability of mail services. Correct configuration of this control will prevent unplanned outages due to being enabled. When maintenance is being performed, care should be taken to clear the check box upon task completion so mail stores are available to users (unmounted mailbox stores are not available to users).
SRG-APP-000246
1 Rule
Exchange mail quota settings must not restrict receiving mail.
Low Severity
Mail quota settings control the maximum sizes of a user's mailbox and the system's response if these limits are exceeded. Mailbox data that is not monitored against a quota increases the risk of mail loss due to filled disk space, which can also render the system unavailable. Failure to allow mail receipt may impede users from receiving mission-critical data.
SRG-APP-000246
1 Rule
Exchange mail quota settings must not restrict sending mail.
Low Severity
Mail quota settings control the maximum sizes of a user's mailbox and the system's response if these limits are exceeded. Mailbox data that is not monitored against a quota increases the risk of mail loss due to filled disk space, which can also render the system unavailable. Multiple controls supply graduated levels of opportunity to respond before risking email service loss. This control prohibits the user from sending an email when the mailbox limit reaches the prohibit send quota value. Note: Best practice for this setting is to prohibit the user from sending email when the mailbox reaches 90 percent of capacity.
SRG-APP-000247
1 Rule
Exchange Message size restrictions must be controlled on Receive connectors.
Low Severity
Email system availability depends in part on best practice strategies for setting tuning configurations. For message size restrictions, multiple places exist to set or override inbound or outbound message size. Failure to control the configuration strategy can result in loss of data or system availability. This setting enables the administrator to control the maximum message size on receive connectors. Using connectors to control size limits may necessitate applying message size limitations in multiple places, with the potential of introducing conflicts and impediments in the mail flow. Changing this setting at the connector overrides the global one. Therefore, if operational needs require it, the connector value may be set lower than the global value with the rationale documented in the Email Domain Security Plan (EDSP).
SRG-APP-000247
1 Rule
The Exchange Receive Connector Maximum Hop Count must be 60.
Low Severity
Email system availability depends in part on best practice strategies for setting tuning configurations. This setting controls the maximum number of hops (email servers traversed) a message may take as it travels to its destination. Part of the original internet protocol implementation, the hop count limit prevents a message being passed in a routing loop indefinitely. Messages exceeding the maximum hop count are discarded undelivered. Recent studies indicate that virtually all messages can be delivered in fewer than 60 hops. If the hop count is set too low, messages may expire before they reach their destinations. If the hop count is set too high, an undeliverable message may cycle between servers, raising the risk of network congestion.
SRG-APP-000247
1 Rule
The Exchange send connector connections count must be limited.
Low Severity
The Exchange Send connector setting controls the maximum number of simultaneous outbound connections allowed for a given SMTP connector and can be used to throttle the SMTP service if resource constraints warrant it. If the limit is too low, connections may be dropped. If the limit is too high, some domains may use a disproportionate resource share, denying access to other domains. Appropriate tuning reduces risk of data delay or loss.
SRG-APP-000247
1 Rule
Exchange receive connectors must control the number of recipients per message.
Low Severity
Email system availability depends in part on best practice strategies for setting tuning configurations. This configuration controls the maximum number of recipients who will receive a copy of a message at one time. This tunable value is related to throughput capacity and can enable the ability to optimize message delivery. Note: There are two types of default Receive connectors: Client Servername: Accepts SMTP connections from all non-MAPI clients, such as POP and IMAP. As POP and IMAP are not authorized for use in DOD, these should not be present. Their default value for "MaxRecipientsPerMessage" is "200". Default Servername: Accepts connections from other Hub Transport servers and any Edge Transport servers. Their default value for "MaxRecipientsPerMessage" is "5000".
SRG-APP-000247
1 Rule
Exchange message size restrictions must be controlled on send connectors.
Low Severity
Email system availability depends in part on best practice strategies for setting tuning configurations. For message size restrictions, multiple places exist to set or override inbound or outbound message size. Failure to control the configuration strategy can result in loss of data or system availability. This setting enables the administrator to control the maximum message size on a Send connector. Using connectors to control size limits may necessitate applying message size limitations in multiple places, with the potential of introducing conflicts and impediments in the mail flow. Changing this setting at the connector overrides the global one. Therefore, if operational needs require it, the connector value may be set lower than the global value with the rationale documented in the Email Domain Security Plan (EDSP).
SRG-APP-000247
1 Rule
The Exchange global inbound message size must be controlled.
Low Severity
Email system availability depends in part on best practice strategies for setting tuning configurations. Message size limits should be set to 10 MB at most but often are smaller, depending on the organization. The key point in message size is that it should be set globally and should not be set to "unlimited". Selecting "unlimited" on "MaxReceiveSize" is likely to result in abuse and can contribute to excessive server disk space consumption. Message size limits may also be applied on SMTP connectors, public folders, and on the user account under Active Directory (AD). Changes at these lower levels are discouraged, as the single global setting is usually sufficient. This practice prevents conflicts that could impact availability and simplifies server administration.
SRG-APP-000247
1 Rule
The Exchange global outbound message size must be controlled.
Low Severity
Email system availability depends in part on best practice strategies for setting tuning configurations. Message size limits should be set to 10 MB at most but often are smaller, depending on the organization. The key point in message size is that it should be set globally and should not be set to "unlimited". Selecting "unlimited" on "MaxReceiveSize" is likely to result in abuse and can contribute to excessive server disk space consumption. Message size limits may also be applied on send and receive connectors, public folders, and on the user account under Active Directory (AD). Changes at these lower levels are discouraged, as the single global setting is usually sufficient. This practice prevents conflicts that could impact availability and simplifies server administration.
SRG-APP-000247
1 Rule
The Exchange Outbound Connection Limit per Domain Count must be controlled.
Low Severity
Email system availability depends in part on best practice strategies for setting tuning configurations. This configuration controls the maximum number of simultaneous outbound connections from a domain as a delivery tuning mechanism. If the limit is too low, connections may be dropped. If the limit is too high, some domains may use a disproportionate resource share, denying access to other domains. Appropriate tuning reduces risk of data delay or loss. By default, a limit of 20 simultaneous outbound connections from a domain should be sufficient. The value may be adjusted if justified by local site conditions.
SRG-APP-000247
1 Rule
The Exchange Outbound Connection Timeout must be 10 minutes or less.
Low Severity
Email system availability depends in part on best practice strategies for setting tuning configurations. This configuration controls the number of idle minutes before the connection is dropped. It works in conjunction with the Maximum Outbound Connections Count setting. Once established, connections may incur delays in message transfer. The default of 10 minutes is a reasonable window in which to resume activities without maintaining idle connections for excessive intervals. If the timeout period is too long, idle connections may be maintained for unnecessarily long time periods, preventing new connections from being established. Sluggish connectivity increases the risk of lost data. A value of "10" or less is optimal.
SRG-APP-000261
1 Rule
Exchange servers must have an approved DOD email-aware virus protection software installed.
High Severity
With the proliferation of trojans, viruses, and spam attaching themselves to email messages (or attachments), it is necessary to have capable email-aware antivirus (AV) products to scan messages and identify any resident malware. Because email messages and their attachments are formatted to the MIME standard, a flat-file AV scanning engine is not suitable for scanning email message stores. Email-aware antivirus engines must be Exchange 2019 compliant. Competent email scanners will have the ability to scan mail stores, attachments (including zip or other archive files) and mail queues and to issue warnings or alerts if malware is detected. As with other AV products, a necessary feature to include is the ability for automatic updates.
SRG-APP-000261
1 Rule
Exchange internal receive connectors must not allow anonymous connections.
Medium Severity
This control is used to limit the servers that may use this server as a relay. If a Simple Mail Transport Protocol (SMTP) sender does not have a direct connection to the internet (for example, an application that produces reports to be emailed), it will need to use an SMTP Receive connector that does have a path to the internet (for example, a local email server) as a relay. SMTP relay functions must be protected so third parties are not able to hijack a relay service for their own purposes. Most commonly, hijacking of relays is done by spammers to disguise the source of their messages and may also be used to cover the source of more destructive attacks. Relays can be restricted in one of three ways: by blocking relays (restrict to a blank list of servers), by restricting use to lists of valid servers, or by restricting use to servers that can authenticate. Because authenticated connections are the most secure for SMTP Receive connectors, it is recommended that relays allow only servers that can authenticate.
SRG-APP-000261
1 Rule
Exchange external/internet-bound automated response messages must be disabled.
Medium Severity
Spam originators, in an effort to refine mailing lists, sometimes monitor transmissions for automated bounce-back messages. Automated messages include such items as "Out of Office" responses, nondelivery messages, and automated message forwarding. Automated bounce-back messages can be used by a third party to determine if users exist on the server. This can result in the disclosure of active user accounts to third parties, paving the way for possible future attacks.
SRG-APP-000261
1 Rule
Exchange must have anti-spam filtering installed.
Medium Severity
Originators of spam messages are constantly changing their techniques to defeat spam countermeasures; therefore, spam software must be constantly updated to address the changing threat. A manual update procedure is labor intensive and does not scale well in an enterprise environment. This risk may be mitigated by using an automatic update capability. Spam protection mechanisms include, for example, signature definitions, rule sets, and algorithms. Exchange 2019 provides both anti-spam and anti-malware protection out of the box. The Exchange 2019 anti-spam and anti-malware product capabilities are limited but still provide some protection.
SRG-APP-000261
1 Rule
Exchange must have anti-spam filtering enabled.
Medium Severity
Originators of spam messages are constantly changing their techniques to defeat spam countermeasures; therefore, spam software must be constantly updated to address the changing threat. A manual update procedure is labor intensive and does not scale well in an enterprise environment. This risk may be mitigated by using an automatic update capability. Spam protection mechanisms include, for example, signature definitions, rule sets, and algorithms. Exchange 2019 provides both anti-spam and anti-malware protection out of the box. The Exchange 2019 anti-spam and anti-malware product capabilities are limited but still provide some protection.
SRG-APP-000261
1 Rule
Exchange must have anti-spam filtering configured.
Medium Severity
Originators of spam messages are constantly changing their techniques to defeat spam countermeasures; therefore, spam software must be constantly updated to address the changing threat. A manual update procedure is labor intensive and does not scale well in an enterprise environment. This risk may be mitigated by using an automatic update capability. Spam protection mechanisms include, for example, signature definitions, rule sets, and algorithms. Exchange 2019 provides both anti-spam and anti-malware protection out of the box. The Exchange 2019 anti-spam and anti-malware product capabilities are limited but still provide some protection.
SRG-APP-000261
1 Rule
Exchange must not send automated replies to remote domains.
Medium Severity
Attackers can use automated messages to determine whether a user account is active, in the office, traveling, and so on. An attacker might use this information to conduct future attacks. Remote users will not receive automated "Out of Office" delivery reports. This setting can be used to determine if all the servers in the organization can send "Out of Office" messages.
SRG-APP-000261
1 Rule
The Exchange Global Recipient Count Limit must be set.
Low Severity
Email system availability depends in part on best practice strategies for setting tuning configurations. The Global Recipient Count Limit field is used to control the maximum number of recipients that can be specified in a single message sent from this server. Its primary purpose is to minimize the chance of an internal sender spamming other recipients, since spam messages often have a large number of recipients. Spam prevention can originate from both outside and inside organizations. While inbound spam is evaluated as it arrives, controls such as this one help prevent spam that might originate inside the organization. The Recipient Count Limit is global to the Exchange implementation. Lower-level refinements are possible; however, in this configuration strategy, setting the value once at the global level facilitates a more available system by eliminating potential conflicts among multiple settings. A value of less than or equal to "5000" is probably larger than is needed for most organizations but is small enough to minimize usefulness to spammers and is easily handled by Exchange. An unexpanded distribution is handled as one recipient. Specifying "unlimited" may result in abuse.
SRG-APP-000272
1 Rule
Exchange antimalware agent must be enabled and configured.
Medium Severity
Microsoft Exchange 2019 offers built-in antimalware protection for messages going through the transport pipeline. When enabled, the default settings are configured to automatically update. Exchange's built-in Malware Agent is not designed to address all malicious code protection workloads. This workload is best handled by third-party antivirus and intrusion prevention software. Sites must use an approved DOD scanner. Exchange Malware software has a limited scanning capability and does not scan files that are downloaded, opened, or executed.
SRG-APP-000272
1 Rule
The Exchange malware scanning agent must be configured for automatic updates.
Medium Severity
Antimalware protection in Exchange Server 2019 helps combat viruses and spyware in an email messaging environment. Viruses infect other programs and data, and they spread throughout computer looking for programs to infect. Spyware gathers personal information (for example, sign-in information and personal data) and sends it back to its author. The antimalware protection in Exchange Server was introduced in Exchange 2013 and is provided by the Transport agent named Malware Agent. The agent scans messages as they travel through the Transport service on a Mailbox server. To ensure increased effectiveness of the Malware Agent, ensuring its signatures are automatically updated is imperative. Not doing so can lead to system compromise. The Malware agent is installed during the initial installation of Microsoft Exchange server and if installed, is set for automatic updates by default.
SRG-APP-000295
1 Rule
The Exchange receive connector timeout must be limited.
Low Severity
Email system availability depends in part on best practice strategies for setting tuning. This configuration controls the number of idle minutes before the connection is dropped. It works in conjunction with the Maximum Inbound Connections Count setting. Connections, once established, may incur delays in message transfer. If the timeout period is too long, there is risk that idle connections may be maintained for unnecessarily long time periods, preventing new connections from being established.
SRG-APP-000340
1 Rule
Role-Based Access Control must be defined for privileged and nonprivileged users.
Medium Severity
Role Based Access Control (RBAC) is the permissions model used in Microsoft Exchange Server 2013, 2016, and 2019. With RBAC, there is no need to modify and manage access control lists (ACLs), which was done in Exchange Server 2007. ACLs created several challenges in Exchange 2007, such as modifying ACLs without causing unintended consequences, maintaining ACL modifications through upgrades, and troubleshooting problems that occurred due to using ACLs in a nonstandard way. RBAC enables users to control, at both broad and granular levels, what administrators and end-users can do. RBAC also enables users to more closely align the roles assigned to users and administrators to the actual roles they hold within the organization. In Exchange 2007, the server permissions model applied only to the administrators who managed the Exchange 2007 infrastructure. Starting with Exchange 2013, RBAC now controls both the administrative tasks that can be performed and the extent to which users can now administer their own mailbox and distribution groups.
SRG-APP-000378
1 Rule
The Exchange application directory must be protected from unauthorized access.
Medium Severity
Default product installations may provide more generous access permissions than are necessary to run the application. By examining and tailoring access permissions to provide the least amount of privilege possible more closely, attack vectors that align with user permissions are less likely to access more highly secured areas.
SRG-APP-000380
1 Rule
An Exchange software baseline copy must exist.
Medium Severity
Exchange software, as with other application software installed on a host system, must be included in a system baseline record and periodically reviewed; otherwise, unauthorized changes to the software may not be discovered. This effort is a vital step to securing the host and the applications, as it is the only method that may provide the ability to detect and recover from otherwise undetected changes, such as those that result from worm or bot intrusions. The Exchange software and configuration baseline is created and maintained for comparison during scanning efforts. Operational procedures must include baseline updates as part of configuration management tasks that change the software and configuration.
SRG-APP-000381
1 Rule
Exchange software must be monitored for unauthorized changes.
Medium Severity
Monitoring software files for changes against a baseline on a regular basis may help detect the possible introduction of malicious code on a system.
SRG-APP-000383
1 Rule
Exchange services must be documented, and unnecessary services must be removed or disabled.
Medium Severity
Unneeded but running services offer attackers an enhanced attack profile, and attackers are constantly watching to discover open ports with running services. By analyzing and disabling unneeded services, the associated open ports become unresponsive to outside queries, and servers become more secure as a result. Exchange Server has role-based server deployment to enable protocol path control and logical separation of network traffic types. For example, a server implemented in the Client Access role (i.e., Outlook Web App [OWA]) is configured and tuned as a web server using web protocols. A client access server exposes only web protocols (HTTP/HTTPS), enabling system administrators to optimize the protocol path and disable all services unnecessary for Exchange web services. Similarly, servers created to host mailboxes are dedicated to that task and must operate only the services needed for mailbox hosting. (Exchange servers must also operate some web services, but only to the degree that Exchange requires the IIS engine in order to function). Because Post Office Protocol 3 (POP3) and Internet Message Access Protocol 4 (IMAP4) clients are not included in the standard desktop offering, they must be disabled. While IMAP4 is restricted, IMAP Secure is not restricted and does not apply to this requirement.
SRG-APP-000391
1 Rule
Exchange Outlook Anywhere clients must use NTLM authentication to access email.
Medium Severity
Identification and authentication provide the foundation for access control. Access to email services applications require NTLM authentication. Outlook Anywhere, if authorized for use by the site, must use NTLM authentication when accessing email. Note: There is a technical restriction in Exchange Outlook Anywhere that requires a direct SSL connection from Outlook to the Certificate Authority (CA) server. There is also a constraint where Microsoft supports that the CA server must participate in the Active Director (AD) domain inside the enclave. For this reason, Outlook Anywhere must be deployed only for enclave-sourced Outlook users.
SRG-APP-000431
1 Rule
The Exchange email application must not share a partition with another application.
Medium Severity
In the same way that added security layers can provide a cumulative positive effect on security posture, multiple applications can provide a cumulative negative effect. A vulnerability and subsequent exploit to one application can lead to an exploit of other applications sharing the same security context. For example, an exploit to a web server process that leads to unauthorized administrative access to the host system can most likely lead to a compromise of all applications hosted by the same system. Email services should be installed on a partition that does not host other applications. Email services should never be installed on a Domain Controller/Directory Services server.
SRG-APP-000435
1 Rule
Exchange must not send delivery reports to remote domains.
Medium Severity
Attackers can use automated messages to determine whether a user account is active, in the office, traveling, and so on. An attacker might use this information to conduct future attacks. Ensure that delivery reports to remote domains are disabled. Before enabling this setting, first configure a remote domain using the Exchange Management Console (EMC) or the New-RemoteDomain cmdlet.
SRG-APP-000435
1 Rule
Exchange must not send nondelivery reports to remote domains.
Medium Severity
Attackers can use automated messages to determine whether a user account is active, in the office, traveling, and so on. An attacker might use this information to conduct future attacks. Ensure that nondelivery reports to remote domains are disabled. Before enabling this setting, first configure a remote domain using the Exchange Management Console (EMC) or the New-RemoteDomain cmdlet.
SRG-APP-000435
1 Rule
The Exchange SMTP automated banner response must not reveal server details.
Medium Severity
Automated connection responses occur as a result of FTP or Telnet connections when connecting to those services. They report a successful connection by greeting the connecting client and stating the name, release level, and (often) additional information regarding the responding product. While useful to the connecting client, connection responses can also be used by a third party to determine operating system or product release levels on the target server. The result can include disclosure of configuration information to third parties, paving the way for possible future attacks. For example, when querying the SMTP service on port 25, the default response looks similar to this one: 220 exchange.mydomain.org Microsoft ESMTP MAIL Service ready at Tuesday, 23 Nov 2021 13:43:00 -0500 Changing the response to hide local configuration details reduces the attack profile of the target.
SRG-APP-000435
1 Rule
Exchange internal send connectors must use an authentication level.
Medium Severity
The Simple Mail Transfer Protocol (SMTP) connector is used by Exchange to send and receive messages from server to server. Several controls work together to provide security between internal servers. This setting controls the encryption method used for communications between servers. With this feature enabled, only servers capable of supporting Transport Layer Security (TLS) will be able to send and receive mail within the domain. The use of secure communication prevents eavesdroppers from reading or modifying communications between mail clients and servers. While sensitive message bodies should be encrypted by the sender at the client, requiring a secure connection from server to server adds protection by encrypting the sender and recipient information that cannot be encrypted by the sender. Individually, channel security and encryption can be compromised by attackers. Used together, email becomes a more difficult target, and security is heightened. Failure to enable this feature gives eavesdroppers an opportunity to read or modify messages between servers.
SRG-APP-000435
1 Rule
Exchange must provide mailbox databases in a highly available and redundant configuration.
Medium Severity
Exchange Server mailbox databases and any data contained in those mailboxes should be protected. This can be accomplished by configuring Mailbox servers and databases for high availability and site resilience. A database availability group (DAG) is a component of the Mailbox server high availability and site resilience framework built into Microsoft Exchange Server 2019. A DAG is a group of Mailbox servers that hosts a set of databases and provides automatic database-level recovery from failures that affect individual servers or databases. A DAG is a boundary for mailbox database replication and database and server switchovers and failovers. Any server in a DAG can host a copy of a mailbox database from any other server in the DAG. When a server is added to a DAG, it works with the other servers in the DAG to provide automatic recovery from failures that affect mailbox databases, such as a disk, server, or network failure.
SRG-APP-000439
1 Rule
The application must protect the confidentiality and integrity of transmitted information.
High Severity
Without protection of the transmitted information, confidentiality and integrity may be compromised since unprotected communications can be intercepted and either read or altered. This requirement applies only to those applications that are either distributed or can allow access to data nonlocally. Use of this requirement will be limited to situations where the data owner has a strict requirement for ensuring data integrity and confidentiality is maintained at every step of the data transfer and handling process. When transmitting data, applications need to leverage transmission protection mechanisms, such as TLS, TLS VPNs, or IPsec. Communication paths outside the physical protection of a controlled boundary are exposed to the possibility of interception and modification. Protecting the confidentiality and integrity of organizational information can be accomplished by physical means (e.g., employing physical distribution systems) or by logical means (e.g., employing cryptographic techniques). If physical means of protection are employed, then logical means (cryptography) do not have to be employed, and vice versa. Satisfies: SRG-APP-000219
SRG-APP-000456
1 Rule
Exchange must have the most current, approved Cumulative Update installed.
Medium Severity
Failure to install the most current Exchange Cumulative Update (CU) leaves a system vulnerable to exploitation. Current CUs correct known security and system vulnerabilities.
SRG-APP-000516
1 Rule
Exchange must be configured in accordance with the security configuration settings based on DOD security configuration or implementation guidance, including STIGs, NSA configuration guides, CTOs, and DTMs.
Medium Severity
Configuring Exchange to implement organization-wide security implementation guides and security checklists ensures compliance with federal standards and establishes a common security baseline across 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 application, including the parameters required to satisfy other security control requirements.