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XCCDF
MS Exchange 2013 Edge Transport Server Security Technical Implementation Guide
MS Exchange 2013 Edge Transport Server Security Technical Implementation Guide
An XCCDF Benchmark
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63 rules organized in 63 groups
SRG-APP-000033
1 Rule
Exchange servers must use approved DoD certificates.
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 must have accepted domains configured.
Medium Severity
Exchange may be configured to accept email for multiple domain names. This setting identifies the domains for which the server will accept mail. This check verifies the email server is not accepting email for unauthorized domains.
SRG-APP-000038
1 Rule
Exchange external Receive connectors must be domain secure-enabled.
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 authentication method used for communications between servers. With this feature enabled, messages can be securely passed from a partner domain securely. 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-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 broken up into 29 main "services", each of which has anywhere from 2 to 26 "categories" of events to be monitored. Moreover, each category may be set to one of four levels of logging: Lowest, Low, CAT II, and High, depending on how much detail one desires. The higher the level of detail, the more disk space required 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 deal 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 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 Hub Transport servers and Edge Transport servers. 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 at the Hub server, the setting will apply to both Hub and Edge roles.
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 exist occasionally. Frequent alerts against this counter may indicate a network or other issue (such as inbound SPAMMER 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-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 non-essential 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 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-000118
1 Rule
Exchange Audit data must be protected against unauthorized access (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-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 non-essential 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 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 to Microsoft must not be sent.
SRG-APP-000119
1 Rule
Exchange audit data must be protected against unauthorized access for modification.
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 audit data must be protected against unauthorized access for 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
The Exchange local machine policy must require signed scripts.
Medium Severity
Scripts, especially those downloaded from untrusted locations, often provide a way for attackers to infiltrate a system. By setting machine policy to prevent unauthorized script executions, unanticipated system impacts can be avoided.
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-000219
1 Rule
Exchange Internet-facing Receive connectors must offer Transport Layer Security (TLS) before using basic authentication.
Medium Severity
Sending unencrypted email over the Internet increases the risk that messages can be intercepted or altered. TLS is designed to protect confidentiality and data integrity by encrypting email messages between servers and thereby reducing the risk of eavesdropping, interception, and alteration. This setting forces Exchange to offer TLS before using basic authentication.
SRG-APP-000247
1 Rule
Exchange Outbound Connection Timeout must be 10 minutes or less.
Medium 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. Connections, once established, 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-000247
1 Rule
Exchange Outbound Connection Limit per Domain Count must be controlled.
Medium 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 and works in conjunction with the Maximum Outbound Connections Count setting 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 the 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
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 megabytes at most but often are smaller, depending on the organization. The key point in message size is that it should be set globally, and it should not be set to "unlimited". Selecting "unlimited" on either field 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 the user account in Active Directory. 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
Exchange Send connector connections count must be limited.
Low Severity
This 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 the risk of data delay or loss.
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 EDSP.
SRG-APP-000247
1 Rule
Exchange Send connectors delivery retries must be controlled.
Low Severity
This setting controls the rate at which delivery attempts from the home domain are retried and user notifications are issued and notes the expiration time when the message will be discarded. If delivery retry attempts are too frequent, servers will generate network congestion. If they are too far apart, messages may remain queued longer than necessary, potentially raising disk resource requirements. The default values of these fields should be adequate for most environments. Administrators may wish to modify the values, but changes should be documented in the System Security Plan. NOTE: Transport configuration settings apply to the organization/global level of the Exchange SMTP path. By checking and setting them at the Hub server, the setting will apply to both Hub and Edge roles.
SRG-APP-000247
1 Rule
Exchange Send connectors must be clearly named.
Low Severity
For Send connectors, unclear naming as to direction and purpose increases risk that messages may not flow as intended, troubleshooting efforts may be impaired, or incorrect assumptions may be made about the completeness of the configuration. Collectively, connectors should account for all connections required for the overall email topology design. Simple Mail Transfer Protocol (SMTP) connectors, when listed, must name purpose and direction clearly, and their counterparts on servers to which they connect should be recognizable as their partners.
SRG-APP-000247
1 Rule
Exchange Receive connector Maximum Hop Count must be 60.
Medium 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 from 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 set too high, an undeliverable message may cycle between servers, raising the risk of network congestion.
SRG-APP-000247
1 Rule
Exchange Receive connectors must be clearly named.
Low Severity
For receive connectors, unclear naming as to direction and purpose increases risk that messages may not flow as intended, troubleshooting efforts may be impaired, or incorrect assumptions may be made about the completeness of the configuration. Collectively, connectors should account for all connections required for the overall email topology design. Simple Mail Transfer Protocol (SMTP) connectors, when listed, must name purpose and direction clearly, and their counterparts on servers to which they connect should be recognizable as their partners.
SRG-APP-000247
1 Rule
Exchange Receive connectors must control the number of recipients chunked on a single message.
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 enable "chunking" on received messages as they arrive at the domain. This is done so large message bodies can be relayed by the remote sender to the Receive connector in multiple, smaller chunks.
SRG-APP-000247
1 Rule
Exchange Receive connectors must control the number of recipients per message.
Medium 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 connecters: "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
The Exchange Internet Receive connector connections count must be set to default.
Low Severity
Email system availability depends in part on best practice strategies for setting tuning configurations. This configuration controls the maximum number of simultaneous inbound connections allowed to the SMTP server. By default, the number of simultaneous inbound connections is 5000. If a limit is set too low, the connections pool may be filled. If attackers perceive the limit is too low, they could deny service to the Simple Mail Transfer Protocol (SMTP) server by using a connection count that exceeds the limit set. By setting the default configuration to 5000, attackers would need many more connections to cause denial of service.
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 practices 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 EDSP.
SRG-APP-000261
1 Rule
Exchange messages with a blank sender field must be rejected.
Medium Severity
By performing filtering at the perimeter, up to 90 percent of spam, malware, and other undesirable messages are eliminated from the message stream rather than admitting them into the mail server environment. Anonymous email (messages with blank sender fields) cannot be replied to. Messages formatted in this way may be attempting to hide their true origin to avoid responses or to spam any receiver with impunity while hiding their source of origination. Rather than spend resources and risk infection while evaluating them, it is recommended that these messages be filtered immediately upon receipt and not forwarded to end users.
SRG-APP-000261
1 Rule
Exchange messages with a blank sender field must be filtered.
Medium Severity
By performing filtering at the perimeter, up to 90 percent of spam, malware, and other undesirable messages are eliminated from the message stream rather than admitting them into the mail server environment. Anonymous email (messages with blank sender fields) cannot be replied to. Messages formatted in this way may be attempting to hide their true origin to avoid responses or to spam any receiver with impunity while hiding their source of origination. Rather than spend resources and risk infection while evaluating them, it is recommended that these messages be filtered immediately upon receipt and not forwarded to end users.
SRG-APP-000261
1 Rule
Exchange filtered messages must be archived.
Medium Severity
By performing filtering at the perimeter, up to 90 percent of spam, malware, and other undesirable messages are eliminated from the message stream rather than admitting them into the mail server environment. This significantly reduces the attack vector for inbound email-borne spam and malware. As messages are filtered, it is prudent to temporarily host them in an archive for evaluation by administrators or users. The archive can be used to recover messages that might have been inappropriately filtered, preventing data loss, and to provide a base of analysis that can provide future filter refinements.
SRG-APP-000261
1 Rule
The Exchange Sender filter must block unaccepted domains.
Medium Severity
Spam origination sites and other sources of suspected email-borne malware have the ability to corrupt, compromise, or otherwise limit availability of email servers. Limiting exposure to unfiltered inbound messages can reduce the risk of spam and malware impacts. The Global Deny list blocks messages originating from specific sources. Most blacklist filtering is done using a commercial Block List service, because eliminating threats from known spammers prevents the messages being evaluated inside the enclave where there is more risk they can do harm. Additional sources should also be blocked to supplement the contents of the commercial Block List service. For example, during a zero-day threat action, entries can be added and then removed when the threat is mitigated. An additional best practice is to enter the enterprise’s home domains in the Deny List, because inbound email with a "from" address of the home domain is very likely to be spoofed spam.
SRG-APP-000261
1 Rule
Exchange nonexistent recipients must not be blocked.
Medium Severity
Spam originators, in an effort to refine mailing lists, sometimes use a technique where they first create fictitious names and then monitor rejected emails for non-existent recipients. Those not rejected are deemed to exist and are used in future spam mailings. To prevent this disclosure of existing email accounts to spammers, email to nonexistent recipients must not be blocked. Instead, it is recommended that all messages be received, then evaluated and disposed of without enabling the sender to determine existent vs. nonexistent recipients.
SRG-APP-000261
1 Rule
The Exchange Sender Reputation filter must be enabled.
Medium Severity
By performing filtering at the perimeter, up to 90 percent of spam, malware, and other undesirable messages are eliminated from the message stream rather than admitting them into the mail server environment. Sender Reputation is antispam functionality that blocks messages according to many characteristics of the sender. Sender Reputation relies on persisted data about the sender to determine what action, if any, to take on an inbound message. This setting enables the Sender Reputation function.
SRG-APP-000261
1 Rule
The Exchange Sender Reputation filter must identify the spam block level.
Medium Severity
By performing filtering at the perimeter, up to 90 percent of spam, malware, and other undesirable messages are eliminated from the message stream rather than admitting them into the mail server environment. Sender Reputation is antispam functionality that blocks messages according to many characteristics of the sender. Sender Reputation relies on persisted data about the sender to determine what action, if any, to take on an inbound message. This setting enables the threshold at which an email will be considered spam.
SRG-APP-000261
1 Rule
Exchange Attachment filtering must remove undesirable attachments by file type.
Medium Severity
By performing filtering at the perimeter, up to 90 percent of spam, malware, and other undesirable messages are eliminated from the message stream rather than admitting them into the mail server environment. Attachments are being used more frequently for different forms of attacks. By filtering undesirable attachments a large percent of malicious code can be prevented from entering the system. Attachments must be controlled at the entry point into the email environment to prevent successful attachment-based attacks. The following is a basic list of known attachments that should be filtered from Internet mail attachments: *.ade *.crt *.jse *.msi *.scr *.wsh *.dir *.adp *.csh *.ksh *.msp *.sct *.htm *.dcr *.app *.exe *.lnk *.mst *.shb *.html *.plg *.asx *.fxp *.mda *.ops *.shs *.htc *.spl *.bas *.hlp *.mdb *.pcd *.url *.mht *.swf *.bat *.hta *.mde *.pif *.vb *.mhtml *.zip *.chm *.inf *.mdt *.prf *.vbe *.shtm *.cmd *.ins *.mdw *.prg *.vbs *.shtml *.com *.isp *.mdz *.reg *.wsc *.stm *.cpl *.js *.msc *.scf *.wsf *.xml
SRG-APP-000261
1 Rule
The Exchange Spam Evaluation filter must be enabled.
Medium Severity
By performing filtering at the perimeter, up to 90 percent of spam, malware, and other undesirable messages may be eliminated from the transport message stream, preventing their entry into the Exchange environment. This significantly reduces the attack vector for inbound email-borne spam and malware. Spam Evaluation filters scan inbound email messages for evidence of spam and other attacks that primarily use "social engineering" techniques. Upon evaluation completion, a rating is assigned to each message estimating the likelihood of its being spam. Upon arrival at the destination mailbox, the junk mail filter threshold (also configurable) determines whether the message will be withheld from delivery, delivered to the junk mail folder, or delivered to the user’s inbox.
SRG-APP-000261
1 Rule
The Exchange Block List service provider must be identified.
Medium Severity
Block List filtering is a sanitization process performed on email messages prior to their arrival at the destination mailbox. By performing this process at the email perimeter, threats can be eliminated outside the enclave, where there is less risk they can do harm. Block List services (sometimes called Reputation Data services) are fee-based data providers that collect the IP addresses of known spammers and other malware purveyors. Block List service subscribers benefit from more effective spam elimination. (Spam is estimated to compose up to 90 percent of inbound mail volume.) Failure to specify a Block List provider risks that manual email administration effort would be needed to maintain and update larger Block Lists than a single email site administrator could conveniently or accurately maintain. The Block List service vendor provides a value for this field, usually the Domain Name System (DNS) suffix for its domain.
SRG-APP-000261
1 Rule
Exchange messages with malformed From address must be rejected.
Medium Severity
Sender Identification (SID) is an email antispam sanitization process. Sender ID uses DNS MX record lookups to verify the Simple Mail Transfer Protocol (SMTP) sending server is authorized to send email for the originating domain. Failure to implement Sender ID risks that spam could be admitted into the email domain that originates from rogue servers. Most spam content originates from domains where the IP address has been spoofed prior to sending, thereby avoiding detection. For example, messages with malformed or incorrect "purported responsible sender" data in the message header could be (best case) created by using RFI noncompliant software but is more likely to be spam.
SRG-APP-000261
1 Rule
The Exchange Recipient filter must be enabled.
Medium Severity
Email system availability depends in part on best practice strategies for setting tuning configurations. Careful tuning reduces the risk that system or network congestion will contribute to availability impacts. Filters that govern inbound email evaluation can significantly reduce spam, phishing, and spoofed emails. Messages from blank senders, known spammers, or zero-day attack modifications must be enabled to be effective.
SRG-APP-000261
1 Rule
The Exchange tarpitting interval must be set.
Medium Severity
Tarpitting is the practice of artificially delaying server responses for specific Simple Mail Transfer Protocol (SMTP) communication patterns that indicate high volumes of spam or other unwelcome messages. The intent of tarpitting is to slow down the communication process for spam batches to reduce the cost effectiveness of sending spam and thwart directory harvest attacks. A directory harvest attack is an attempt to collect valid email addresses from a particular organization so the email addresses can be added to a spam database. A program can be written to collect email addresses that return a "Recipient OK" SMTP response and discard all email addresses that return a "User unknown" SMTP response. Tarpitting makes directory harvest attacks too costly to automate efficiently.
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, relay hijacking 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 Simple Mail Transfer Protocol (SMTP) IP Allow List entries must be empty.
Medium Severity
Email system availability depends in part on best practice strategies for setting tuning configurations. Careful tuning reduces the risk that system or network congestion will contribute to availability impacts. Filters that govern inbound email evaluation can significantly reduce spam, phishing, and spoofed emails. Filters for messages from blank senders, known spammers, or zero-day attack modifications must be enabled to be effective. Having items identified in the Allow List causes other spam evaluation steps to be bypassed and therefore should be used only with an abundance of caution. If spammers were to learn of entries in the Allow List, it could enable them to plan a denial of service attack (or other attack) by spoofing that source.
SRG-APP-000261
1 Rule
The Exchange Simple Mail Transfer Protocol (SMTP) IP Allow List Connection filter must be enabled.
Medium Severity
Email system availability depends in part on best practice strategies for setting tuning configurations. Careful tuning reduces the risk that system or network congestion will contribute to availability impacts. Filters that govern inbound email evaluation can significantly reduce spam, phishing, and spoofed emails. Filters for messages from blank senders, known spammers, or zero-day attack modifications must be enabled to be effective. Having items identified in the Allow List causes other spam evaluation steps to be bypassed and therefore should be used only with an abundance of caution. If spammers were to learn of entries in the Allow List, it could enable them to plan a denial of service attack (or other attack) by spoofing that source.
SRG-APP-000261
1 Rule
The Exchange Simple Mail Transfer Protocol (SMTP) Sender filter must be enabled.
Medium Severity
Email system availability depends in part on best practices strategies for setting tuning configurations. Careful tuning reduces the risk that system or network congestion will contribute to availability impacts. Filters that govern inbound email evaluation can significantly reduce spam, phishing, and spoofed emails. Filters for messages from blank senders, known spammers, or zero-day attack modifications must be enabled to be effective. Failure to enable the filter will result in no action taken. This setting should always be enabled.
SRG-APP-000261
1 Rule
Exchange must have antispam filtering installed.
Medium Severity
Originators of spam messages are constantly changing their techniques in order to defeat spam countermeasures; therefore, spam software must be constantly updated to address the changing threat. Spam protection mechanisms include, for example, signature definitions, rule sets, and algorithms. Exchange 2013 provides both antispam and antimalware protection out of the box. The Exchange 2013 antispam and antimalware product capabilities are limited but still provide some protection.
SRG-APP-000261
1 Rule
Exchange must have antispam filtering enabled.
Medium Severity
Originators of spam messages are constantly changing their techniques in order to defeat spam countermeasures; therefore, spam software must be constantly updated to address the changing threat. Spam protection mechanisms include, for example, signature definitions, rule sets, and algorithms. Exchange 2013 provides both antispam and antimalware protection out of the box. The Exchange 2013 antispam and antimalware product capabilities are limited but still provide some protection.
SRG-APP-000261
1 Rule
Exchange must have antispam filtering configured.
Medium Severity
Originators of spam messages are constantly changing their techniques in order 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 2013 provides both antispam and antimalware protection out of the box. The Exchange 2013 antispam and antimalware product capabilities are limited but still provide some protection.
SRG-APP-000261
1 Rule
Exchange Sender Identification Framework must be enabled.
Medium Severity
Email is only as secure as the recipient. When the recipient is an email server accepting inbound messages, authenticating the sender enables the receiver to better assess message quality and to validate the sending domain as authentic. One or more authentication techniques used in combination can be effective in reducing spam, phishing, and forger attacks. The Sender ID Framework (SIDF) receiver accesses specially formatted DNS records (SPF format) that contain the IP address of authorized sending servers for the sending domain that can be compared to data in the email message header. Receivers are able to validate the authenticity of the sending domain, helping to avoid receiving inbound messages from phishing or other spam domains.
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 more closely provide the least amount of privilege possible, attack vectors that align with user permissions are less likely to access more highly secured areas.
SRG-APP-000380
1 Rule
The 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 POP3 and IMAP4 clients are not included in the standard desktop offering, they must be disabled.
SRG-APP-000431
1 Rule
Exchange software must be installed on a separate partition from the OS.
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
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 about 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, Version: 6.0.3790.211 ready at Wed, 2 Feb 2005 23:40:00 -0500 Changing the response to hide local configuration details reduces the attack profile of the target.
SRG-APP-000435
1 Rule
Exchange must provide redundancy.
High Severity
Denial of Service (DoS) is a condition when a resource is not available for legitimate users. When this occurs, the organization either cannot accomplish its mission or must operate at degraded capacity. This requirement addresses the configuration of applications to mitigate the impact of DoS attacks that have occurred or are ongoing on application availability. For each application, known and potential DoS attacks must be identified and solutions for each type implemented. A variety of technologies exist to limit or, in some cases, eliminate the effects of DoS attacks (e.g., limiting processes or restricting the number of sessions the application opens at one time). Employing increased capacity and bandwidth, combined with service redundancy, may reduce the susceptibility to some DoS attacks.
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-000439
1 Rule
Exchange internal Receive connectors must require encryption.
High Severity
The Simple Mail Transfer Protocol (SMTP) Receive connector is used by Exchange to send and receive messages from server to server using SMTP protocol. This setting controls the encryption strength used for client connections to the SMTP Receive connector. With this feature enabled, only clients capable of supporting secure communications will be able to send mail using this SMTP server. Where secure channels are required, encryption can also be selected. 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 the client to the server adds protection by encrypting the sender and recipient information that cannot be encrypted by the sender. Individually, channel security and encryption have been 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 the client and server.
SRG-APP-000439
1 Rule
Exchange internal Send connectors must require encryption.
High 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-000456
1 Rule
Exchange must have the most current, approved service pack installed.
Medium Severity
The organization (including any contractor to the organization) must promptly install security-relevant software updates (e.g., patches, service packs, hot fixes). Flaws discovered during security assessments, continuous monitoring, incident response activities, or information system error handling must also be addressed.
SRG-APP-000278
1 Rule
The applications built-in Malware Agent must be disabled.
Medium Severity
Malicious code protection mechanisms include, but are not limited, to, anti-virus and malware detection software. In order to minimize potential negative impact to the organization that can be caused by malicious code, it is imperative that malicious code is identified and eradicated. Malicious code includes viruses, worms, Trojan horses, and Spyware. It is not enough to simply have the software installed; this software must periodically scan the system to search for malware on an organization-defined frequency. Exchange's built-in Malware Agent is not designed to address all malicious code protection workloads. This workload is best handled by third-party anti-virus and intrusion prevention software. Site must utilize 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-000278
1 Rule
A DoD-approved third party Exchange-aware malicious code protection application must be implemented.
Medium Severity
Malicious code protection mechanisms include, but are not limited, to, anti-virus and malware detection software. In order to minimize potential negative impact to the organization that can be caused by malicious code, it is imperative that malicious code is identified and eradicated. Malicious code includes viruses, worms, Trojan horses, and Spyware. It is not enough to simply have the software installed; this software must periodically scan the system to search for malware on an organization-defined frequency. Exchange's built-in Malware Agent is not designed to address all malicious code protection workloads. This workload is best handled by third-party anti-virus and intrusion prevention software. Site must utilize 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-000001
1 Rule
Exchange must limit the Receive connector timeout.
Medium Severity
Email system availability depends in part on best practices 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 connections may be maintained for unnecessarily long time periods, preventing new connections from being established.