cyberstrikeus 2.2.15 Ensure mail transfer agent is configured for local-only mode (Scored)
Profile Applicability
- Level 1 - Server
- Level 1 - Workstation
Description
Mail Transfer Agents (MTA), such as sendmail and Postfix, are used to listen for incoming mail and transfer the messages to the appropriate user or mail server. If the system is not intended to be a mail server, it is recommended that the MTA be configured to only process local mail.
Rationale
The software for all Mail Transfer Agents is complex and most have a long history of security issues. While it is important to ensure that the system can process local mail messages, it is not necessary to have the MTA's daemon listening on a port unless the server is intended to be a mail server that receives and processes mail from other systems.
Audit Procedure
Run the following command and verify that the MTA is not listening on any non-loopback address (127.0.0.1 or ::1):
netstat -an | grep LIST | grep ":25[[:space:]]"
Expected output:
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN
Expected Result
The MTA should only be listening on the loopback address (127.0.0.1:25 or ::1:25). No non-loopback addresses should appear.
Remediation
Edit /etc/postfix/main.cf and add the following line to the RECEIVING MAIL section. If the line already exists, change it to look like the line below:
inet_interfaces = loopback-only
Restart postfix:
service postfix restart
Note: This recommendation is designed around the postfix mail server, depending on your environment you may have an alternative MTA installed such as sendmail. If this is the case consult the documentation for your installed MTA to configure the recommended state.
Default Value
Postfix listens on all interfaces by default.
References
- CIS Controls: 9.1 Limit Open Ports, Protocols, and Services
Profile
- Level 1 - Server
- Level 1 - Workstation