Software Engineering Institute

Sendmail is a widely used mail transfer agent (MTA) that is included with IBM AIX. According to IBM:

The default configuration files for sendmail enable three sendmail options which allow arbitrary users to route email via sendmail. The options are "promiscuous_relay", "accept_unresolvable_domains" and "accept_unqualified_senders".
In this configuration, sendmail accepts messages addressed from and to non-local email domains.

Any remote user can route mail through sendmail on a vulnerable IBM AIX system. This configuration is called an "open relay" and such systems are frequently abused to deliver unsolicited commercial email (UCE) or SPAM.

Modify sendmail Configuration
Configure sendmail not to accept messages addressed from and to foreign domains on behalf of unauthenticated users. IBM has released an advisory and updated configuration files for sendmail on AIX:

ftp://aix.software.ibm.com/aix/efixes/security/sendmail_3_mod.tar.Z
From the IBM advisory:

Note that this change should only be applied to systems using the default configuration files. Administrators who use customized configuration files will lose their customizations. The changes made by this modification will be lost if a migration install is done; it will be necessary to re-apply the change. This change will be made permanent in a future release of AIX.

Block or Restrict Access

Using a firewall or similar technology, block access to vulnerable AIX sendmail systems (SMTP 25/tcp) from untrusted networks such as the Internet.