Title

SMTP service must not have the EXPN or VRFY features active on AIX systems. (Cat II impact)

Discussion

The SMTP EXPN function allows an attacker to determine if an account exists on a system, providing significant assistance to a brute force attack on user accounts. EXPN may also provide additional information concerning users on the system, such as the full names of account owners. The VRFY (Verify) command allows an attacker to determine if an account exists on a system, providing significant assistance to a brute force attack on user accounts. VRFY may provide additional information about users on the system, such as the full names of account owners.

Check Content

Check the "PrivacyOptions" parameter in "/etc/mail/sendmail.cf": # grep -v "^#" /etc/mail/sendmail.cf |grep -i privacyoptions The above command should yield the following output: O PrivacyOptions=goaway The "O PrivacyOptions" should have the "goaway" option (covering both noexpn and novrfy). If the "O PrivacyOptions" value does not contain "goaway", this is a finding.

Fix Text

Edit the "sendmail.cf" file and add or edit the following line: O PrivacyOptions=goaway Restart the "Sendmail" service: # startsrc -s sendmail -a "-bd -q30m"

Additional Identifiers

Rule ID: SV-215415r508663_rule

Vulnerability ID: V-215415

Group Title: SRG-OS-000480-GPOS-00227

Expert Comments

CCIs tied to check.

Controls tied to check. These are derived from the CCIs shown above.