SMTP service must not have the EXPN or VRFY features active on AIX systems.

Severity	Group ID	Group Title	Version	Rule ID	Date	STIG Version
medium	V-215415	SRG-OS-000480-GPOS-00227	AIX7-00-003117	SV-215415r991589_rule	2024-08-16	3

Description
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.

Description

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.

ℹ️ Check
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
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"