The SMTP service must not have the EXPN feature active.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-4692	GEN004660	SV-28402r1_rule	ECSC-1	Low

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.

STIG	Date
AIX 6.1 SECURITY TECHNICAL IMPLEMENTATION GUIDE	2018-09-18

Details

Check Text ( C-655r2_chk )
Perform the following to determine if EXPN is disabled: # telnet localhost 25 expn root If the command does not return a 500 error code of command unrecognized, this is a finding. OR Locate the sendmail.cf configuration file by: # find / -name sendmail.cf -print # grep -v "^#" \|grep -i privacyoptions The O PrivacyOptions should have the noexpn or the goaway option (covering both noexpn and novrfy). If the EXPN command is not disabled, this is a finding.

Check Text ( C-655r2_chk )

Perform the following to determine if EXPN is disabled:

# telnet localhost 25
expn root

If the command does not return a 500 error code of command unrecognized, this is a finding.

OR

Locate the sendmail.cf configuration file by:

# find / -name sendmail.cf -print
# grep -v "^#" |grep -i privacyoptions

The O PrivacyOptions should have the noexpn or the goaway option (covering both noexpn and novrfy).
If the EXPN command is not disabled, this is a finding.

Fix Text (F-25673r1_fix)
Edit the sendmail.cf file and add or edit the following line: O PrivacyOptions=goaway Restart the Sendmail service.