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The SMTP service must not have the EXPN feature active.


Finding ID Version Rule ID IA Controls Severity
V-89669 VRAU-SL-000625 SV-100319r1_rule Medium
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.
VMware vRealize Automation 7.x SLES Security Technical Implementation Guide 2018-10-12


Check Text ( C-89361r1_chk )
Use the following command to check if EXPN is disabled:

# grep -v "^#" /etc/sendmail.cf |grep -i PrivacyOptions

If "noexpn" is not returned, this is a finding.
Fix Text (F-96411r1_fix)
Add "noexpn" to the "PrivacyOptions" flag in /etc/sendmail.cf