Information
The recommendation is to ensure that PrivacyOptions includes at least three settings:
- authwarnings (a default)
- novrfy
- noexpn
The sendmail deamon has a history of security vulnerabilities. The recommendation is to modify default sendmail settings that otherwise may provide information that can be used by an attacker.
- novrfy: No Verify: do not verify valid email addresses. This can be used by attackers, e.g., phishing attacks.
- noexpn: no expansion: do not verify/expand email list addresses - providing attackers with a list of valid email addresses.
Solution
Create a backup copy of /etc/mail/sendmail.cf :
cp -p /etc/mail/sendmail.cf /etc/mail/sendmail.cf.pre_cis
Edit:
vi /etc/mail/sendmail.cf
Replace:
O PrivacyOptions=authwarnings
With:
O PrivacyOptions=authwarnings,noexpn,novrfy
Or - append noexpn,novrfy at then end of the current PrivacyOptions settings (assuming authwarnings is already included).