SV-35076r1: The SMTP service must not have the EXPN feature active. | HP-UX 11.31 Security Technical Implementation Guide

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Assessment Title

Rule Text

The SMTP service must not have the EXPN feature active.

Description

Discussion Text

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.

How this relates to the STIG checklist: STIG vs SCAP benchmark.

IA Controls

NIST SP 800-53

Not mappedCCI-000366

Additional Details

False Positives

False positives may occur with the SMTP EXPN check. According to RFC821, it is acceptable for a server to respond with a 250 (success) or 550 (failure) when the server supports the EXPN command. For example, some servers return "550 EXPN command not available," meaning the command is not supported and the machine is not vulnerable. However, a result of "550 That is a mailing list, not a user" would be a failure code, but not an indication of an error, and the machine would be vulnerable. If a false positive is suspected, check your log file for the response from the server.

Responsibility

System Administrator

IA Controls

ECSC-1

References

DPMS Target HP-UX 11.31 DISA DPMS Target HP-UX 11.31 2171

SCC Automation Reference

The reconstructed evaluation script behind this rule's OVAL check(s) — every module a compound check combines, with this rule's own concrete values and DISA's SCC tool's real per-module mechanism curated from static teardown of SCC releases.

#!/bin/bash
# Reconstructed from oval:mil.disa.fso.hpux11:def:271
# The SMTP service must not have the EXPN feature active.
# Per-module read mechanism curated from static teardown of DISA's SCC tool
# (SCAP Compliance Checker) -- see the SCC Automation Reference above.

# expn disabled
# textfilecontent54_object (textfilecontent54_test)
test_1() {
  (sed -En 's/^O PrivacyOptions=([^
]*)/\1/p' '/etc/mail/sendmail.cf') | grep -Pq '(^|,)[ 	]*(noexpn|goaway)[ 	]*(,|$)'
}
# sendmail not installed
# file_object (file_test)
test_2() {
  ! ( test -e '/usr/sbin/sendmail' &>/dev/null )
}

if ( test_1 || test_2 ); then
  echo "PASS"
else
  echo "FAIL"
fi

Assessment Script Powered by AISandbox-executed

A standalone Shell script to assess this control automatically. Review before running it in your environment.

Syntax-checked and actually executed in an isolated sandbox (not the real system this checks) — confirmed to run without errors.

#!/bin/sh
# OVAL Definition: oval:mil.disa.fso.hpux11:def:271


if ! grep -qE "(^|,)[ \t]*(noexpn|goaway)[ \t]*(,|$)" /etc/mail/sendmail.cf || [ ! -p "/etc/mail/sendmail.cf" ]; then
    if ! grep -qE "^\O PrivacyOptions=([^\r\n]*)$" /etc/mail/sendmail.cf; then
        echo "FAIL"
        exit 1
    fi
fi

if ! [ -x /usr/sbin/sendmail ]; then
    echo "PASS"
    exit 0
fi

echo "FAIL"
exit 1

Remediation

Edit the /etc/mail/sendmail.cf file and add or edit the following line:
O PrivacyOptions=goaway

Then restart the Sendmail service.