cis-ubuntu1204-v110-13-17

name	cis-ubuntu1204-v110-13-17
description	Check for Duplicate User Names
category	cis-os-hardening
version	1.1.0
author	cyberstrike-official
tags	["cis","ubuntu",12.04,"linux","user-management","duplicate","username","accountability"]
cis_id	13.17
cis_benchmark	CIS Ubuntu 12.04 LTS Server Benchmark v1.1.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

13.17 Check for Duplicate User Names (Scored)

Profile Applicability

• Level 1

Description

Although the useradd program will not let you create a duplicate user name, it is possible for an administrator to manually edit the /etc/passwd file and change the user name.

Rationale

If a user is assigned a duplicate user name, it will create and have access to files with the first UID for that username in /etc/passwd. For example, if "test4" has a UID of 1000 and a subsequent "test4" entry has a UID of 2000, logging in as "test4" will use UID 1000. Effectively, the UID is shared, which is a security problem.

Audit Procedure

Using Command Line

This script checks to make sure all user names in the /etc/passwd file are unique.

#!/bin/bash
cat /etc/passwd | /usr/bin/cut -f1 -d":" | /usr/bin/sort -n | /usr/bin/uniq -c |\
while read x ; do
    [ -z "${x}" ] && break
    set - $x
    if [ $1 -gt 1 ]; then
        uids=`/usr/bin/awk -F: '($1 == n) { print $3 }' n=$2 \
            /etc/passwd | xargs`
        echo "Duplicate User Name ($2): ${uids}"
    fi
done

Expected Result

No output should be returned. Any output indicates duplicate user names exist in /etc/passwd.

Remediation

Using Command Line

Based on the results of the script, establish unique user names for the users. File ownerships will automatically reflect the change as long as the users have unique UIDs.

Default Value

The useradd command prevents creation of duplicate user names by default.

References