Red Hat and Linux libuser /etc/passwd Direct Modification Vulnerability (CVE-2015-3246) Advisory

1. Package Upgrade (Recommended)

Update the libuser and usermode packages to the latest secured versions via your package manager.

For RHEL / CentOS systems:

sudo yum update libuser usermode

For Debian / Ubuntu systems (if applicable):

sudo apt-get update && sudo apt-get install --only-upgrade libuser

2. Temporary Mitigation (If patching is delayed)

If immediate patching is not feasible, mitigate the risk by removing the Setuid bit from the userhelper binary to prevent unprivileged execution. (Note: This may impact some GUI-based user configuration tools.)

# Remove Setuid permission from userhelper
sudo chmod u-s /usr/sbin/userhelper

This vulnerability (CVE-2015-3246) exists in the libuser library, which is used for implementing user and group account administration tasks. The root cause lies in the library directly modifying the /etc/passwd file instead of performing atomic file operations (such as writing to a temporary file and renaming it).

If a local attacker triggers an error or interruption during this direct write operation, the /etc/passwd file can be left in an inconsistent or corrupted state, leading to a system-wide Denial of Service (DoS) where users can no longer log in.

Because the userhelper program in the usermode package runs with Setuid root privileges and utilizes libuser, local unprivileged users can exploit this behavior. When chained with CVE-2015-3245 (an out-of-bounds/validation issue in userhelper), this vulnerability allows local attackers to escalate their privileges to root. Due to its high impact and inclusion in the CISA KEV catalog, immediate remediation is highly recommended.