CVE-2017-9525

What it actually does

The postinst maintainer script performs chown and chmod on paths that can be controlled via symlinks, allowing a user in the crontab group to redirect operations to arbitrary files and escalate to root.

Why it matters to the business

Local attackers can obtain full root access on affected Debian and Ubuntu systems, enabling complete compromise of the host and any data or services it runs.

How an attack unfolds

An attacker in the crontab group creates a symlink in a location processed by the postinst script, then triggers package installation or upgrade so that chown/chmod operations follow the link and grant elevated ownership or permissions on attacker-chosen files.

How to fix it

Upgrade the cron package to a version that safely handles file paths without following untrusted symlinks; on Debian and Ubuntu apply the security updates released after the vulnerable 3.0pl1-128 and 3.0pl1-128ubuntu2 releases respectively.

How to confirm the fix worked

Verify the installed cron package version is newer than the vulnerable releases, confirm the postinst script no longer performs chown/chmod on paths under user control, and test that package upgrades complete without following attacker-controlled symlinks.

Typical attack path

1. Obtain membership in the crontab group on a vulnerable system
2. Create a malicious symlink pointing to a privileged target file
3. Trigger execution of the cron package postinst script via apt or dpkg
4. Abuse the resulting root-owned file to obtain a root shell or persistent access

How to detect it

Review dpkg and apt history logs for cron package installations or upgrades performed by non-root users, and inspect system logs around those times for unexpected chown or chmod activity on sensitive paths.

What your security team should do

• Immediately update cron on all Debian and Ubuntu systems to a patched release
• Restrict membership in the crontab group to only trusted administrative accounts
• Monitor package management logs for unexpected cron upgrades or postinst executions
• Apply the principle of least privilege for all local user accounts