CVE-2021-31523
📋 TL;DR
This vulnerability allows local users on Debian systems with xscreensaver 5.42+dfsg1-1 to escalate privileges due to improper capability assignment. The /usr/libexec/xscreensaver/sonar file has cap_net_raw enabled, which conflicts with Mesa 3D Graphics library dependencies. Only local authenticated users on affected Debian installations are impacted.
💻 Affected Systems
- Debian xscreensaver
📦 What is this software?
Xscreensaver by Xscreensaver Project
⚠️ Risk & Real-World Impact
Worst Case
Local privilege escalation to root, allowing complete system compromise and persistence.
Likely Case
Local user gains elevated privileges to access restricted files or perform unauthorized actions.
If Mitigated
Minimal impact with proper privilege separation and capability restrictions in place.
🎯 Exploit Status
Exploitation requires local access and knowledge of the capability misuse; proof-of-concept details are publicly available in security advisories.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 5.42+dfsg1-1+deb11u1 or later
Vendor Advisory: https://security-tracker.debian.org/tracker/CVE-2021-31523
Restart Required: No
Instructions:
1. Update package: sudo apt update && sudo apt upgrade xscreensaver 2. Verify installation: dpkg -l xscreensaver 3. No restart needed, but restart xscreensaver processes if running.
🔧 Temporary Workarounds
Remove cap_net_raw capability
linuxManually remove the problematic capability from the sonar binary to prevent privilege escalation.
sudo setcap -r /usr/libexec/xscreensaver/sonar
Disable xscreensaver sonar module
linuxPrevent execution of the vulnerable sonar module by disabling it in xscreensaver configuration.
Edit ~/.xscreensaver and remove or comment out 'sonar' from programs list
🧯 If You Can't Patch
- Restrict local user access to systems with vulnerable xscreensaver installation.
- Implement strict privilege separation and monitor for unusual privilege escalation attempts.
🔍 How to Verify
Check if Vulnerable:
Check if xscreensaver version is 5.42+dfsg1-1 and sonar binary has cap_net_raw: getcap /usr/libexec/xscreensaver/sonarCheck Version:
dpkg -l xscreensaver | grep ^iiVerify Fix Applied:
Verify xscreensaver version is patched (5.42+dfsg1-1+deb11u1 or later) and sonar has no capabilities: getcap /usr/libexec/xscreensaver/sonar should return empty.📡 Detection & Monitoring
Log Indicators:
- Unusual privilege escalation attempts from xscreensaver processes
- Failed capability checks in system logs
Network Indicators:
- None - local exploitation only
SIEM Query:
process.name:"sonar" AND event.action:"capability_raised"