CVE-2026-25704: cosmic-greeter Privilege Escalation Flaw

CVE-2026-25704 Overview

A Privilege Dropping / Lowering Errors combined with a Time-of-check Time-of-use (TOCTOU) Race Condition vulnerability has been identified in cosmic-greeter, the display manager/greeter component for the COSMIC desktop environment. This vulnerability allows an attacker with local access to regain privileges that should have been dropped and abuse them through the racy checking logic present in the application.

The vulnerability stems from improper handling of privilege transitions, where a race condition exists between the time privileges are checked and when they are actually used. An attacker can exploit this timing window to maintain elevated privileges that should have been relinquished, potentially leading to unauthorized system access and privilege escalation.

Critical Impact

Local attackers can exploit the TOCTOU race condition to regain dropped privileges, potentially compromising system integrity and gaining unauthorized elevated access on affected Linux systems running cosmic-greeter.

Affected Products

• cosmic-greeter versions prior to the fix in pull request #426
• Pop!_OS systems using vulnerable cosmic-greeter versions
• Linux distributions utilizing cosmic-greeter as their display manager

Discovery Timeline

• 2026-03-30 - CVE-2026-25704 published to NVD
• 2026-03-30 - Last updated in NVD database

Technical Details for CVE-2026-25704

Vulnerability Analysis

This vulnerability combines two distinct weakness patterns: CWE-271 (Privilege Dropping / Lowering Errors) and a Time-of-check Time-of-use (TOCTOU) race condition. In cosmic-greeter, when the application attempts to drop privileges after performing privileged operations, a race condition exists that allows an attacker to exploit the timing gap between the privilege check and the actual privilege drop operation.

Display managers like cosmic-greeter typically run with elevated privileges to handle authentication and session management. After successful authentication, the process should drop privileges before handing control to the user session. The vulnerability occurs because the privilege-dropping logic contains a race condition that can be exploited to maintain elevated privileges.

The local attack vector requires the attacker to have some level of access to the system already, but the successful exploitation could result in confidentiality and integrity impacts along with high availability impact to the system.

Root Cause

The root cause lies in the improper synchronization of privilege state transitions within cosmic-greeter. When the application performs a time-of-check operation to verify privilege requirements and subsequently attempts to lower privileges, a timing window exists where the privilege state can be manipulated. This occurs because the check and the actual privilege modification are not performed atomically, creating a race condition that can be exploited by a local attacker.

Attack Vector

The attack requires local access to the system and exploits the TOCTOU window in the privilege-dropping mechanism. An attacker would need to:

1. Identify the precise timing window where the privilege check occurs
2. Execute carefully timed operations to win the race condition
3. Maintain or regain the elevated privileges that should have been dropped
4. Leverage the retained privileges to perform unauthorized actions

The exploitation mechanism targets the asynchronous nature of the privilege transition, where an attacker can interfere with the expected state change between the check and use phases. Technical details regarding the specific exploitation method can be found in the SUSE Bugzilla entry and the GitHub pull request #426 that addresses this vulnerability.

Detection Methods for CVE-2026-25704

Indicators of Compromise

• Unexpected privilege retention by cosmic-greeter processes after user session initialization
• Anomalous process execution under elevated privileges associated with the greeter component
• Unusual timing patterns in authentication and session establishment logs
• Evidence of rapid repeated authentication attempts that may indicate race condition exploitation

Detection Strategies

• Monitor cosmic-greeter process privilege transitions using Linux audit framework (auditd) with rules targeting privilege change syscalls
• Implement process monitoring to detect cosmic-greeter running with unexpected privilege levels after authentication completion
• Configure alerting for any cosmic-greeter child processes that maintain root or elevated privileges beyond expected timeframes
• Review system logs for patterns indicating rapid privilege state changes or anomalous greeter behavior

Monitoring Recommendations

• Enable comprehensive audit logging for privilege-related system calls (setuid, setgid, setresuid, setresgid) involving cosmic-greeter
• Deploy endpoint detection solutions capable of monitoring process privilege state transitions in real-time
• Establish baseline behavior patterns for cosmic-greeter privilege operations to identify deviations
• Configure SentinelOne Singularity platform to monitor for suspicious privilege escalation patterns on Linux endpoints

How to Mitigate CVE-2026-25704

Immediate Actions Required

• Update cosmic-greeter to the patched version that includes the fix from pull request #426
• Review system configurations for any unnecessary privilege grants to the cosmic-greeter component
• Implement additional access controls to limit local access to systems running vulnerable versions
• Consider temporarily using an alternative display manager if immediate patching is not possible

Patch Information

The vulnerability is addressed in cosmic-greeter through the changes implemented in GitHub pull request #426. Users should update to the version that includes this fix. For Pop!_OS users, ensure your system is up to date by running the standard system update process. Additional tracking information is available through the SUSE Bugzilla entry for CVE-2026-25704.

Workarounds

• Restrict local access to systems running vulnerable cosmic-greeter versions to trusted users only
• Implement mandatory access control policies (SELinux/AppArmor) to constrain cosmic-greeter privilege boundaries
• Monitor systems closely for exploitation attempts while awaiting patch deployment
• Consider disabling cosmic-greeter and using TTY-based login as a temporary measure for high-security environments

# Check cosmic-greeter version and update on Pop!_OS/Debian-based systems
apt list --installed | grep cosmic-greeter
sudo apt update && sudo apt upgrade cosmic-greeter

# Enable audit logging for privilege-related syscalls
sudo auditctl -a always,exit -F arch=b64 -S setuid -S setgid -S setresuid -S setresgid -F exe=/usr/bin/cosmic-greeter -k cosmic_priv_drop