CVE-2026-25704 is a vulnerability identified in the cosmic-greeter component of the pop-os Linux distribution. The issue stems from improper handling of privilege dropping, specifically a race condition classified as a Time-of-Check to Time-of-Use (TOCTOU) flaw. In this context, cosmic-greeter attempts to lower its privileges during execution to reduce risk, but due to a timing window in the checking logic, an attacker can exploit the race condition to regain privileges that should have been relinquished. This can lead to unauthorized privilege escalation, allowing a low-privileged local attacker to execute code or perform actions with higher privileges than intended. The vulnerability is tracked under CWE-271 (Privilege Dropping Errors) and CWE-367 (TOCTOU Race Condition). The flaw affects versions of cosmic-greeter before the fix merged in pull request #426 on the pop-os GitHub repository. The CVSS 4.0 vector indicates the attack requires local access (AV:L), high attack complexity (AC:H), no authentication (AT:N), low privileges (PR:L), no user interaction (UI:N), and impacts confidentiality, integrity, and availability to a limited extent (VC:L, VI:L, VA:H). No public exploits have been reported yet, but the vulnerability poses a risk in environments where local users have limited privileges but could attempt privilege escalation. The issue highlights the challenges in securely implementing privilege dropping and the risks of race conditions in security-critical code.

The primary impact of CVE-2026-25704 is unauthorized privilege escalation on systems running vulnerable versions of cosmic-greeter. This can compromise system confidentiality, integrity, and availability by allowing attackers to execute privileged operations, potentially leading to full system compromise. Organizations relying on pop-os with cosmic-greeter as their login manager or greeter face risks of local attackers bypassing intended privilege restrictions. This is particularly concerning in multi-user environments such as shared workstations, development machines, or educational institutions where local user accounts exist. The vulnerability could facilitate lateral movement or persistence by attackers who gain initial low-level access. Although exploitation requires local access and has high complexity, the potential for privilege escalation elevates the threat level. No known exploits in the wild reduce immediate risk, but the vulnerability should be addressed promptly to prevent future attacks. The impact is more significant in environments with less stringent local user controls or where cosmic-greeter is deployed in critical infrastructure or sensitive systems.

To mitigate CVE-2026-25704, organizations should apply the official patch or update cosmic-greeter to the fixed version once available from the pop-os maintainers. Until a patch is deployed, restrict local user access to trusted personnel only and enforce strict user privilege separation. Employ mandatory access controls (e.g., SELinux, AppArmor) to limit the ability of local users to manipulate or interfere with the greeter process. Conduct regular audits of local accounts and remove unnecessary privileges. Consider using alternative greeter or login managers that do not exhibit this vulnerability if immediate patching is not feasible. Monitor system logs for suspicious privilege escalation attempts and implement host-based intrusion detection systems to detect anomalous behavior. Educate users and administrators about the risks of local privilege escalation and the importance of applying security updates promptly. Finally, review and harden the overall privilege dropping and process isolation mechanisms in the environment to reduce the attack surface for race conditions.