gvfsd-admin Privilege Escalation via TOCTOU Race Condition

Executive Summary

This vulnerability is a Time-of-Check Time-of-Use (TOCTOU) race condition in the gvfs admin backend. The privileged gvfsd-admin daemon uses chown() on a path in a user-controlled directory to change ownership of private D-Bus sockets. A local attacker can replace the socket path with a symbolic link to a root-owned file like /etc/pam.d/su. The daemon follows the symlink and changes ownership of the target file to the attacker's user ID, allowing modification of critical system files and leading to full local privilege escalation to root.

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Detection Guidance

Detecting this vulnerability requires monitoring for suspicious activity around gvfsd-admin socket creation. Use inotifywait to monitor the directory where sockets are created (typically /run/user/$UID/gvfs-admin). Commands like 'inotifywait -m /run/user/$(id -u)/gvfs-admin' can help detect socket creation events. Check for unexpected ownership changes on critical system files like /etc/pam.d/su using 'ls -l /etc/pam.d/su'.

Additionally, audit logs for gvfsd-admin processes with 'auditctl -w /usr/libexec/gvfsd-admin -p x -k gvfs-admin' and review logs with 'ausearch -k gvfs-admin'.

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Impact Analysis

This vulnerability allows an authenticated local attacker to gain root privileges by exploiting a race condition. Successful exploitation could enable the attacker to modify critical system files, install malware, or perform other actions requiring root access. It requires local access, the ability to start the gvfsd-admin daemon, and winning a timing race condition.

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Compliance Impact

This vulnerability could lead to unauthorized root access, compromising system integrity and confidentiality. For GDPR, it may result in unauthorized access to personal data. For HIPAA, it could allow access to protected health information. Compliance violations may occur due to the inability to maintain system security and data protection controls.

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Mitigation Strategies

Immediately restrict access to gvfsd-admin by adjusting Polkit rules to require password authentication for starting the daemon. Alternatively, remove execute permissions from the binary with 'chmod -x /usr/libexec/gvfsd-admin'. Disable the gvfsd-admin service if not needed with 'systemctl disable --now gvfs-daemon'. Monitor system files for unauthorized ownership changes.

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