CVE-2026-50261: Xorg-x11-server: xorg-x11-server-xwayland: xorg-x11-server: use-after-free in syncchangecounter()

Synopsis

A use-after-free vulnerability affects the X.Org X server and Xwayland in the SyncChangeCounter() function. The flaw is reached locally by an authenticated low-privilege user who sets up multiple SyncCounters and destroys them via a second client connection while changes are in flight; no network access is required. Successful exploitation crashes the X server or, if the server runs as root (a common configuration), allows the attacker to escalate privileges to root. No upstream fix has been published yet; HarborGuard is tracking the advisory for patch availability.

HarborGuard Coverage

Exploit Conditions

• Network reachabilityNot required

  The attacker needs an existing shell or process on the host; no network path to the service is required.

• AuthenticationRequired

  Any low-privilege local account is sufficient to open X client connections and trigger the vulnerability.

• Victim interactionNot required

  No user interaction is needed; the attacker drives the exploit entirely through their own client connections.

• Attack complexityDetail

  The exploit is reliable and condition-free once local access is obtained; no race conditions or special memory layout requirements are noted in the CVSS scoring.

Blast Radius

• Crashes the X server, disrupting all graphical sessions and display-dependent services on the host.
• If the X server runs as root (a common deployment posture on RHEL systems), the attacker gains full root-level code execution on the host.
• With root access, the attacker reads any file on the filesystem, including credentials, certificates, and application secrets stored on disk.
• With root access, the attacker modifies or deletes any file on the filesystem, including system binaries, configuration, and persistent application data.