CVE-2025-26596: TigerVNC Buffer Overflow Vulnerability

CVE-2025-26596 Overview

CVE-2025-26596 is a heap-based buffer overflow vulnerability in X.Org Server and Xwayland. The flaw resides in the XKB (X Keyboard) extension, where the length computation in XkbSizeKeySyms() does not match the data written by XkbWriteKeySyms(). This inconsistency allows an authenticated local attacker to overflow a heap buffer, potentially corrupting adjacent memory. The vulnerability affects Red Hat Enterprise Linux 7, 8, and 9, as well as TigerVNC, which embeds Xorg components. Successful exploitation can lead to code execution in the context of the X server process, which frequently runs with elevated privileges.

Critical Impact
A local attacker with an active X session can trigger a heap overflow in the X server, leading to memory corruption and potential privilege escalation on affected Linux systems.

Affected Products

X.Org X Server
X.Org Xwayland
TigerVNC
Red Hat Enterprise Linux 7, 8, and 9

Discovery Timeline

2025-02-25 - CVE-2025-26596 published to NVD
2026-06-29 - Last updated in NVD database

Technical Details for CVE-2025-26596

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow classified as [CWE-787] (Out-of-bounds Write). It exists in the XKB extension of the X.Org Server, which handles keyboard symbol mapping. Two functions responsible for allocating and serializing keyboard symbol data disagree on the required buffer length. XkbSizeKeySyms() calculates one size for the buffer, while XkbWriteKeySyms() writes a larger amount of data into that buffer. The mismatch causes an out-of-bounds write on the heap, corrupting adjacent memory structures. Because the X server processes keyboard state on behalf of connected clients, a local attacker with access to the X session can craft XKB requests that trigger the mismatched allocation and write.

Root Cause

The root cause is a divergence between the size-calculation logic in XkbSizeKeySyms() and the serialization logic in XkbWriteKeySyms(). Both functions must produce identical byte counts for the buffer to hold all written data. When these routines drift out of sync, the writer serializes more bytes than the sizer requested, producing a classic heap overflow.

Attack Vector

Exploitation requires local access with low privileges and an active X connection. An attacker sends crafted XKB requests to the X server that trigger the buffer allocation path in XkbSizeKeySyms(). When the server subsequently serializes key symbol data through XkbWriteKeySyms(), the write exceeds the allocated buffer boundary. Because Xorg often runs as root on Linux endpoints, memory corruption in this process can escalate to local root privileges.

No verified public proof-of-concept code is available. See the Red Hat CVE Analysis for technical details on the affected code paths.

Detection Methods for CVE-2025-26596

Indicators of Compromise

Unexpected crashes or segmentation faults in Xorg, Xwayland, or Xvnc processes recorded in system logs or coredumpctl.
Abnormal SIGSEGV or SIGABRT signals originating from the X server, particularly near XKB request handling.
Local user processes issuing unusual volumes of XKB extension requests immediately preceding X server termination.

Detection Strategies

Monitor package inventory to identify hosts running unpatched versions of xorg-x11-server, xwayland, or tigervnc-server.
Correlate X server crash events with preceding local user session activity to identify potential exploitation attempts.
Deploy behavioral monitoring for privilege escalation patterns following X server anomalies, such as unexpected child processes spawned by Xorg running as root.

Monitoring Recommendations

Enable abrt or systemd-coredump to capture and retain X server core dumps for forensic analysis.
Ingest /var/log/Xorg.*.log and journal entries for the display manager into a centralized log platform for anomaly detection.
Alert on unexpected restarts of graphical sessions or display managers such as gdm, sddm, or lightdm.

How to Mitigate CVE-2025-26596

Immediate Actions Required

Apply vendor security updates for xorg-x11-server, xwayland, and tigervnc on all affected Red Hat Enterprise Linux 7, 8, and 9 systems.
Inventory endpoints and multi-user servers running X sessions to prioritize patching where local user accounts exist.
Restart affected graphical sessions after patching to ensure the vulnerable process is fully replaced.

Patch Information

Red Hat has released fixes across multiple advisories, including RHSA-2025:2500, RHSA-2025:2502, RHSA-2025:2861, RHSA-2025:3976, RHSA-2025:7163, and RHSA-2025:7458. Debian LTS shipped corresponding updates as noted in the Debian LTS Announcement. Additional bug tracking is available in Red Hat Bug Report #2345256.

Workarounds

Restrict local shell and remote desktop access on multi-user systems until patches are applied.
On servers that do not require a graphical stack, remove or disable xorg-x11-server and tigervnc-server packages.
Where feasible, prefer Wayland-native sessions without Xwayland for workloads that do not require X11 compatibility.

bash

# Apply security updates on Red Hat Enterprise Linux
sudo dnf update xorg-x11-server-Xorg xorg-x11-server-Xwayland tigervnc-server

# Verify installed versions after patching
rpm -q xorg-x11-server-Xorg xorg-x11-server-Xwayland tigervnc-server

# Restart the display manager to load patched binaries
sudo systemctl restart gdm