CVE-2025-26597: TigerVNC Buffer Overflow Vulnerability

CVE-2025-26597 Overview

A buffer overflow vulnerability has been identified in X.Org Server and Xwayland affecting the XkbChangeTypesOfKey() function. This flaw occurs when the function is called with a 0 group parameter, which resizes the key symbols table to 0 while leaving key actions unchanged. A subsequent call to the same function with a non-zero group value triggers a buffer overflow because the key actions array has an incorrect size, leading to potential memory corruption.

Critical Impact

This vulnerability allows a local attacker with low privileges to potentially achieve code execution or cause system instability by corrupting memory through the buffer overflow condition in X.Org/Xwayland display server components.

Affected Products

• X.Org X Server (all vulnerable versions)
• X.Org Xwayland (all vulnerable versions)
• TigerVNC (all vulnerable versions)
• Red Hat Enterprise Linux 7.0, 8.0, and 9.0

Discovery Timeline

• February 25, 2025 - CVE-2025-26597 published to NVD
• November 3, 2025 - Last updated in NVD database

Technical Details for CVE-2025-26597

Vulnerability Analysis

This buffer overflow vulnerability exists within the X Keyboard Extension (XKB) implementation in both X.Org Server and Xwayland. The flaw is rooted in improper memory management within the XkbChangeTypesOfKey() function, which is responsible for modifying keyboard type mappings.

When a client application calls XkbChangeTypesOfKey() with a group count of zero, the function performs a resize operation on the key symbols table, effectively setting its size to zero. However, the corresponding key actions array—which should maintain synchronized sizing with the symbols table—is not properly updated during this operation. This creates a dangerous state where the internal data structures have mismatched sizes.

The vulnerability manifests when the function is subsequently called with a non-zero group value. At this point, the code attempts to access or modify the key actions array based on the newly specified group count, but the array remains at its original size from before the zero-group call. This size mismatch results in out-of-bounds memory writes, constituting a classic buffer overflow condition.

Root Cause

The root cause is a failure to properly synchronize memory allocations between the key symbols table and key actions array within XkbChangeTypesOfKey(). When the group parameter is zero, only the symbols table is resized while the actions array retains its previous allocation, creating an inconsistent state that can be exploited through subsequent function calls.

Attack Vector

This is a local attack vector requiring low-privilege access to the system. An attacker would need to be able to send specially crafted XKB requests to the X server, which typically requires local access or a connection to the display server. The attack sequence involves:

1. Establishing a connection to the X.Org or Xwayland display server
2. Sending an XKB request that triggers XkbChangeTypesOfKey() with a group value of 0
3. Subsequently sending another XKB request with a non-zero group value
4. The resulting buffer overflow can corrupt adjacent memory, potentially leading to code execution

The vulnerability mechanism involves improper bounds checking in the XKB keyboard extension code. For detailed technical analysis, refer to Red Hat Bugzilla Report #2345255.

Detection Methods for CVE-2025-26597

Indicators of Compromise

• Unexpected crashes or segmentation faults in Xorg or Xwayland processes
• Anomalous XKB-related requests appearing in X server logs
• Memory corruption warnings or errors in system logs related to display server components
• Unusual process behavior following X server restarts or display reconfigurations

Detection Strategies

• Monitor X server logs for unusual XKB extension activity or malformed requests
• Implement memory integrity monitoring for Xorg and Xwayland processes
• Deploy endpoint detection solutions capable of identifying buffer overflow exploitation attempts
• Track system calls related to X server memory operations for anomalous patterns

Monitoring Recommendations

• Enable verbose logging for X.Org and Xwayland to capture XKB extension requests
• Configure crash dump analysis for automated detection of exploitation attempts
• Implement file integrity monitoring on X server binaries and libraries
• Monitor for unauthorized modifications to keyboard configuration files

How to Mitigate CVE-2025-26597

Immediate Actions Required

• Apply vendor-provided security patches for X.Org Server, Xwayland, and TigerVNC immediately
• Review and restrict access to X display servers where possible
• Consider temporarily disabling XKB extension functionality if patches cannot be immediately applied
• Audit systems for signs of exploitation before and after patching

Patch Information

Multiple vendors have released security updates addressing this vulnerability:

Red Hat Enterprise Linux:

• RHSA-2025:2500 - X.Org server security update
• RHSA-2025:2502 - Xwayland security update
• RHSA-2025:2861 through RHSA-2025:2880 - Additional component updates
• RHSA-2025:7163, RHSA-2025:7165, and RHSA-2025:7458 - Latest security updates

Debian:

• Security updates available via Debian LTS Security Announcement

For complete vulnerability details, see Red Hat CVE Resource for CVE-2025-26597.

Workarounds

• Restrict local access to systems running vulnerable X.Org or Xwayland versions
• Implement network segmentation to limit exposure of systems with display servers
• Use application sandboxing to isolate X server processes from critical system components
• Consider temporarily switching to Wayland-native compositors where XKB extension usage can be controlled

# Check installed X.Org server version on RHEL/CentOS
rpm -qa | grep -E "xorg-x11-server|xwayland|tigervnc"

# Update affected packages on RHEL-based systems
sudo dnf update xorg-x11-server-Xorg xorg-x11-server-Xwayland tigervnc-server

# Verify update was applied
rpm -q xorg-x11-server-Xorg xorg-x11-server-Xwayland