CVE-2025-49175: X Rendering Extension DoS Vulnerability

CVE-2025-49175 Overview

A flaw was found in the X Rendering extension's handling of animated cursors in X.Org Server. When a client provides no cursors while creating an animated cursor, the server incorrectly assumes at least one cursor is present. This assumption leads to an out-of-bounds read condition that can result in information disclosure or cause the X server to crash, impacting system availability.

Critical Impact

Local attackers with access to the X server can trigger an out-of-bounds read by sending specially crafted animated cursor requests with zero cursors, potentially causing denial of service or leaking sensitive memory contents.

Affected Products

• X.Org Server (xorg-server)
• Red Hat Enterprise Linux (multiple versions)
• Debian GNU/Linux

Discovery Timeline

• June 17, 2025 - CVE-2025-49175 published to NVD
• December 11, 2025 - Last updated in NVD database

Technical Details for CVE-2025-49175

Vulnerability Analysis

This vulnerability resides in the X Rendering (Render) extension, specifically within the code path that handles animated cursor creation. The flaw is classified as CWE-125 (Out-of-bounds Read), a memory safety issue that occurs when the software reads data from a location that is outside the intended buffer boundaries.

The X server's animated cursor functionality allows clients to define cursors that cycle through multiple frames. When processing a request to create an animated cursor, the server code expects the client to provide at least one cursor frame. However, there is no validation to ensure the cursor count is greater than zero before proceeding with memory operations.

When a malicious or buggy client sends a request with zero cursors, the server attempts to access cursor data at invalid memory locations. This out-of-bounds read can expose sensitive information from adjacent memory regions or trigger a segmentation fault that crashes the X server process.

Root Cause

The root cause is a missing input validation check in the animated cursor handling code within the Render extension. The code path assumes that the number of cursors provided will always be at least one, without explicitly verifying this condition. This assumption creates a boundary condition error where the server proceeds to read from an array using an index that would be valid only if cursors were actually present.

The fix involves adding proper validation to check that the cursor count is non-zero before attempting to access cursor data. The patch at commit 0885e0b26225 addresses this by implementing the necessary boundary check.

Attack Vector

The attack vector is local, requiring the attacker to have access to send requests to the X server. This can be accomplished by:

1. A local user running a malicious X client application
2. A compromised application with X server access sending crafted requests
3. Remote exploitation through X11 forwarding if the server accepts remote connections

The attacker crafts an AnimateCursor request via the Render extension protocol, specifying zero cursors in the request. When the X server processes this malformed request, it attempts to read cursor data from memory without first validating that any cursors exist, triggering the out-of-bounds read.

The exploitation does not require elevated privileges—any authenticated user with access to the X display can trigger the vulnerability. The impact includes potential information disclosure from leaked memory contents and denial of service through server crashes.

Detection Methods for CVE-2025-49175

Indicators of Compromise

• Unexpected X server (Xorg) crashes or restarts without apparent cause
• Core dumps from the Xorg process showing segmentation faults in cursor-related functions
• Log entries in /var/log/Xorg.*.log indicating crashes during animated cursor operations
• Multiple rapid X server restart cycles indicating potential exploitation attempts

Detection Strategies

• Monitor for Xorg process crashes and analyze core dumps for stack traces involving the Render extension cursor handling code
• Implement system audit rules to track X server restarts and correlate with user activity
• Deploy endpoint detection to identify unusual X protocol traffic patterns, particularly AnimateCursor requests with malformed parameters
• Review X server logs for error messages related to cursor allocation failures

Monitoring Recommendations

• Enable detailed X server logging to capture protocol-level errors and crashes
• Configure system monitoring to alert on Xorg process terminations and automatic restarts
• Implement application whitelisting to restrict which processes can connect to the X server
• Use SentinelOne Singularity Platform to detect and respond to exploitation attempts targeting system services

How to Mitigate CVE-2025-49175

Immediate Actions Required

• Apply security patches from your Linux distribution vendor immediately
• Review and apply Red Hat security advisories (RHSA-2025:9303 through RHSA-2025:10410) for affected RHEL systems
• Debian users should apply updates per the Debian LTS Announcement
• Verify X server version post-patching to confirm the fix is applied

Patch Information

Multiple Linux distributions have released patches addressing this vulnerability. The upstream fix is available in the Freedesktop GitLab Commit 0885e0b26225. Red Hat has published numerous security advisories including RHSA-2025:9303, RHSA-2025:9304, RHSA-2025:9305, and RHSA-2025:9306 for various RHEL versions. Consult the Red Hat CVE Report for the complete list of available patches.

Workarounds

• Restrict access to the X server by ensuring only trusted local users can connect
• Disable the Render extension if not required (may impact graphical functionality)
• Consider using Wayland instead of X11 where possible, as this vulnerability is specific to the X server
• Implement network segmentation to prevent remote X11 connections if X11 forwarding is enabled

# Verify X server version after patching
Xorg -version

# Check if Render extension is enabled
xdpyinfo | grep -i render

# Restrict X server access to local connections only (if SSH X11 forwarding is not needed)
# Add to /etc/X11/xorg.conf.d/security.conf:
# Section "ServerFlags"
#     Option "DisableXAce" "off"
# EndSection