CVE-2025-49176: Big Requests Extension DOS Vulnerability

CVE-2025-49176 Overview

A critical integer overflow vulnerability has been discovered in the Big Requests extension of the X.Org Server. The flaw exists in the request length validation logic, where the request length is multiplied by 4 before being checked against the maximum allowed size. This arithmetic operation can cause an integer overflow condition, effectively allowing an attacker to bypass the size check and submit maliciously crafted requests that exceed intended boundaries.

Critical Impact

Local attackers with low privileges can exploit this integer overflow to bypass size validation checks, potentially leading to high integrity and availability impact through memory corruption or denial of service conditions.

Affected Products

• X.Org Server (xserver)
• Red Hat Enterprise Linux distributions (multiple versions)
• Debian Linux distributions

Discovery Timeline

• June 17, 2025 - CVE-2025-49176 published to NVD
• December 11, 2025 - Last updated in NVD database

Technical Details for CVE-2025-49176

Vulnerability Analysis

This vulnerability is classified as CWE-190 (Integer Overflow or Wraparound). The Big Requests extension in X.Org Server is designed to handle requests that exceed the standard 262140-byte limit of the X protocol. When processing these extended requests, the server performs a multiplication operation on the request length to calculate the actual byte size.

The fundamental issue lies in the validation sequence: the request length value is multiplied by 4 (since X protocol lengths are measured in 4-byte units) before being compared against the maximum permissible size. On systems where the result of this multiplication exceeds the maximum value representable by the integer type used, the value wraps around to a small positive number or potentially zero. This wrapped value then passes the size check, allowing the server to process a request that should have been rejected.

Successful exploitation requires local access and low-level privileges, as the attacker must be able to send X protocol requests to the server. The vulnerability can lead to memory corruption scenarios where the server allocates insufficient buffer space for the actual data being processed, resulting in out-of-bounds memory operations.

Root Cause

The root cause is an unsafe arithmetic operation in the Big Requests extension where integer multiplication is performed without proper overflow checking. When a request length value is provided such that multiplying it by 4 causes an integer overflow, the resulting wrapped value bypasses the size validation check. This is a classic integer overflow vulnerability where the code assumes the multiplication result will always be larger than the input, which is not true when overflow occurs.

Attack Vector

The attack requires local access to the target system with the ability to communicate with the X.Org Server. An attacker would craft a malicious X protocol request with a carefully chosen length value that, when multiplied by 4, causes an integer overflow. The attacker needs low privileges to execute this attack, as standard X client applications can send these requests. No user interaction is required for exploitation. The attack could result in memory corruption affecting both the integrity and availability of the X server process.

The exploitation flow involves:

1. Establishing a connection to the X.Org Server
2. Sending a Big Requests extension request with a length value calculated to overflow when multiplied by 4
3. The overflowed value passes the size check
4. The server processes data beyond intended boundaries

Detection Methods for CVE-2025-49176

Indicators of Compromise

• Unexpected X.Org Server crashes or restarts, particularly in the Big Requests extension handler
• Memory corruption errors in X server logs related to request processing
• Unusual large request patterns in X protocol traffic analysis
• Segmentation faults or memory access violations in Xorg process logs

Detection Strategies

• Monitor X.Org Server logs for abnormal request sizes or processing errors
• Implement system auditing to track X server process crashes and restarts
• Deploy memory protection tools to detect out-of-bounds access attempts in Xorg processes
• Use intrusion detection systems to flag unusual patterns in local X protocol communications

Monitoring Recommendations

• Enable detailed logging for X.Org Server to capture request handling events
• Configure system monitoring to alert on Xorg process anomalies or unexpected terminations
• Review audit logs for users making repeated connections to the X server with unusual request patterns

How to Mitigate CVE-2025-49176

Immediate Actions Required

• Apply vendor-provided security patches immediately from Red Hat, Debian, or your distribution's package manager
• Review system access controls to limit which users can connect to the X.Org Server
• Consider restricting X server access to only trusted local users where possible
• Monitor X server processes for any signs of exploitation attempts

Patch Information

Multiple vendors have released patches to address this vulnerability. The fix involves adding proper integer overflow checks before the multiplication operation in the Big Requests extension. The patches can be found in the following resources:

• X.Org GitLab Commit 03731b3
• X.Org GitLab Commit 4fc4d76

Red Hat has issued multiple security advisories including RHSA-2025:9303, RHSA-2025:9304, and numerous others. Debian has also released patches via their LTS announcement.

Workarounds

• Restrict access to the X server by configuring appropriate xhost or xauth settings to limit client connections
• If feasible, run the X server with reduced privileges or in a containerized environment to limit impact
• Consider using Wayland as an alternative display server where application compatibility allows
• Implement network segmentation to prevent untrusted systems from accessing X server instances

# Restrict X server access to only the local user
xhost -
xhost +SI:localuser:$(whoami)

# Verify current X server access permissions
xhost

# Check installed X.Org Server version for patching status
Xorg -version