CVE-2016-7953 – CVE-2016-7953 is a buffer underflow vulnerabili... (How to Fix)

Q: What is the severity of CVE-2016-7953?

CVE-2016-7953 has a CVSS score of 9.8 (CRITICAL). CVE-2016-7953 is a buffer underflow vulnerability in X.org's libXvMC library that allows remote X servers to potentially execute arbitrary code or cause denial of service. The vulnerability affects systems using X Window System with libXvMC before version 1.0.10. Attackers could exploit this by sending specially crafted empty strings to trigger memory corruption.

📋 TL;DR

CVE-2016-7953 is a buffer underflow vulnerability in X.org's libXvMC library that allows remote X servers to potentially execute arbitrary code or cause denial of service. The vulnerability affects systems using X Window System with libXvMC before version 1.0.10. Attackers could exploit this by sending specially crafted empty strings to trigger memory corruption.

💻 Affected Systems

Products:

X.org libXvMC
Applications using libXvMC for video acceleration

Versions: All versions before 1.0.10

Operating Systems: Linux, Unix-like systems with X Window System

Default Config Vulnerable: ✅ No

Notes: Only vulnerable if X server accepts network connections (disabled by default in modern distributions) and libXvMC is used.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Remote code execution leading to complete system compromise, privilege escalation, or persistent backdoor installation.

🟠

Likely Case

Denial of service causing X server crashes or application instability.

🟢

If Mitigated

Limited impact if network access to X servers is restricted and proper memory protections are enabled.

🌐 Internet-Facing: MEDIUM - Requires X server network accessibility, which is uncommon in default configurations.

🏢 Internal Only: MEDIUM - Internal attackers could exploit if X servers accept network connections.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ⚠️ Yes

Complexity: MEDIUM

Exploitation requires network access to X server and knowledge of libXvMC usage patterns.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 1.0.10

Vendor Advisory: https://lists.x.org/archives/xorg-announce/2016-October/002720.html

Restart Required: Yes

Instructions:

1. Update libXvMC package to version 1.0.10 or later. 2. Restart X server and affected applications. 3. For source installations: apply commit 2cd95e7da8367cccdcdd5c9b160012d1dec5cbdb and recompile.

🔧 Temporary Workarounds

Disable X Server Network Access

linux

Prevent remote exploitation by disabling X server network listening.

sudo systemctl disable x11-common
Edit /etc/X11/xinit/xserverrc to remove -listen tcp
Use xhost - or xhost -localhost

🧯 If You Can't Patch

Restrict network access to X servers using firewall rules (block TCP port 6000-6009)
Use X11 forwarding via SSH instead of direct X server network connections

🔍 How to Verify

Check if Vulnerable:

Check libXvMC version: dpkg -l | grep libxvmc or rpm -qa | grep libXvMC

Check Version:

pkg-config --modversion xvmc

Verify Fix Applied:

Verify version is 1.0.10 or higher and check for commit 2cd95e7da8367cccdcdd5c9b160012d1dec5cbdb in source installations.

📡 Detection & Monitoring

Log Indicators:

X server segmentation faults
libXvMC-related crash reports in /var/log/Xorg*.log

Network Indicators:

Unexpected connections to X server ports (6000+)
Malformed X protocol packets

SIEM Query:

source="Xorg.log" AND ("segmentation fault" OR "libXvMC")

📊 Metadata

CVE ID: CVE-2016-7953

CVSS v3 Score: 9.8 (CRITICAL)

CVSS Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CWE: CWE-119

Published: December 13, 2016

Last Updated: April 12, 2025

🔗 References

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