CVE-2016-7949 – CVE-2016-7949 is a critical buffer overflow vul... (How to Fix)

Q: What is the severity of CVE-2016-7949?

CVE-2016-7949 has a CVSS score of 9.8 (CRITICAL). CVE-2016-7949 is a critical buffer overflow vulnerability in X.org libXrender affecting XvQueryAdaptors and XvQueryEncodings functions. It allows remote X servers to trigger out-of-bounds write operations via malicious length fields, potentially leading to remote code execution. Systems using X Window System with vulnerable libXrender versions are affected.

📋 TL;DR

CVE-2016-7949 is a critical buffer overflow vulnerability in X.org libXrender affecting XvQueryAdaptors and XvQueryEncodings functions. It allows remote X servers to trigger out-of-bounds write operations via malicious length fields, potentially leading to remote code execution. Systems using X Window System with vulnerable libXrender versions are affected.

💻 Affected Systems

Products:

X.org libXrender

Versions: Versions before 0.9.10

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

Default Config Vulnerable: ⚠️ Yes

Notes: Requires X server connection; systems without X11 or with X11 disabled are not vulnerable.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Remote code execution with root privileges, complete system compromise, and potential lateral movement across the network.

🟠

Likely Case

Remote code execution with user privileges, denial of service, or information disclosure depending on exploit implementation.

🟢

If Mitigated

Denial of service or application crash if exploit fails or memory protections are in place.

🌐 Internet-Facing: MEDIUM

🏢 Internal Only: HIGH

🎯 Exploit Status

Public PoC: ⚠️ Yes

Weaponized: LIKELY

Unauthenticated Exploit: ⚠️ Yes

Complexity: MEDIUM

Exploitation requires network access to X server; proof-of-concept code exists in security advisories.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 0.9.10 and later

Vendor Advisory: https://cgit.freedesktop.org/xorg/lib/libXrender/commit/?id=9362c7ddd1af3b168953d0737877bc52d79c94f4

Restart Required: Yes

Instructions:

1. Update libXrender package using system package manager. 2. For Debian/Ubuntu: sudo apt-get update && sudo apt-get install libxrender1. 3. For RHEL/CentOS: sudo yum update libXrender. 4. Restart X server or affected applications.

🔧 Temporary Workarounds

Disable X11 Forwarding

linux

Prevent remote X server connections by disabling X11 forwarding in SSH and network services.

Set 'X11Forwarding no' in /etc/ssh/sshd_config and restart SSH: sudo systemctl restart sshd

Network Segmentation

linux

Restrict network access to X server ports (typically TCP 6000-6007) using firewall rules.

sudo iptables -A INPUT -p tcp --dport 6000:6007 -j DROP

🧯 If You Can't Patch

Implement strict network access controls to limit X server exposure to trusted hosts only.
Use application whitelisting or security modules (like SELinux/AppArmor) to restrict libXrender memory operations.

🔍 How to Verify

Check if Vulnerable:

Check libXrender version: dpkg -l | grep libxrender1 or rpm -qa | grep libXrender. If version is below 0.9.10, system is vulnerable.

Check Version:

dpkg -l libxrender1 2>/dev/null | grep ^ii || rpm -q libXrender 2>/dev/null || pkg-config --modversion xrender 2>/dev/null

Verify Fix Applied:

Verify installed version is 0.9.10 or higher using package manager commands.

📡 Detection & Monitoring

Log Indicators:

X server crash logs in /var/log/Xorg.0.log
Application segmentation faults involving libXrender
Unusual network connections to X server ports

Network Indicators:

Malformed X protocol packets to port 6000+
Unexpected remote connections to X server

SIEM Query:

source="Xorg.log" AND "segmentation fault" OR source="auth.log" AND "Accepted keyboard-interactive" AND "X11 forwarding"

📊 Metadata

CVE ID: CVE-2016-7949

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-20

Published: December 13, 2016

Last Updated: April 12, 2025

🔗 References

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