CVE-2025-27091: Cisco OpenH264 Buffer Overflow Vulnerability

CVE-2025-27091 Overview

CVE-2025-27091 is a critical heap overflow vulnerability in the Cisco OpenH264 codec library that could allow remote, unauthenticated attackers to execute arbitrary code on vulnerable systems. The vulnerability exists in the decoding functions and is triggered by a race condition between Sequence Parameter Set (SPS) memory allocation and subsequent non-IDR NAL unit memory usage. This flaw affects both Scalable Video Coding (SVC) and Advanced Video Coding (AVC) modes in OpenH264 versions 2.5.0 and earlier.

Critical Impact
Remote attackers can exploit this heap overflow vulnerability by crafting malicious video bitstreams, potentially leading to arbitrary code execution on victim systems without authentication.

Affected Products

Cisco OpenH264 versions 2.5.0 and earlier
Applications using OpenH264 SVC (Scalable Video Coding) mode
Applications using OpenH264 AVC (Advanced Video Coding) mode

Discovery Timeline

2025-02-20 - CVE-2025-27091 published to NVD
2025-05-06 - Last updated in NVD database

Technical Details for CVE-2025-27091

Vulnerability Analysis

This heap overflow vulnerability (CWE-122, CWE-787) stems from a race condition in the OpenH264 decoding pipeline. The flaw occurs between two critical operations: when memory is allocated for a Sequence Parameter Set (SPS) and when a subsequent non-Instantaneous Decoder Refresh (non-IDR) Network Abstraction Layer (NAL) unit attempts to use that memory. Due to improper synchronization, the NAL unit may access the SPS memory before allocation is complete or after it has been modified, resulting in heap corruption.

The vulnerability is particularly dangerous because it exists in the core decoding functionality used for H.264 video processing. Any application leveraging OpenH264 for video decoding—including web browsers, video conferencing software, and media players—may be susceptible to exploitation.

Root Cause

The root cause is a race condition (TOCTOU - Time-of-Check Time-of-Use) between the SPS memory allocation routine and NAL unit processing. When processing crafted video bitstreams, the timing window between these operations can be exploited to cause the decoder to read or write to improperly allocated heap memory, leading to heap buffer overflow conditions.

Attack Vector

An attacker exploits this vulnerability through the network by crafting a malicious H.264 video bitstream. The attack requires user interaction—the victim must process the malicious video content, either by opening a video file, streaming media content, or participating in a video call that delivers the exploit payload.

The malicious bitstream is specifically designed to trigger the race condition between SPS allocation and non-IDR NAL unit processing. When successful, the attacker can:

Cause the decoding application to crash (denial of service)
Achieve arbitrary code execution by carefully manipulating heap memory layout
Potentially gain control over the victim's system with the privileges of the decoding application

Detection Methods for CVE-2025-27091

Indicators of Compromise

Unexpected crashes in applications using OpenH264 for video decoding
Abnormal memory allocation patterns in video processing services
Receipt of malformed or unusually structured H.264 video files from untrusted sources
Heap corruption signatures in crash dumps from media players or video conferencing applications

Detection Strategies

Monitor for abnormal process behavior in applications that utilize OpenH264 codec library
Implement memory corruption detection tools such as AddressSanitizer in development/testing environments
Deploy endpoint detection and response (EDR) solutions capable of identifying heap overflow exploitation attempts
Analyze video files from untrusted sources for malformed NAL unit structures before processing

Monitoring Recommendations

Enable crash reporting and analysis for all applications utilizing video decoding functionality
Monitor system logs for repeated application crashes associated with video processing
Implement network monitoring to detect suspicious video content delivery patterns
Review application dependencies to identify all software using vulnerable OpenH264 versions

How to Mitigate CVE-2025-27091

Immediate Actions Required

Upgrade OpenH264 to version 2.6.0 or later immediately
Audit all applications and systems for dependencies on OpenH264 codec library
Restrict processing of video content from untrusted sources until patching is complete
Enable application sandboxing where possible to limit potential exploitation impact

Patch Information

Cisco has released OpenH264 version 2.6.0 which contains the fix for this vulnerability. The patch was developed through collaboration between Meta security researchers and the OpenH264 maintainers. Users should upgrade to version 2.6.0 or later as soon as possible. Detailed release information is available in the OpenH264 v2.6.0 Release Notes and the GitHub Security Advisory.

Workarounds

No official workarounds are available for this vulnerability according to Cisco
Upgrading to OpenH264 version 2.6.0 or later is the only recommended remediation
As a temporary measure, avoid processing video content from untrusted or unknown sources
Consider disabling H.264 decoding functionality in non-essential applications until patching is complete

bash

# Verify OpenH264 version and upgrade
# Check current OpenH264 version in your application dependencies
pkg-config --modversion openh264

# For systems using package managers, update to the latest version
# Example for Ubuntu/Debian (when available in repositories)
sudo apt update && sudo apt install --only-upgrade libopenh264-7

# Verify the updated version
pkg-config --modversion openh264