CVE-2026-6751: Mozilla Firefox Use-After-Free Vulnerability

CVE-2026-6751 Overview

CVE-2026-6751 is an uninitialized memory vulnerability affecting the Audio/Video Web Codecs component in Mozilla Firefox and Thunderbird. This memory safety flaw occurs when the browser processes media content through the Web Codecs API, potentially allowing attackers to read sensitive memory contents or cause unexpected application behavior through specially crafted web content.

Critical Impact

Attackers could exploit uninitialized memory in the Web Codecs component to potentially leak sensitive information, corrupt data, or achieve code execution through malicious media content delivered via the network.

Affected Products

• Mozilla Firefox (versions prior to 150)
• Mozilla Firefox ESR (versions prior to 140.10)
• Mozilla Thunderbird (versions prior to 150 and ESR versions prior to 140.10)

Discovery Timeline

• 2026-04-21 - CVE-2026-6751 published to NVD
• 2026-04-22 - Last updated in NVD database

Technical Details for CVE-2026-6751

Vulnerability Analysis

This vulnerability (CWE-457: Use of Uninitialized Variable) exists within the Web Codecs API implementation in Mozilla's browser engines. The Web Codecs API provides low-level access to audio and video encoding and decoding primitives, enabling web developers to create sophisticated media applications. When processing certain media codec operations, the affected components fail to properly initialize memory buffers before use.

Uninitialized memory vulnerabilities are particularly dangerous because the contents of these memory regions are unpredictable and may contain sensitive data from previous operations. In the context of browser security, this could include fragments of previously processed data, authentication tokens, or other confidential information that could be exposed to malicious scripts.

Root Cause

The root cause is the failure to properly initialize memory allocations within the Audio/Video Web Codecs component before they are read or processed. When codec operations request memory buffers, the allocation routine does not zero-initialize or otherwise sanitize the memory contents, leaving residual data from previous heap operations accessible.

Attack Vector

This vulnerability is exploitable remotely through network-delivered content. An attacker could craft a malicious webpage or email containing specially formatted media content that triggers the uninitialized memory condition. When a victim visits the malicious page or views the email in Thunderbird, the browser processes the media through the vulnerable Web Codecs API.

The attack does not require any privileges or user interaction beyond visiting the malicious content. A successful exploit could allow the attacker to read portions of uninitialized memory, potentially disclosing sensitive information, or manipulate the memory contents to influence program execution flow.

For technical details on the vulnerability mechanism, refer to Mozilla Bug #2025883.

Detection Methods for CVE-2026-6751

Indicators of Compromise

• Unusual memory access patterns in Firefox or Thunderbird processes related to media decoding
• Unexpected crashes in the Web Codecs component with memory access violations
• Anomalous network requests following media content processing that could indicate data exfiltration
• Browser process memory dumps showing evidence of information leakage

Detection Strategies

• Monitor for abnormal Web Codecs API usage patterns in browser telemetry
• Deploy endpoint detection solutions to identify exploitation attempts targeting browser memory corruption
• Implement network traffic analysis to detect malicious media content delivery
• Use browser extension or content security policies to restrict untrusted media codec operations

Monitoring Recommendations

• Enable enhanced crash reporting for Mozilla products to capture memory-related exceptions
• Monitor system event logs for browser process anomalies during media playback
• Track browser version deployments to ensure vulnerable versions are identified and prioritized for updates
• Configure SentinelOne agents to monitor for memory corruption exploitation techniques in browser processes

How to Mitigate CVE-2026-6751

Immediate Actions Required

• Update Mozilla Firefox to version 150 or later immediately
• Update Mozilla Firefox ESR to version 140.10 or later
• Update Mozilla Thunderbird to version 150 or later
• Update Mozilla Thunderbird ESR to version 140.10 or later
• Prioritize updates on systems that access untrusted web content or email

Patch Information

Mozilla has released security patches addressing this vulnerability in the following versions:

• Firefox 150 - Full fix implemented (MFSA-2026-30)
• Firefox ESR 140.10 - Backported fix (MFSA-2026-32)
• Thunderbird 150 - Full fix implemented (MFSA-2026-33)
• Thunderbird 140.10 - Backported fix (MFSA-2026-34)

Organizations should prioritize deploying these updates through their software management systems. For enterprise environments, leverage Mozilla's Extended Support Release (ESR) channel for managed browser deployments.

Workarounds

• Consider temporarily disabling JavaScript on untrusted sites to prevent Web Codecs API exploitation
• Use browser content blocking extensions to restrict media content from untrusted sources
• Implement network-level filtering to block known malicious domains serving exploit content
• Configure email clients to disable automatic rendering of HTML content with embedded media

# Verify Firefox version on Linux/macOS
firefox --version
# Expected output: Mozilla Firefox 150.0 or higher

# Verify Thunderbird version
thunderbird --version
# Expected output: Thunderbird 150.0 or higher

# For enterprise deployment, update via package manager
# Debian/Ubuntu
sudo apt update && sudo apt install firefox thunderbird

# Fedora/RHEL
sudo dnf update firefox thunderbird