CVE-2026-6747: Mozilla Firefox Use-After-Free Vulnerability

CVE-2026-6747 Overview

CVE-2026-6747 is a use-after-free vulnerability discovered in the WebRTC component of Mozilla Firefox and Thunderbird. This memory corruption flaw occurs when the WebRTC subsystem improperly handles object lifetimes, potentially allowing an attacker to access memory after it has been freed. The vulnerability affects multiple versions of Firefox (including ESR), Thunderbird, and Thunderbird ESR, and has been addressed in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10.

Critical Impact

This use-after-free vulnerability in the WebRTC component can be exploited remotely without authentication, potentially leading to denial of service conditions through memory corruption.

Affected Products

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

Discovery Timeline

• April 21, 2026 - CVE-2026-6747 published to NVD
• April 22, 2026 - Last updated in NVD database

Technical Details for CVE-2026-6747

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption vulnerability that occurs when a program continues to use a pointer after the memory it references has been freed. In the context of the WebRTC component, this condition arises during WebRTC session handling where object references are not properly managed during the lifecycle of media connections or peer-to-peer communication sessions.

Use-after-free vulnerabilities in browser components like WebRTC are particularly concerning because they operate in the context of web content processing. When freed memory is subsequently reallocated and overwritten with attacker-controlled data, and the dangling pointer is later dereferenced, this can lead to unpredictable behavior including application crashes and potential denial of service.

Root Cause

The root cause of this vulnerability lies in improper memory management within the WebRTC component. Specifically, object lifecycle handling fails to properly track when WebRTC-related objects are deallocated, leaving dangling pointers that can be accessed after the underlying memory has been freed. This typically occurs in complex asynchronous operations common in real-time communication protocols where multiple threads or callbacks may hold references to objects that are prematurely destroyed.

Attack Vector

The attack vector for CVE-2026-6747 is network-based, requiring no user interaction or authentication. An attacker could exploit this vulnerability by crafting malicious web content that triggers the vulnerable code path in the WebRTC component. When a user visits a malicious website or views malicious content in Thunderbird (such as an HTML email), the specially crafted WebRTC operations could trigger the use-after-free condition.

The exploitation scenario involves manipulating WebRTC API calls to create a race condition or specific sequence of events that causes the WebRTC component to access freed memory. This can result in denial of service through application crash. For detailed technical information, refer to the Mozilla Bug Report #2021769.

Detection Methods for CVE-2026-6747

Indicators of Compromise

• Unexpected Firefox or Thunderbird crashes during WebRTC-enabled web sessions or video conferencing
• Application crash dumps indicating memory access violations in WebRTC-related modules
• Repeated crashes when accessing specific websites that utilize WebRTC functionality
• Memory corruption errors logged by the browser's crash reporter

Detection Strategies

• Monitor browser crash reports for patterns indicating WebRTC component failures
• Implement endpoint detection rules to identify anomalous WebRTC activity or repeated browser crashes
• Deploy network monitoring to detect unusual WebRTC signaling patterns that may indicate exploitation attempts
• Review system logs for signs of memory corruption in firefox.exe or thunderbird.exe processes

Monitoring Recommendations

• Enable crash reporting in Firefox and Thunderbird to capture detailed information about any crashes
• Monitor endpoint security telemetry for repeated browser crashes that may indicate active exploitation
• Implement browser version auditing to identify systems running vulnerable versions
• Configure SentinelOne to alert on suspicious browser behavior patterns associated with memory corruption exploits

How to Mitigate CVE-2026-6747

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
• Verify updates across all managed systems using software inventory tools

Patch Information

Mozilla has released security patches addressing this vulnerability across multiple product lines. Organizations should apply the following updates as documented in the official security advisories:

• Mozilla Security Advisory MFSA-2026-30
• Mozilla Security Advisory MFSA-2026-32
• Mozilla Security Advisory MFSA-2026-33
• Mozilla Security Advisory MFSA-2026-34

Workarounds

• Disable WebRTC functionality in Firefox by navigating to about:config and setting media.peerconnection.enabled to false
• Block access to untrusted websites that may attempt to exploit WebRTC functionality
• Consider using browser isolation technologies for high-risk browsing scenarios
• Implement network segmentation to limit the impact of potential browser compromises

# Firefox WebRTC disable via user preferences (user.js)
user_pref("media.peerconnection.enabled", false);
user_pref("media.navigator.enabled", false);