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

CVE-2026-6747 Overview

CVE-2026-6747 is a use-after-free vulnerability [CWE-416] in the WebRTC component of Mozilla Firefox and Mozilla Thunderbird. The flaw allows remote attackers to trigger memory corruption through specially crafted WebRTC traffic, resulting in a denial-of-service condition. The vulnerability requires no privileges and no user interaction beyond visiting an attacker-controlled page or processing malicious real-time communication content. Mozilla addressed the issue in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10.

Critical Impact

A network-based attacker can exploit the WebRTC use-after-free condition to crash the browser process or potentially achieve further memory corruption outcomes, disrupting availability for affected users.

Affected Products

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

Discovery Timeline

• 2026-04-21 - CVE-2026-6747 published to NVD
• 2026-04-22 - Last updated in NVD database

Technical Details for CVE-2026-6747

Vulnerability Analysis

The vulnerability resides in the WebRTC (Web Real-Time Communication) component used by Firefox and Thunderbird. WebRTC enables peer-to-peer audio, video, and data exchange directly within the browser. A use-after-free [CWE-416] condition occurs when the component releases a memory object but retains a dangling reference that is subsequently dereferenced.

Attackers can reach the vulnerable code path remotely over the network by serving malicious WebRTC content or initiating crafted signaling exchanges with a target. Successful exploitation primarily impacts availability, causing the affected process to terminate. Use-after-free conditions in browser media stacks have historically been chained with heap manipulation primitives to escalate toward code execution, so timely patching is required.

Root Cause

The root cause is improper object lifetime management within the WebRTC subsystem. Code paths that release a referenced object fail to invalidate all outstanding pointers, allowing reuse of the freed memory region. Mozilla addressed the defect across the Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10 release lines.

Attack Vector

Exploitation occurs over the network without authentication. A victim browsing a malicious site, or a Thunderbird client rendering attacker-influenced WebRTC content, can trigger the vulnerable code path. No user interaction beyond normal content consumption is required.

No public proof-of-concept exploit is currently available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. See Mozilla Bug #2021769 for technical details once the bug becomes publicly accessible.

Detection Methods for CVE-2026-6747

Indicators of Compromise

• Unexpected crashes of firefox.exe, thunderbird.exe, or their Linux and macOS equivalents involving WebRTC modules in stack traces.
• Crash reports referencing components such as libwebrtc, mozilla::dom::RTCPeerConnection, or related media transport functions.
• Outbound connections from browser processes to suspicious STUN, TURN, or signaling endpoints not associated with legitimate services.

Detection Strategies

• Inventory installed Firefox and Thunderbird versions across endpoints and flag any builds older than Firefox 150, Firefox ESR 140.10, or Thunderbird 140.10 / 150.
• Correlate browser process crashes with concurrent navigation events or WebRTC session establishment in proxy and DNS logs.
• Monitor for execution of child processes spawned from browser parents shortly after WebRTC-related crashes, which may indicate exploitation attempts beyond denial of service.

Monitoring Recommendations

• Forward Windows Error Reporting and macOS CrashReporter telemetry to a central log store to identify clusters of WebRTC-related failures.
• Alert on connections to attacker-controlled signaling servers and on anomalous use of UDP port ranges typical of WebRTC media flows.
• Track browser version compliance through endpoint management tooling and generate alerts when out-of-date instances are observed.

How to Mitigate CVE-2026-6747

Immediate Actions Required

• Update Firefox to version 150 or later on all managed endpoints.
• Update Firefox ESR deployments to version 140.10 or later.
• Update Thunderbird to version 150 or version 140.10 on all systems that process email or chat content.
• Validate browser version compliance through endpoint management and remediate any out-of-date instances.

Patch Information

Mozilla has released fixed builds for all affected products. Refer to the official advisories for download and changelog details: MFSA-2026-30, MFSA-2026-32, MFSA-2026-33, and MFSA-2026-34.

Workarounds

• Disable WebRTC functionality in Firefox by setting media.peerconnection.enabled to false in about:config where business workflows allow.
• Restrict outbound STUN and TURN traffic at the network perimeter for environments that do not require WebRTC.
• Apply browser policy controls through enterprise configuration to enforce updates automatically until all endpoints are patched.

# Configuration example: Firefox enterprise policy to disable WebRTC peer connections
# /etc/firefox/policies/policies.json
{
  "policies": {
    "Preferences": {
      "media.peerconnection.enabled": {
        "Value": false,
        "Status": "locked"
      }
    },
    "DisableAppUpdate": false
  }
}