CVE-2025-13020: Mozilla Firefox Use-After-Free Vulnerability

CVE-2025-13020 Overview

CVE-2025-13020 is a use-after-free vulnerability in the WebRTC Audio/Video component of Mozilla Firefox and Thunderbird. The flaw resides in the real-time communication subsystem that handles audio and video streams in the browser. An attacker who convinces a user to visit a crafted web page can trigger memory corruption that may lead to arbitrary code execution within the content process. Mozilla addressed the issue in Firefox 145, Firefox ESR 140.5, Thunderbird 145, and Thunderbird 140.5. The weakness is classified under [CWE-416] Use After Free.

Critical Impact

Successful exploitation can lead to remote code execution in the renderer process after a user visits a malicious page that abuses WebRTC audio or video streams.

Affected Products

• Mozilla Firefox versions prior to 145
• Mozilla Firefox ESR versions prior to 140.5
• Mozilla Thunderbird versions prior to 145 and prior to 140.5

Discovery Timeline

• 2025-11-11 - CVE-2025-13020 published to NVD
• 2026-04-13 - Last updated in NVD database

Technical Details for CVE-2025-13020

Vulnerability Analysis

The vulnerability is a use-after-free condition within Firefox's WebRTC Audio/Video pipeline. WebRTC (Web Real-Time Communication) enables peer-to-peer audio and video streaming directly inside the browser. The component manages reference-counted media objects across multiple threads, including capture, encoding, and transport stages. When an object is released while another execution path retains a dangling pointer, subsequent access reads or writes freed memory. An attacker can shape heap allocations to place controlled data into the reclaimed region, converting the dangling pointer access into arbitrary read or write primitives. From there, the attacker can pivot to code execution in the sandboxed content process.

Root Cause

The defect stems from incorrect object lifetime management in the WebRTC audio and video stack. A media resource is freed while a separate caller still holds a reference, violating the invariant that all pointer holders are notified before deallocation. Mozilla tracked the underlying defect in Mozilla Bug Report #1995686.

Attack Vector

Exploitation requires the victim to load attacker-controlled web content with JavaScript enabled. The malicious page initiates a WebRTC session and manipulates audio or video stream lifecycle events to trigger the freed-object access. The attack is network-reachable and requires user interaction in the form of visiting the page. No authentication is needed. For Thunderbird users, the same code path is reachable when remote content is rendered. Refer to Mozilla Security Advisory MFSA-2025-87 and Mozilla Security Advisory MFSA-2025-88 for vendor details.

// No public proof-of-concept code is available for CVE-2025-13020.
// Mozilla has not disclosed exploit details. See Mozilla Bug #1995686
// for the upstream tracking entry.

Detection Methods for CVE-2025-13020

Indicators of Compromise

• Firefox or Thunderbird processes crashing with access-violation signatures inside xul.dll or libxul.so WebRTC symbols.
• Unexpected child content-process spawns following navigation to untrusted sites that initiate getUserMedia or RTCPeerConnection calls.
• Outbound connections from a browser content process to unfamiliar STUN/TURN endpoints shortly before a crash.

Detection Strategies

• Inventory installed Firefox and Thunderbird versions across endpoints and flag any build older than Firefox 145, Firefox ESR 140.5, or Thunderbird 140.5/145.
• Correlate browser crash telemetry with web-proxy logs to identify users who visited pages immediately before a content-process fault.
• Hunt for post-exploitation behavior such as a browser process writing to autostart locations or spawning shell interpreters.

Monitoring Recommendations

• Forward Windows Error Reporting (WER) and macOS crash logs to a central SIEM to detect WebRTC-related faults at scale.
• Monitor EDR telemetry for process-injection or memory-write anomalies originating from firefox.exe or thunderbird.exe child processes.
• Track DNS and TLS SNI logs for newly registered domains hosting WebRTC test pages, which are common staging grounds for browser exploits.

How to Mitigate CVE-2025-13020

Immediate Actions Required

• Update Firefox to version 145 or later on all managed endpoints.
• Update Firefox ESR to 140.5 and Thunderbird to 140.5 or 145, depending on the deployed branch.
• Restart browsers after patch deployment to ensure the vulnerable libraries are unloaded from memory.
• Validate patch status through software inventory tooling and remediate any drift detected.

Patch Information

Mozilla released fixes in Firefox 145, Firefox ESR 140.5, Thunderbird 145, and Thunderbird 140.5. Patch details are documented in Mozilla Security Advisory MFSA-2025-87, Mozilla Security Advisory MFSA-2025-88, Mozilla Security Advisory MFSA-2025-90, and Mozilla Security Advisory MFSA-2025-91.

Workarounds

• Disable WebRTC where business workflows allow by setting media.peerconnection.enabled to false in about:config.
• Restrict access to camera and microphone permissions via enterprise policy until patches are applied.
• Apply web-proxy or DNS filtering to block known malicious sites that abuse browser memory corruption flaws.

# Firefox enterprise policy example to disable WebRTC until patched
# Place in policies.json under the Firefox distribution directory
{
  "policies": {
    "Preferences": {
      "media.peerconnection.enabled": {
        "Value": false,
        "Status": "locked"
      }
    }
  }
}