CVE-2026-4714: Mozilla Firefox Buffer Overflow Vulnerability

CVE-2026-4714 Overview

CVE-2026-4714 is a high-severity vulnerability affecting the Audio/Video component in Mozilla Firefox, Firefox ESR, and Thunderbird. The flaw stems from incorrect boundary conditions [CWE-754] in media processing logic. A remote attacker can deliver crafted media content over the network to trigger the issue without authentication or user interaction. Successful exploitation results in a high-impact availability loss, consistent with a crash or denial-of-service condition in the browser process. Mozilla addressed the issue in Firefox 149, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

Critical Impact
Remote attackers can trigger a denial-of-service condition in Firefox and Thunderbird through crafted audio/video content processed by the affected media component.

Affected Products

Mozilla Firefox (versions prior to 149)
Mozilla Firefox ESR (versions prior to 140.9)
Mozilla Thunderbird (versions prior to 149 and 140.9)

Discovery Timeline

2026-03-24 - CVE-2026-4714 published to NVD
2026-04-13 - Last updated in NVD database

Technical Details for CVE-2026-4714

Vulnerability Analysis

The vulnerability resides in the Audio/Video component used by Firefox and Thunderbird to decode and render media streams. Mozilla classifies the defect as an incorrect boundary condition, mapped to [CWE-754] (Improper Check for Unusual or Exceptional Conditions). When the affected code path processes specific media data, it fails to validate boundary conditions correctly, leading to an exceptional state that the media stack does not handle safely. The result is an availability impact on the application process. Confidentiality and integrity are not affected, according to the CVSS vector provided by Mozilla.

Root Cause

The root cause is improper validation of boundary conditions inside the Audio/Video subsystem. Media parsers and decoders rely on tight invariants about buffer sizes, sample counts, and stream metadata. When a boundary check is missing or incorrect, malformed input can drive the code into an unexpected state. Mozilla shipped the corrected validation logic in Firefox 149, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9. Technical specifics are tracked in the upstream bug, restricted while patches roll out.

Attack Vector

The attack vector is network-based and requires no privileges or user interaction beyond loading attacker-controlled content. An attacker hosts a malicious page or embeds crafted media in an email rendered by Thunderbird. When the vulnerable client processes the media, the boundary condition triggers and the application enters an exceptional state. The vulnerability does not yield code execution or data disclosure based on the published CVSS metrics. No public proof-of-concept or exploit is available at the time of writing. The EPSS probability is low, reflecting limited expected exploitation activity in the near term.

Verified exploitation code is not publicly available. For technical specifics, refer to the Mozilla Bug Report #2018126 and the Mozilla Security Advisory MFSA-2026-20.

Detection Methods for CVE-2026-4714

Indicators of Compromise

Unexpected Firefox or Thunderbird process crashes correlated with rendering specific media content from untrusted origins.
Crash reports referencing the Audio/Video component or media decoder modules submitted to Mozilla crash reporting.
Repeated browser tab or content process terminations on pages embedding HTML5 <audio> or <video> elements from unfamiliar hosts.

Detection Strategies

Inventory endpoints running Firefox and Thunderbird and flag versions below Firefox 149, Firefox ESR 140.9, or Thunderbird 149/140.9.
Correlate process crash telemetry with web proxy logs to identify domains serving media that triggers client-side faults.
Hunt for outbound connections to newly registered domains hosting media files immediately preceding browser crash events.

Monitoring Recommendations

Forward Firefox and Thunderbird crash telemetry, including faulting module and stack hash, into centralized logging for trend analysis.
Monitor EDR process termination events for firefox.exe, thunderbird.exe, and their content child processes during media playback.
Track endpoint software inventory continuously to detect drift from patched browser builds.

How to Mitigate CVE-2026-4714

Immediate Actions Required

Upgrade Firefox to version 149, Firefox ESR to 140.9, Thunderbird to 149, or Thunderbird ESR to 140.9 across all managed endpoints.
Enforce automatic updates for Mozilla products through enterprise policy to ensure timely patch delivery.
Identify any internal applications or kiosks embedding Gecko or Mozilla media stacks and validate that they also receive the fix.

Patch Information

Mozilla published fixes in Firefox 149, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9. Patch details and affected version ranges are documented in Mozilla Security Advisory MFSA-2026-20, Mozilla Security Advisory MFSA-2026-22, Mozilla Security Advisory MFSA-2026-23, and Mozilla Security Advisory MFSA-2026-24.

Workarounds

Restrict autoplay and remote media loading in Thunderbird by disabling remote content in messages until patches are deployed.
Use enterprise browser policies to block media playback from untrusted origins where feasible.
Apply network-level controls to limit access to untrusted streaming or file-hosting domains until all endpoints are patched.

bash

# Verify installed Firefox version on Linux endpoints
firefox --version

# Example Windows policy fragment to enforce automatic updates
# policies.json placed in the Firefox distribution directory
{
  "policies": {
    "DisableAppUpdate": false,
    "AppAutoUpdate": true
  }
}