CVE-2026-2447: Mozilla Firefox Buffer Overflow Vulnerability

CVE-2026-2447 Overview

CVE-2026-2447 is a heap buffer overflow [CWE-122] in the libvpx video codec library bundled with Mozilla Firefox and Thunderbird. The flaw allows attackers to corrupt heap memory when the browser processes a crafted VP8 or VP9 video stream. Successful exploitation can lead to arbitrary code execution within the affected application process. Mozilla addressed the issue in Firefox 147.0.4, Firefox ESR 140.7.1, Firefox ESR 115.32.1, Thunderbird 140.7.2, and Thunderbird 147.0.2. The vulnerability requires user interaction, such as loading a malicious webpage or opening an HTML email that auto-renders embedded video content.

Critical Impact

Remote attackers can trigger heap memory corruption in Firefox or Thunderbird by serving crafted video content, potentially leading to arbitrary code execution in the renderer process.

Affected Products

• Mozilla Firefox versions prior to 147.0.4
• Mozilla Firefox ESR versions prior to 140.7.1 and 115.32.1
• Mozilla Thunderbird versions prior to 140.7.2 and 147.0.2

Discovery Timeline

• 2026-02-16 - CVE-2026-2447 published to NVD
• 2026-04-13 - Last updated in NVD database

Technical Details for CVE-2026-2447

Vulnerability Analysis

The vulnerability resides in libvpx, the reference implementation Mozilla uses to decode VP8 and VP9 video streams. A crafted bitstream causes the decoder to write beyond the bounds of a heap-allocated buffer during frame processing. The out-of-bounds write corrupts adjacent heap metadata or object pointers, which an attacker can leverage to influence control flow. Because video decoding occurs automatically whenever Firefox or Thunderbird renders supported media, exploitation requires only that a user visit a malicious page or open an email containing the payload.

Root Cause

The root cause is improper bounds validation inside the libvpx decoder when handling attacker-controlled frame dimensions or chunk sizes. Insufficient checks allow the decoder to compute an allocation smaller than the data it later copies into the buffer. This mismatch produces a classic heap buffer overflow [CWE-122] in a high-throughput media path that is reachable without authentication.

Attack Vector

The attack vector is network-based and requires user interaction. An attacker hosts a webpage containing a malicious VP8 or VP9 video, or sends an HTML email to a Thunderbird user that embeds the same content. When the application decodes the video, the overflow triggers in the content process. Successful exploitation grants code execution at the privilege level of the browser content sandbox, providing a foothold for sandbox-escape chains.

No verified public proof-of-concept is available for this vulnerability. Refer to the Mozilla Bug Report #2014390 for additional technical context.

Detection Methods for CVE-2026-2447

Indicators of Compromise

• Unexpected crashes of firefox.exe or thunderbird.exe referencing vpx_codec or libvpx modules in crash dumps.
• Child content processes spawning unusual descendants such as command shells or scripting interpreters after rendering web video.
• Outbound connections from browser or mail-client processes to newly registered or low-reputation domains shortly after media playback.

Detection Strategies

• Hunt for browser process crashes with faulting modules tied to libvpx and correlate with the URLs or message IDs loaded immediately before the crash.
• Inspect web proxy and email gateway logs for .webm, .mkv, or video/webm content sourced from unverified origins delivered to internal users.
• Monitor endpoint telemetry for anomalous process trees originating from firefox or thunderbird content processes, which typically should not launch interactive binaries.

Monitoring Recommendations

• Centralize Firefox and Thunderbird version inventory and alert on hosts running versions below the fixed releases.
• Forward browser and mail-client crash telemetry to the SIEM to detect repeated exploitation attempts across the fleet.
• Track egress traffic from endpoints immediately after media-rich page loads to surface post-exploitation command-and-control behavior.

How to Mitigate CVE-2026-2447

Immediate Actions Required

• Upgrade Firefox to 147.0.4 or later, Firefox ESR to 140.7.1 or 115.32.1, Thunderbird to 140.7.2, and Thunderbird 147 users to 147.0.2.
• Apply distribution-level updates such as the Debian LTS Announcement February 2026 on managed Linux endpoints.
• Restart all browser and mail-client processes after patching to ensure vulnerable libraries are unloaded from memory.

Patch Information

Mozilla published fixes in Mozilla Security Advisory MFSA-2026-10 and Mozilla Security Advisory MFSA-2026-11. The fixed versions are Firefox 147.0.4, Firefox ESR 140.7.1, Firefox ESR 115.32.1, Thunderbird 140.7.2, and Thunderbird 147.0.2. Administrators using enterprise deployment policies should push the updated MSI or PKG installers through their existing software distribution tooling.

Workarounds

• Configure Thunderbird to display messages in plain text, which prevents automatic rendering of embedded HTML video content.
• Use group policy or policies.json to disable autoplay and restrict media playback to trusted origins until patches are deployed.
• Apply web filtering at the proxy layer to block video/webm content from uncategorized or low-reputation domains during the patch window.

# Example Firefox enterprise policy to disable autoplay until patched
# /etc/firefox/policies/policies.json
{
  "policies": {
    "Permissions": {
      "Autoplay": {
        "Default": "block-audio-video"
      }
    }
  }
}