CVE-2026-6751: Mozilla Firefox Use-After-Free Flaw

CVE-2026-6751 Overview

CVE-2026-6751 is an uninitialized memory vulnerability in the Audio/Video: Web Codecs component of Mozilla Firefox and Thunderbird. The flaw allows attackers to access memory contents that were never properly initialized, potentially exposing sensitive process data through crafted media content. Mozilla addressed the issue in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10. The vulnerability is classified under [CWE-457] (Use of Uninitialized Variable) and carries network-based exploitation characteristics requiring no authentication or user interaction.

Critical Impact

Remote attackers can trigger uninitialized memory reads by serving crafted Web Codecs content, potentially leaking sensitive data from browser memory across confidentiality, integrity, and availability dimensions.

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-2026-6751 published to NVD
• 2026-04-22 - Last updated in NVD database

Technical Details for CVE-2026-6751

Vulnerability Analysis

The vulnerability resides in the Audio/Video: Web Codecs component, which handles encoding and decoding of audio and video streams within Firefox and Thunderbird. The component fails to initialize memory regions before use, allowing residual data from previous allocations to be processed or exposed. Attackers can craft malicious media content that triggers code paths leveraging this uninitialized state. Because Web Codecs is exposed to web content through JavaScript APIs, exploitation requires only that a victim load a hostile page or open a message rendering active media content.

Root Cause

The root cause maps to [CWE-457] Use of Uninitialized Variable. A memory buffer or structure within the Web Codecs decoding pipeline is allocated but not zeroed or populated before being read. The consumed data reflects whatever previously occupied that memory region, including potentially sensitive content such as pointers, tokens, or fragments of other documents processed by the renderer.

Attack Vector

Exploitation occurs over the network. An attacker hosts a webpage containing crafted Web Codecs media, or embeds such content in an HTML email rendered by Thunderbird. When the victim's browser or mail client processes the content, the uninitialized memory is read and may influence rendering behavior or be exfiltrated through side channels available to web content.

No verified public proof-of-concept exists at this time. Technical details are tracked in Mozilla Bug Report #2025883 and the corresponding Mozilla Foundation Security Advisories.

Detection Methods for CVE-2026-6751

Indicators of Compromise

• Outdated Firefox installations reporting versions below 150 or ESR builds below 140.10 in browser telemetry or asset inventory.
• Thunderbird clients running versions below 150 or 140.10 across the environment.
• Renderer process crashes or anomalous memory access patterns coincident with Web Codecs API usage.

Detection Strategies

• Inventory all endpoints for installed Firefox and Thunderbird versions using software asset management tooling.
• Monitor browser and mail client process telemetry for unexpected child process behavior or crash dumps referencing Web Codecs modules.
• Correlate web proxy logs with newly observed domains serving media content to users running vulnerable browser versions.

Monitoring Recommendations

• Enable centralized logging of browser version data through endpoint management or EDR agents.
• Track Mozilla advisory feeds MFSA-2026-30, MFSA-2026-32, MFSA-2026-33, and MFSA-2026-34 for related disclosures.
• Alert on Thunderbird rendering of remote media content from untrusted senders.

How to Mitigate CVE-2026-6751

Immediate Actions Required

• Update Firefox to version 150 or later on all managed endpoints.
• Update Firefox ESR deployments to 140.10 or later.
• Upgrade Thunderbird installations to 150 or 140.10 as appropriate to the deployment channel.
• Verify update enforcement policies in enterprise management tools to prevent rollback to vulnerable versions.

Patch Information

Mozilla released fixes in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10. Refer to the Mozilla Security Advisory MFSA-2026-30 and related advisories for complete patch metadata and verification details.

Workarounds

• Disable remote content rendering in Thunderbird preferences for messages from untrusted senders until patching is complete.
• Restrict access to untrusted websites through web filtering or browser isolation where immediate patching is not feasible.
• Apply enterprise policies that block JavaScript execution on untrusted origins to reduce exposure to the Web Codecs API.

# Configuration example: enforce minimum Firefox version via policies.json
{
  "policies": {
    "DisableAppUpdate": false,
    "AppAutoUpdate": true,
    "OverrideFirstRunPage": "",
    "BlockAboutConfig": false
  }
}