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FirefoxWeb browser · Mozilla

CVE-2026-8946

HIGH · 7.5 CVSS v3.1 Published 2026-05-19
Fix available
A fix is available. Upgrade to 115.36.0 / 140.11 or later.
See remediation →
84/100
Remediation priority · High
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Incorrect boundary conditions in the Audio/Video: Web Codecs component. This vulnerability was fixed in Firefox 151, Firefox ESR 115.36, Firefox ESR 140.11, Thunderbird 151, and Thunderbird 140.11.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

A boundary condition vulnerability exists in the Web Codecs component used for audio/video processing in Firefox and Thunderbird. Incorrect boundary checks during media decoding could lead to out-of-bounds memory access, potentially causing crashes or information disclosure. The issue affects multiple product versions across Firefox (151, ESR 115.36, ESR 140.11) and Thunderbird (151, 140.11).

MitigationUpdate affected Mozilla products to version 151 or later (Firefox), ESR 115.36/140.11 or later, or Thunderbird 140.11 or later to remediate this vulnerability.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
FirefoxWeb browser
Affected:< 115.36.0< 151.0.0>= 140.0, < 140.11.0
ThunderbirdApplication
Affected:< 140.11

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check Firefox version
    In Firefox, navigate to Menu > Help > About Firefox, or enter 'about:support' in the address bar. The version number is displayed at the top of the page.
    Affected if The installed version is any of: < 115.36.0, >= 115.36.0 but < 140.0, >= 140.0 but < 140.11.0, or >= 140.0 but < 151.0.0
  2. Check Thunderbird version
    In Thunderbird, navigate to Menu > Help > About Thunderbird. The version number is displayed in the dialog that opens.
    Affected if The installed version is < 140.11
  3. Verify Web Codecs is accessible
    In Firefox, enter 'about:config' in the address bar and search for 'WebCodecs' to confirm the feature is enabled. The media.webcodecs.enabled preference controls this.
    Affected if The WebCodecs feature is enabled (media.webcodecs.enabled is set to true) on an affected Firefox version
  4. Check for media playback usage
    Web Codecs is used when processing audio/video via JavaScript APIs (VideoDecoder, AudioDecoder, etc.). Check if any web content or extension uses media decoding features.
    Affected if Any audio/video content is being processed through the Web Codecs API on an affected version

You are affected if you run Firefox with a version less than 115.36.0, between 115.36.0-139.x, or between 140.0-140.10.x, or Thunderbird version less than 140.11, AND the Web Codecs feature is enabled (it is enabled by default) and you process media content.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 115.36.0 / 140.11 / 140.11.0 or later
Fixed in 115.36.0140.11140.11.0
Interim mitigation

Update affected Mozilla products to version 151 or later (Firefox), ESR 115.36/140.11 or later, or Thunderbird 140.11 or later to remediate this vulnerability.

Recommended fix High confidence

Firefox ESR 115.36.0+, Firefox 140.11.0+, Firefox 151.0.0+, Thunderbird 140.11+, Thunderbird 151+

  1. Identify the currently installed Firefox version by navigating to Help > About Firefox
  2. If using Firefox 115.x (ESR), upgrade to Firefox ESR 115.36.0 or later
  3. If using Firefox 140.x, upgrade to Firefox 140.11.0 or later
  4. If using Firefox 141-150, upgrade to Firefox 151.0.0 or later
  5. Identify the currently installed Thunderbird version by navigating to Help > About Thunderbird
  6. If using Thunderbird 140.x, upgrade to Thunderbird 140.11 or later
  7. If using earlier Thunderbird versions, upgrade to Thunderbird 151 or later
  8. Restart the application after upgrade to apply changes
Caveat Standard version upgrade; ensure compatibility with any site-specific plugins or extensions before upgrading

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Firefox Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA1.0 h
10.0 hours of engineering $1,750
Get the upgrade done

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-8946 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data