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FirefoxWeb browser · Mozilla

CVE-2026-4685

HIGH · 7.5 CVSS v3.1 Published 2026-03-24
Fix available
A fix is available. Upgrade to 115.34.0 / 140.9.0 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 Graphics: Canvas2D component. This vulnerability was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

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 · moderate confidence

A boundary condition vulnerability exists in Firefox and Thunderbird's Canvas2D graphics component. Incorrect boundary checks in Canvas2D rendering operations could allow memory access beyond intended buffers, potentially leading to information disclosure or memory corruption. This high-severity issue was addressed through patches in the specified Firefox and Thunderbird releases.

MitigationApply the vendor-supplied updates: Firefox 149 or later, Firefox ESR 115.34/140.9 or later, or Thunderbird 149/140.9 or later. For applications embedding affected browser engines, ensure the updated rendering libraries are integrated.

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:< 115.34.0< 149.0>= 128.0, < 140.9.0

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
None
Integrity
None
Availability
High

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

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. Identify installed Firefox or Thunderbird version
    In Firefox: Menu > Help > About Firefox. In Thunderbird: Menu > Help > About Thunderbird. Note the full version number displayed (e.g., 140.0, 128.5.2, 115.12.1).
    Affected if The installed version falls into any of these ranges: < 115.34.0, OR >= 128.0 AND < 140.9.0, OR < 149.0 (for non-ESR releases). Compare your exact version against these bounds.
  2. Confirm Canvas2D feature is enabled
    Canvas2D is enabled by default in Firefox and Thunderbird. No explicit configuration toggle is needed. This check is informational: if the browser can render web content, Canvas2D is available.
    Affected if Canvas2D is a core rendering component enabled by default in affected versions. No workaround to disable it without breaking web content rendering.
  3. Verify the Graphics component is present
    In Firefox: type "about:support" in the address bar and look for the Graphics section. Check that Canvas2D is listed as active. In Thunderbird: same procedure for mail content with embedded HTML/canvas.
    Affected if The Graphics: Canvas2D component is present and operational in the affected version range, which is required for the boundary condition vulnerability to be exploitable.

You are affected if your installed Firefox or Thunderbird version is below 115.34.0, between 128.0 and 140.9.0, or below 149.0 (non-ESR), and the Canvas2D rendering feature is enabled (default state).

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 115.34.0 / 140.9.0 / 149.0 or later
Fixed in 115.34.0140.9.0149.0
Interim mitigation

Apply the vendor-supplied updates: Firefox 149 or later, Firefox ESR 115.34/140.9 or later, or Thunderbird 149/140.9 or later. For applications embedding affected browser engines, ensure the updated rendering libraries are integrated.

Recommended fix High confidence

Firefox 149 (or Firefox ESR 115.34 / Firefox ESR 140.9 depending on current version)

  1. 1. Check the current Firefox version by navigating to about:support in the address bar
  2. 2. If running Firefox < 115.34.0, upgrade to Firefox ESR 115.34 or later
  3. 3. If running Firefox >= 128.0 and < 140.9.0, upgrade to Firefox ESR 140.9 or Firefox 149
  4. 4. If running Firefox >= 115.34.0 and < 128.0, upgrade to Firefox 149
  5. 5. Download the appropriate fixed version from the official Mozilla website (www.mozilla.org)
  6. 6. Close all Firefox instances and install the updated version
  7. 7. Restart Firefox and verify the version via about:support
Caveat Upgrading may change browser settings, extensions may need re-validation, syncing credentials required

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

Fix this in Firefox Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
6.0 hours of engineering $1,060
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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-2026-4685 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