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FirefoxWeb browser · Mozilla

CVE-2026-5731

CRITICAL · 9.8 CVSS v3.1 Published 2026-04-07
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
100/100
Remediation priority · Urgent
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
Memory safety bugs present in Firefox ESR 115.34.0, Firefox ESR 140.9.0, Thunderbird ESR 140.9.0, Firefox 149.0.1 and Thunderbird 149.0.1. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability was fixed in Firefox 149.0.2, Firefox ESR 115.34.1, Firefox ESR 140.9.1, Thunderbird 149.0.2, and Thunderbird 140.9.1.

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

Memory safety bugs in Firefox ESR 115.34.0, Firefox ESR 140.9.0, Thunderbird ESR 140.9.0, Firefox 149.0.1 and Thunderbird 149.0.1 allowed memory corruption that could potentially be exploited for arbitrary code execution.

MitigationUpgrade to Firefox 149.0.2, Firefox ESR 115.34.1, Firefox ESR 140.9.1, Thunderbird 149.0.2, or Thunderbird 140.9.1 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:= 115.34.0= 140.9.0= 149.0.1
ThunderbirdApplication
Affected:= 140.9.0= 149.0.1

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
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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. Check if Mozilla Firefox is installed
    On Windows, check Start menu or Program Files for Firefox, or run 'firefox --version' in command prompt. On macOS, check /Applications or run 'firefox --version' in Terminal. On Linux, run 'firefox --version' or check your package manager.
    Affected if Firefox version is 115.34.0, 140.9.0, or 149.0.1
  2. Check if Mozilla Thunderbird is installed
    On Windows, check Start menu or Program Files for Thunderbird, or run 'thunderbird --version' in command prompt. On macOS, check /Applications or run 'thunderbird --version' in Terminal. On Linux, run 'thunderbird --version' or check your package manager.
    Affected if Thunderbird version is 140.9.0 or 149.0.1
  3. Compare installed Firefox version to affected versions
    If Firefox is installed, compare the version number you obtained (115.34.0, 140.9.0, or 149.0.1) against the list of affected versions. On Windows, you can also right-click firefox.exe > Properties > Details to see the version.
    Affected if Installed Firefox matches exactly 115.34.0, 140.9.0, or 149.0.1
  4. Compare installed Thunderbird version to affected versions
    If Thunderbird is installed, compare the version number you obtained (140.9.0 or 149.0.1) against the list of affected versions. On Windows, you can also right-click thunderbird.exe > Properties > Details to see the version.
    Affected if Installed Thunderbird matches exactly 140.9.0 or 149.0.1

A system is affected if either Mozilla Firefox version 115.34.0, 140.9.0, or 149.0.1 is installed, or Mozilla Thunderbird version 140.9.0 or 149.0.1 is installed.

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
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Upgrade to Firefox 149.0.2, Firefox ESR 115.34.1, Firefox ESR 140.9.1, Thunderbird 149.0.2, or Thunderbird 140.9.1 to remediate this vulnerability.

Recommended fix High confidence

Firefox 149.0.2, Firefox ESR 115.34.1, Firefox ESR 140.9.1, Thunderbird 149.0.2, or Thunderbird 140.9.1 depending on current branch

  1. Identify which Firefox or Thunderbird version is currently installed (Help > About Firefox/Thunderbird)
  2. If running Firefox 115.34.0, upgrade to Firefox ESR 115.34.1
  3. If running Firefox 140.9.0, upgrade to Firefox ESR 140.9.1
  4. If running Firefox 149.0.1, upgrade to Firefox 149.0.2
  5. If running Thunderbird 140.9.0, upgrade to Thunderbird 140.9.1
  6. If running Thunderbird 149.0.1, upgrade to Thunderbird 149.0.2
  7. Restart the application after upgrade
  8. Verify the version by checking Help > About to confirm the fixed version is installed
Caveat Standard minor version upgrades typically have no breaking changes; however, always test in a non-production environment first

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

Fix this in Firefox Scoped from the published advisory
  • Consultation2.0 h
  • Implementation3.0 h
  • Testing2.0 h
  • Review / QA1.0 h
8.0 hours of engineering $1,420
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Scan for this in your stack

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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-2026-5731 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

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