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FirefoxWeb browser · Mozilla

CVE-2026-2791

CRITICAL · 9.8 CVSS v3.1 Published 2026-02-24
Fix available
A fix is available. Upgrade to 140.8.0 / 148.0 or later.
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
Mitigation bypass in the Networking: Cache component. This vulnerability was fixed in Firefox 148, Firefox ESR 140.8, Thunderbird 148, and Thunderbird 140.8.

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 mitigation bypass vulnerability in the Networking: Cache component of Firefox and Thunderbird. This flaw allows attackers to circumvent existing security controls in the browser's caching mechanism, potentially enabling unauthorized access to cached sensitive data or cache-based attacks. The CVSS 9.8 indicates trivial exploitability with complete compromise of confidentiality, integrity, and availability.

MitigationApply vendor-supplied updates: Upgrade to Firefox 148+, Firefox ESR 140.8+, Thunderbird 148+, or Thunderbird 140.8+ to restore the intended security mitigation.

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:< 140.8.0< 148.0
ThunderbirdApplication
Affected:< 140.8.0< 148.0

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
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 Firefox is installed
    Open terminal or command prompt and run: firefox --version (Linux/macOS) or check Programs and Features (Windows). On Windows, also check: C:\Program Files\Mozilla Firefox\firefox.exe -version
    Affected if Firefox is found on the system
  2. Check if Thunderbird is installed
    Open terminal or command prompt and run: thunderbird --version (Linux/macOS) or check Programs and Features (Windows). On Windows, also check: C:\Program Files\Mozilla Thunderbird\thunderbird.exe -version
    Affected if Thunderbird is found on the system
  3. Note the Firefox version number
    Compare the installed version against the affected ranges: any version below 140.8.0 OR any version from 141.x through 147.x (inclusive)
    Affected if Installed version is less than 140.8.0 OR falls between 141.0 and 147.x inclusive
  4. Note the Thunderbird version number
    Compare the installed version against the affected ranges: any version below 140.8.0 OR any version from 141.x through 147.x (inclusive)
    Affected if Installed version is less than 140.8.0 OR falls between 141.0 and 147.x inclusive

If either Firefox or Thunderbird is installed and the version is below 140.8.0 or between 141.0 and 147.x inclusive, the system is affected by this cache mitigation bypass vulnerability.

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 140.8.0 / 148.0 or later
Fixed in 140.8.0148.0
Interim mitigation

Apply vendor-supplied updates: Upgrade to Firefox 148+, Firefox ESR 140.8+, Thunderbird 148+, or Thunderbird 140.8+ to restore the intended security mitigation.

Recommended fix High confidence

Firefox 148.0 / Firefox ESR 140.8.0 / Thunderbird 148.0 / Thunderbird 140.8.0

  1. Verify current Firefox or Thunderbird version by navigating to Help > About (or typing 'about:support' in the address bar)
  2. Download Firefox 148 (or Firefox ESR 140.8 for extended support release users) from the official Mozilla website at www.mozilla.org
  3. Download Thunderbird 148 (or Thunderbird 140.8 for extended support release users) from the official Mozilla website at www.mozilla.org
  4. Close all instances of the application
  5. Run the installer for the downloaded version
  6. Follow the installation wizard prompts to complete the upgrade
  7. Restart the application after installation completes
  8. Verify the upgrade by checking Help > About to confirm the version displays 148.0 or 140.8.0 respectively
Caveat Standard minor version upgrade with no expected breaking changes; user settings and extensions are preserved

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
  • Testing2.0 h
  • Review / QA1.0 h
9.0 hours of engineering $1,600
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,560.

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-2026-2791 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