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FirefoxWeb browser · Mozilla

CVE-2021-29976

HIGH · 8.8 CVSS v3.1 Published 2021-08-05
Fix available
A fix is available. Upgrade to 78.12 / 90.0 or later.
See remediation →
95/100
Remediation priority · Urgent
Remotely reachable No privileges

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
Mozilla developers reported memory safety bugs present in code shared between Firefox and Thunderbird. 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 affects Thunderbird < 78.12, Firefox ESR < 78.12, and Firefox < 90.

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 bug in code shared between Firefox and Thunderbird allowed memory corruption. With sufficient effort, this could be exploited to run arbitrary code.

MitigationUpdate Thunderbird to 78.12+, Firefox ESR to 78.12+, and Firefox to 90+ to obtain the fixed versions.

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:< 90.0
Firefox EsrWeb browser
Affected:< 78.12
ThunderbirdApplication
Affected:< 78.12

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
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/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 Firefox version (Stable/Beta)
    Open Firefox, click the menu button (three horizontal lines), then Help > About Firefox. The version number is displayed at the top of the dialog.
    Affected if The displayed version is less than 90.0 (for example, 89.x or earlier)
  2. Check Firefox ESR version
    Open Firefox ESR, click the menu button, then Help > About Firefox ESR. The version number is displayed at the top of the dialog.
    Affected if The displayed version is less than 78.12 (for example, 78.11 or earlier)
  3. Check Thunderbird version
    Open Thunderbird, click the menu button (three horizontal lines), then Help > About Thunderbird. The version number is displayed at the top of the dialog.
    Affected if The displayed version is less than 78.12 (for example, 78.11 or earlier)
  4. Verify via command line (optional)
    Run 'firefox --version' or 'thunderbird --version' in a terminal/command prompt to retrieve the installed version programmatically.
    Affected if The version output is below 90.0 for Firefox, or below 78.12 for Firefox ESR/Thunderbird

You are affected if any installed instance of Firefox (Stable/Beta), Firefox ESR, or Thunderbird has a version number lower than the respective fixed version (90.0, 78.12, or 78.12).

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 78.12 / 90.0 or later
Fixed in 78.1290.0
Interim mitigation

Update Thunderbird to 78.12+, Firefox ESR to 78.12+, and Firefox to 90+ to obtain the fixed versions.

Recommended fix High confidence

Firefox 90.0, Firefox ESR 78.12, or Thunderbird 78.12 depending on product

  1. 1. Back up any important local data (bookmarks, passwords, emails if using Thunderbird)
  2. 2. Close all instances of Firefox or Thunderbird
  3. 3. For Firefox: Navigate to Help > About Firefox to trigger automatic update, or download Firefox 90.0 from the official Mozilla website
  4. 4. For Firefox ESR: Download Firefox ESR 78.12 from Mozilla's ESR channel
  5. 5. For Thunderbird: Download Thunderbird 78.12 from the official Mozilla Thunderbird website
  6. 6. Install the updated version
  7. 7. Restart the application and verify it runs without errors
  8. 8. Restore any backed-up data if needed

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

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
5.0 hours of engineering $860
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,376.

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-2021-29976 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