FirefoxWeb browser · Mozilla
CVE-2021-29946
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 · uneditedPorts that were written as an integer overflow above the bounds of a 16-bit integer could have bypassed port blocking restrictions when used in the Alt-Svc header. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.
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 confidenceA vulnerability in Firefox and Thunderbird allows port numbers exceeding 16-bit integer bounds (65535) to cause an integer overflow when processed in the Alt-Svc header, bypassing configured port blocking restrictions.
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
< 88.0
Firefox EsrWeb browser
< 78.10< 78.10CVSS 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 checksWork through these to decide whether this CVE applies to you.
-
Check Firefox version on WindowsOpen Firefox, click the menu (three horizontal lines), select Help, then About Firefox. The version number is displayed.Affected if The version shown is earlier than 88.0
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Check Firefox version on LinuxRun the command: firefox --versionAffected if The version output is earlier than 88.0
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Check Firefox ESR version on WindowsOpen Firefox ESR, click the menu, select Help, then About Mozilla Firefox. The version will show ESR.Affected if The version shown is earlier than 78.10 (e.g., 78.9.x)
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Check Firefox ESR version on LinuxRun the command: firefox-esr --versionAffected if The version output is earlier than 78.10
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Check Thunderbird version on WindowsOpen Thunderbird, click the menu, select Help, then About Thunderbird.Affected if The version shown is earlier than 78.10
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Check Thunderbird version on LinuxRun the command: thunderbird --versionAffected if The version output is earlier than 78.10
A user is affected if they are running any version of Firefox below 88.0, Firefox ESR below 78.10, or Thunderbird below 78.10, since the integer overflow in Alt-Svc header parsing can cause port blocking to be bypassed.
Generated from the published advisory. Verify against your own configuration.
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 · scoped78.1088.0
Upgrade to Firefox ESR 78.10, Thunderbird 78.10, Firefox 88, or later versions to patch the integer overflow in Alt-Svc header port validation.
Firefox 88.0, Firefox ESR 78.10, or Thunderbird 78.10 (or later)
- 1. Open Firefox/Thunderbird and navigate to Help > About [Application Name]
- 2. The About window will automatically check for and download the latest update
- 3. Wait for the download to complete, then restart the application when prompted
- 4. Alternatively, download the fixed version directly from the official Mozilla website: www.mozilla.org
- 5. For enterprise deployments, use the ESR version 78.10 or later for extended support
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Firefox
Scoped from the published advisory
- Consultation2.0 h
- Implementation6.0 h
- Testing3.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,664.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-29946 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-29946 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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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.
- 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