FirefoxWeb browser · Mozilla
CVE-2021-29981
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 · uneditedAn issue present in lowering/register allocation could have led to obscure but deterministic register confusion failures in JITted code that would lead to a potentially exploitable crash. This vulnerability affects Firefox < 91 and Thunderbird < 91.
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 register allocation bug in the SpiderMonkey JIT compiler causes deterministic register confusion in compiled JavaScript code, potentially leading to exploitable crashes. This is a memory safety vulnerability in the just-in-time compilation process affecting Firefox and Thunderbird versions prior to 91.
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
< 91.0< 91.0CVSS 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.
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Identify installed Mozilla productCheck if Firefox or Thunderbird is installed on the system by locating the application executable or checking system package informationAffected if Neither Firefox nor Thunderbird is installed - not applicable
-
Determine installed versionRetrieve the version number of Firefox or Thunderbird (typically available via application menu Help > About, command line with --version, or system package manager)Affected if Version is 91.0 or higher - not affected; version is below 91.0 - potentially affected
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Confirm JIT compiler is enabledCheck browser or mail client configuration for JavaScript JIT compiler settings. In Firefox: about:config, look for javascript.options.jit.content and related JIT preferences. In Thunderbird: similar configuration paths. The vulnerability exists in the JIT compiler, so JIT must be enabled for the flaw to applyAffected if JIT is disabled - not affected; JIT is enabled (default state) with version below 91 - affected
User is affected if they have Firefox or Thunderbird installed with version below 91.0 and the JIT compiler is enabled (which is the default configuration).
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 · scoped91.0
Update Firefox to version 91 or later and Thunderbird to version 91 or later to obtain the patched JIT compiler with corrected register allocation logic.
Firefox 91.0 / Thunderbird 91.0
- Upgrade Firefox to version 91.0 or later
- If using Thunderbird, upgrade Thunderbird to version 91.0 or later
- After upgrade, verify the version by navigating to 'about:Firefox' (or 'about:Thunderbird') in the URL bar to confirm the installed version
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
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
Free · runs locallyCheck whether your project pulls in CVE-2021-29981 — 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-2021-29981 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