FirefoxWeb browser · Mozilla
CVE-2021-29955
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 · uneditedA transient execution vulnerability, named Floating Point Value Injection (FPVI) allowed an attacker to leak arbitrary memory addresses and may have also enabled JIT type confusion attacks. (A related vulnerability, Speculative Code Store Bypass (SCSB), did not affect Firefox.). This vulnerability affects Firefox ESR < 78.9 and Firefox < 87.
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 confidenceCVE-2021-29955 is a transient execution vulnerability (Floating Point Value Injection/FPVI) in Firefox's JavaScript engine that exploits speculative execution to leak arbitrary memory addresses. The vulnerability also potentially enabled JIT type confusion attacks through speculative execution side channels.
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
< 87.0
Firefox EsrWeb browser
< 78.9CVSS 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
- High
- Privileges
- None
- User interaction
- Required
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- None
- Availability
- None
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:N/A:N
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.
-
Confirm Firefox is installedCheck for Firefox browser on the system. On Windows: look in Program Files or via registry. On Linux: check /usr/bin/firefox or package manager. On macOS: check /Applications/Firefox.appAffected if Firefox browser is present on the system
-
Determine installed Firefox versionOpen Firefox and navigate to Help > About Firefox, or run 'firefox --version' from command line. Note the full version number displayed (e.g., 86.0.1 or 78.8.0)Affected if Version number is displayed and can be compared against affected ranges
-
Identify if Firefox is Standard or ESR releaseIn the About dialog, ESR versions display 'Firefox ESR' alongside the version number. The command line output may also indicate ESR (e.g., 'Mozilla Firefox 78.8.0 ESR')Affected if Firefox ESR is in use and version is below 78.9, or standard Firefox is below 87.0
-
Compare version against affected rangesFor standard Firefox: verify if version is less than 87.0 (e.g., 86.x, 85.x, etc.). For Firefox ESR: verify if version is less than 78.9 (e.g., 78.8.x, 77.x, etc.)Affected if Standard Firefox version < 87.0 OR Firefox ESR version < 78.9
The environment is affected if Firefox (standard release) version is below 87.0, or if Firefox ESR version is below 78.9, as these contain the vulnerable JavaScript engine with the unpatched transient execution vulnerability.
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.
From vendor data78.987.0
Update Firefox to version 87 or later (or Firefox ESR to 78.9 or later) to receive the vendor patch addressing this transient execution vulnerability.
Fix this in Firefox
Scoped from the published advisory
- Consultation1.0 h
- Implementation2.0 h
- Testing1.0 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 $1,424.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-29955 — 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-2021-29955 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
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.
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