FirefoxWeb browser · Mozilla
CVE-2021-29977
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 · uneditedMozilla developers reported memory safety bugs present in Firefox 89. 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 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 · moderate confidenceMemory safety vulnerability in Firefox 89 involving multiple memory corruption bugs. Mozilla developers identified these bugs as showing evidence of memory corruption with the potential for exploitation to achieve arbitrary code execution. Affects Firefox versions prior to 90.
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
< 90.0CVSS 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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Check Firefox version via internal pageOpen a new tab and navigate to about:Firefox or go to Help > About Firefox. The version number is displayed on this page.Affected if The displayed version is below 90.0 (for example 89.x, 88.x, etc.)
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Check Firefox version via command lineOpen a terminal or command prompt and run: firefox --version (Linux/macOS) or "C:\Program Files\Mozilla Firefox\firefox.exe" -v (Windows).Affected if The version output shows a number lower than 90.0
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Check Firefox version via system package managerOn Linux systems, query the installed package: dpkg -l firefox (Debian/Ubuntu) or rpm -q firefox (RHEL/Fedora) or pacman -Q firefox (Arch).Affected if The listed package version is below 90.0
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Check Firefox version on Windows via file propertiesNavigate to C:\Program Files\Mozilla Firefox\ and right-click firefox.exe to view Properties > Details. The File Version is shown there.Affected if The file version shown is below 90.0.0
You are affected if any method shows Firefox installed with a version number less than 90.0.
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 · scoped90.0
Upgrade to Firefox 90 or later to remediate the vulnerability. No other mitigation options are described; users should apply the vendor patch promptly.
Firefox 90.0
- 1. Back up any important browser data (bookmarks, saved passwords, etc.)
- 2. Close all instances of Firefox
- 3. Download Firefox 90.0 or later from the official Mozilla website (www.mozilla.org)
- 4. Run the installer and follow the prompts to upgrade
- 5. Alternatively, use your system's package manager to update Firefox (e.g., apt-get update && apt-get install firefox on Debian/Ubuntu, or dnf update firefox on Fedora)
- 6. After installation, verify the version by navigating to about:Firefox in the address bar
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
- 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 $1,200.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-29977 — 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-2021-29977 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