FirefoxWeb browser · Mozilla
CVE-2021-29964
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 locally-installed hostile program could send `WM_COPYDATA` messages that Firefox would process incorrectly, leading to an out-of-bounds read. *This bug only affects Firefox on Windows. Other operating systems are unaffected.*. This vulnerability affects Thunderbird < 78.11, Firefox < 89, and Firefox ESR < 78.11.
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 locally-installed hostile program on Windows can send WM_COPYDATA messages to Firefox, which the browser processes incorrectly, causing an out-of-bounds read. This is a memory safety vulnerability in Firefox's Windows message handling subsystem.
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
< 89.0
Firefox EsrWeb browser
< 78.11< 78.11CVSS 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
- Local
- Complexity
- Low
- Privileges
- None
- User interaction
- Required
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:N/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. On Windows, look in Program Files for Mozilla Firefox or Mozilla Thunderbird folders, or check the application list in Programs and Features.Affected if Product is Firefox or Thunderbird on Windows
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Check Firefox versionOpen Firefox, click the menu button (three horizontal lines), select Help, then About Firefox. The version number will be displayed.Affected if Firefox version is below 89.0 (for example, 88.x or earlier)
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Check Firefox ESR versionOpen Firefox ESR, click the menu button, select Help, then About Firefox ESR. The version number will be displayed.Affected if Firefox ESR version is below 78.11
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Check Thunderbird versionOpen Thunderbird, click the menu button (three horizontal lines), select Help, then About Thunderbird. The version number will be displayed.Affected if Thunderbird version is below 78.11
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Verify Windows platformConfirm the operating system is Windows. This vulnerability only affects the Windows platform.Affected if The browser is running on Windows - this vulnerability does not affect macOS or Linux
You are affected if you run Firefox < 89.0, Firefox ESR < 78.11, or Thunderbird < 78.11 on Windows, because the browser will process WM_COPYDATA messages from local programs and trigger the out-of-bounds read.
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 data78.1189.0
Update affected Firefox versions to 89+, Thunderbird to 78.11+, and Firefox ESR to 78.11+. As a defense-in-depth measure, restrict local program installation privileges to prevent untrusted applications from sending messages to browsers.
Fix this in Firefox
Scoped from the published advisory
- Consultation1.0 h
- Implementation2.0 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 $1,664.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-29964 — 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-2021-29964 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
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