FirefoxWeb browser · Mozilla
CVE-2021-29987
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 · uneditedAfter requesting multiple permissions, and closing the first permission panel, subsequent permission panels will be displayed in a different position but still record a click in the default location, making it possible to trick a user into accepting a permission they did not want to. *This bug only affects Firefox on Linux. Other operating systems are unaffected.*. 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 confidenceThis is a UI positioning vulnerability in Firefox and Thunderbird on Linux where after closing an initial permission dialog, subsequent permission panels display in a different screen position but click events are still recorded at the default location, enabling clickjacking attacks to trick users into granting unintended permissions.
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
- None
- Availability
- None
CVSS:3.1/AV:N/AC:L/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.
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Check Firefox version on LinuxOpen Firefox, go to Help > About Firefox, or run 'firefox --version' in terminalAffected if Version is below 91.0 (e.g., 90.x, 89.x, etc.)
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Check Thunderbird version on LinuxOpen Thunderbird, go to Help > About Thunderbird, or run 'thunderbird --version' in terminalAffected if Version is below 91.0 (e.g., 90.x, 89.x, etc.)
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Confirm operating system is LinuxRun 'uname -a' or check system settings - this vulnerability only affects Linux systemsAffected if Running on Linux (the flaw does not affect Windows or macOS)
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Verify permission dialogs are enabledCheck browser/Thunderbird settings for permission prompts (e.g., camera, microphone, location access). These are typically found in Privacy & Security settings under PermissionsAffected if Permission prompts are enabled and user encounters dialogs for camera, microphone, location, or other permissions
User is affected if running Firefox or Thunderbird version below 91.0 on Linux, with permission dialogs enabled and likely having interacted with an initial permission prompt followed by subsequent ones.
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
Upgrade Firefox to version 91 or later, and Thunderbird to version 91 or later, to receive the patch that corrects the permission dialog click registration logic.
Firefox 91.0 / Thunderbird 91.0
- 1. Open Firefox or Thunderbird
- 2. Click the menu button (three horizontal lines) and select 'Help' > 'About Firefox' or 'About Thunderbird'
- 3. The application will automatically check for updates and download version 91.0 or later
- 4. Once the download completes, click 'Restart to update Firefox/Thunderbird'
- 5. After restarting, verify the version is 91.0 or higher via Help > About
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-2021-29987 — 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-2021-29987 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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