FirefoxWeb browser · Mozilla
CVE-2021-29951
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 · uneditedThe Mozilla Maintenance Service granted SERVICE_START access to BUILTIN|Users which, in a domain network, grants normal remote users access to start or stop the service. This could be used to prevent the browser update service from operating (if an attacker spammed the 'Stop' command); but also exposed attack surface in the maintenance service. *Note: This issue only affected Windows operating systems older than Win 10 build 1709. Other operating systems are unaffected.*. This vulnerability affects Thunderbird < 78.10.1, Firefox < 87, and Firefox ESR < 78.10.1.
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 confidenceThe Mozilla Maintenance Service for Windows incorrectly granted SERVICE_START access to the BUILTIN|Users group, which in domain network environments allowed any remote user to start or stop the service. This ACL misconfiguration could be exploited to prevent browser updates by repeatedly stopping the service, and exposed additional attack surface in the maintenance service. Only affects Windows versions older than Win 10 build 1709.
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
< 78.10.1< 78.10.1CVSS 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
- None
- Scope
- Unchanged
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/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 installed Firefox versionOpen Firefox, go to Help > About Firefox, or run 'firefox -v' from command line, or check Programs and FeaturesAffected if Version is below 87.0 (or below 78.10.1 for ESR)
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Check installed Thunderbird versionOpen Thunderbird, go to Help > About Thunderbird, or check Programs and FeaturesAffected if Version is below 78.10.1
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Verify Mozilla Maintenance Service existsOpen Services console (services.msc) or run 'sc query maintenance' from elevated command prompt, look for service named 'Mozilla Maintenance Service'Affected if The service is installed and present on the system
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Check SERVICE_START ACL on the Mozilla Maintenance ServiceRun 'sc sdshow maintenance' from an elevated command prompt and examine the DACL; look for 'S-1-5-32-545' (BUILTIN\Users) with SERVICE_START permission grantedAffected if The DACL grants SERVICE_START access to BUILTIN\Users (S-1-5-32-545) or Everyone group
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Verify Windows version is older than build 1709Run 'winver' or check System Properties, or run 'systeminfo | findstr /B /C:"OS Name" /C:"Build"'Affected if Running Windows 10 older than Fall Creators Update (build 1709)
Your environment is affected if you have Firefox < 87.0, Firefox ESR < 78.10.1, or Thunderbird < 78.10.1 installed with the Mozilla Maintenance Service present, and the service's ACL grants SERVICE_START permissions to standard users.
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 data78.10.187.0
Update affected installations to Firefox 87, Firefox ESR 78.10.1, Thunderbird 78.10.1, or later versions. For unpatched systems, restrict SERVICE_START permissions on the Mozilla Maintenance Service to administrators only via sc or the Services console.
Fix this in Firefox
Scoped from the published advisory
- Consultation2.0 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 $1,744.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-29951 — 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-2021-29951 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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- 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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