FirefoxWeb browser · Mozilla
CVE-2020-35112
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 · uneditedIf a user downloaded a file lacking an extension on Windows, and then "Open"-ed it from the downloads panel, if there was an executable file in the downloads directory with the same name but with an executable extension (such as .bat or .exe) that executable would have been launched instead. *Note: This issue only affected Windows operating systems. Other operating systems are unaffected.*. This vulnerability affects Firefox < 84, Thunderbird < 78.6, and Firefox ESR < 78.6.
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 confidenceOn Windows, when a user downloads a file lacking an extension and opens it from the downloads panel, the browser incorrectly searches for and launches an executable with the same base name plus an executable extension (.exe, .bat, etc.) residing in the downloads directory. This allows arbitrary code execution by placing a maliciously named executable in the downloads folder.
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
< 84.0
Firefox EsrWeb browser
< 78.6.0< 78.6.0CVSS 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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Confirm Windows environmentDetermine if the system is running Windows (this vulnerability only affects Windows platforms). Check OS via system information or 'winver' command.Affected if The system is running Windows - other operating systems are not affected by this flaw.
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Check Firefox versionOpen Firefox, click Help > About Firefox, or navigate to about:support. Note the version number displayed.Affected if The installed version is less than 84.0 on Windows.
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Check Firefox ESR versionOpen Firefox ESR, click Help > About Firefox ESR, or navigate to about:support. Note the version number with 'ESR' designation.Affected if The installed ESR version is less than 78.6.0 on Windows.
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Check Thunderbird versionOpen Thunderbird, click Help > About Thunderbird, or navigate to about:support. Note the version number displayed.Affected if The installed version is less than 78.6.0 on Windows.
A user is affected if they are running Firefox < 84.0, Firefox ESR < 78.6.0, or Thunderbird < 78.6.0 on a Windows system, and they use the Open action from the downloads panel on files without extensions.
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 data78.6.084.0
Upgrade Firefox to version 84 or later, or Thunderbird/Firefox ESR to version 78.6 or later. Until patched, avoid using the 'Open' action for extensionless files on Windows; instead save files to a known location and open them manually.
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-2020-35112 — 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-2020-35112 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