FirefoxWeb browser · Mozilla
CVE-2021-24002
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 · uneditedWhen a user clicked on an FTP URL containing encoded newline characters (%0A and %0D), the newlines would have been interpreted as such and allowed arbitrary commands to be sent to the FTP server. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.
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 confidenceFirefox and Thunderbird fail to properly sanitize FTP URLs containing URL-encoded newline characters (%0A and %0D). When processed, these encoded characters are decoded into actual newline characters, allowing attackers to inject arbitrary FTP commands through crafted URLs. This enables command injection against FTP servers when users click malicious links.
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
< 88.0
Firefox EsrWeb browser
< 78.10< 78.10CVSS 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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Identify installed Firefox versionOpen Firefox and navigate to Help > About Firefox, or type 'about:support' in the address bar and locate the Version field under Application BasicsAffected if Version number is less than 88.0 (or less than 78.10 for Firefox ESR)
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Identify installed Thunderbird versionOpen Thunderbird and navigate to Help > About Thunderbird to display the version numberAffected if Version number is less than 78.10
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Check if FTP URL handling is in useDetermine whether users in your environment frequently handle or click FTP links, as this is the attack vector; inspect browser or mail client settings for FTP protocol handlers if accessibleAffected if Users process or click FTP URLs in the browser or mail client, as the vulnerability is triggered specifically when handling FTP URLs with encoded newlines
You are affected if any installed version of Firefox is below 88.0, Firefox ESR is below 78.10, or Thunderbird is below 78.10, AND users in your environment click or process FTP links.
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 · scoped78.1088.0
Upgrade to Firefox ESR 78.10+, Thunderbird 78.10+, or Firefox 88+ to receive the patch. Until then, instruct users to avoid clicking untrusted FTP links.
Firefox 88.0, Firefox ESR 78.10, or Thunderbird 78.10 (depending on product)
- Identify the currently installed Mozilla product (Firefox, Firefox ESR, or Thunderbird) and its version number
- Navigate to the official Mozilla website at www.mozilla.org to download the latest version
- For Firefox users: Download and install Firefox 88.0 or later
- For Firefox ESR users: Download and install Firefox ESR 78.10 or later
- For Thunderbird users: Download and install Thunderbird 78.10 or later
- Restart the application after installation
- Verify the installed version matches or exceeds the fixed release (Firefox 88.0, Firefox ESR 78.10, or Thunderbird 78.10)
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Firefox
Scoped from the published advisory
- Consultation3.0 h
- Implementation2.0 h
- Testing4.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,072.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-24002 — 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-2021-24002 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