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FirefoxWeb browser · Mozilla

CVE-2021-38504

HIGH · 8.8 CVSS v3.1 Published 2021-12-08
Fix available
A fix is available. Upgrade to 91.3.0 / 94.0 or later.
See remediation →
95/100
Remediation priority · Urgent
Remotely reachable No privileges

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 · unedited
When interacting with an HTML input element's file picker dialog with webkitdirectory set, a use-after-free could have resulted, leading to memory corruption and a potentially exploitable crash. This vulnerability affects Firefox < 94, Thunderbird < 91.3, and Firefox ESR < 91.3.

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 confidence

A use-after-free vulnerability exists in Firefox and Thunderbird when using the file picker dialog with the webkitdirectory attribute set on an HTML input element. This causes memory corruption and potentially exploitable crashes due to improper memory management during directory selection operations.

MitigationUpgrade to Firefox 94+, Thunderbird 91.3+, or Firefox ESR 91.3+ to patch the vulnerability. Organizations should deploy the security update across all affected installations.

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
Affected:< 94.0
Firefox EsrWeb browser
Affected:< 91.3.0
Debian LinuxOperating system
Affected:= 9.0= 10.0= 11.0
ThunderbirdApplication
Affected:< 91.3.0

CVSS 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 checks

Work through these to decide whether this CVE applies to you.

  1. Check installed Firefox version
    Navigate to about:firefox in Firefox browser or run 'firefox --version' from command line
    Affected if Version is below 94.0 (e.g., 93.x or earlier) or below ESR 91.3.0 for ESR releases
  2. Check installed Thunderbird version
    Navigate to about:thunderbird in Thunderbird or run 'thunderbird --version' from command line
    Affected if Version is below 91.3.0
  3. Check Debian package versions for Firefox
    Run 'dpkg -l firefox' or 'apt-cache policy firefox' on Debian-based systems
    Affected if Package version indicates Firefox installed is earlier than 94.0-1 (Debian 9/10/11)
  4. Check Debian package versions for Thunderbird
    Run 'dpkg -l thunderbird' or 'apt-cache policy thunderbird' on Debian-based systems
    Affected if Package version indicates Thunderbird installed is earlier than 1:91.3.0-1 (Debian 9/10/11)
  5. Identify webkitdirectory usage in local environment
    Search local HTML files or web applications for '<input type="file" webkitdirectory>' or 'webkitdirectory' attribute usage
    Affected if Local applications or test pages use the webkitdirectory attribute and are loaded in vulnerable browser versions

Your environment is affected if you run Firefox below 94.0 (or below ESR 91.3.0), Thunderbird below 91.3.0, or Debian-packaged versions from 9/10/11 that have not been patched, AND you or users interact with web pages using the webkitdirectory attribute in file input elements.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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 · scoped
Upgrade available Upgrade to 91.3.0 / 94.0 or later
Fixed in 91.3.094.0
Interim mitigation

Upgrade to Firefox 94+, Thunderbird 91.3+, or Firefox ESR 91.3+ to patch the vulnerability. Organizations should deploy the security update across all affected installations.

Recommended fix High confidence

Firefox 94.0, Firefox ESR 91.3.0, or Thunderbird 91.3.0

  1. Upgrade Firefox to version 94.0 or later
  2. Upgrade Firefox ESR to version 91.3.0 or later
  3. Upgrade Thunderbird to version 91.3.0 or later
  4. For Debian systems, apply security updates: 'apt update && apt upgrade' or 'apt-get update && apt-get upgrade' to install the patched Mozilla packages

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Implementation2.0 h
  • Testing1.0 h
  • Review / QA1.0 h
5.0 hours of engineering $890
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,424.

Scan for this in your stack

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-38504 in production — separate from our analysis above.

No notes yet

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What this is

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.

What belongs here
  • 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