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FirefoxWeb browser · Mozilla

CVE-2021-38496

HIGH · 8.8 CVSS v3.1 Published 2021-11-03
Fix available
A fix is available. Upgrade to 78.15 / 91.2 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
During operations on MessageTasks, a task may have been removed while it was still scheduled, resulting in memory corruption and a potentially exploitable crash. This vulnerability affects Thunderbird < 78.15, Thunderbird < 91.2, Firefox ESR < 91.2, Firefox ESR < 78.15, and Firefox < 93.

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

During MessageTask operations in Firefox and Thunderbird, a race condition allows a task to be removed from memory while still scheduled, resulting in use-after-free memory corruption that can be exploited for code execution.

MitigationUpdate to Firefox 93+, Firefox ESR 91.2+, Thunderbird 91.2+, or Thunderbird 78.15+ to obtain vendor patches.

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:< 93.0>= 91.0, < 91.2
Firefox EsrWeb browser
Affected:< 78.15
Debian LinuxOperating system
Affected:= 9.0= 10.0= 11.0
ThunderbirdApplication
Affected:< 78.15>= 91.0, < 91.2

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 if Mozilla Firefox is installed
    On Windows, check C:\Program Files\Mozilla Firefox\firefox.exe or run 'firefox --version' in command prompt. On Linux, run 'firefox --version' or check your package manager. On macOS, check /Applications/Firefox.app and get version via Finder or 'defaults read /Applications/Firefox.app/Contents/Info.plist CFBundleShortVersionString'
    Affected if Firefox is present on the system
  2. Determine installed Firefox version
    Run 'firefox --version' or check Help > About Firefox in the application menu. For Windows, right-click firefox.exe > Properties > Details tab to see File Version
    Affected if Version is less than 93.0, OR falls between 91.0 and 91.2 (inclusive of 91.0, exclusive of 91.2), OR for ESR versions less than 78.15
  3. Check for Firefox ESR installation
    Run 'firefox --version' and look for 'ESR' in the output, or check package manager listings for Firefox ESR package
    Affected if Firefox ESR version is installed and is less than 78.15
  4. Check if Mozilla Thunderbird is installed
    On Windows, check C:\Program Files\Mozilla Thunderbird\thunderbird.exe or run 'thunderbird --version'. On Linux, run 'thunderbird --version' or check package manager. On macOS, check /Applications/Thunderbird.app
    Affected if Thunderbird is present on the system
  5. Determine installed Thunderbird version
    Run 'thunderbird --version' or check Help > About Thunderbird. On Windows, right-click thunderbird.exe > Properties > Details for version info
    Affected if Version is less than 78.15, OR falls between 91.0 and 91.2 (inclusive of 91.0, exclusive of 91.2)

A user is affected if they have Firefox (including ESR) version below 93.0 or Thunderbird version below 78.15, with specific attention to the 91.x branch versions that must be 91.2 or higher to be patched.

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 78.15 / 91.2 / 93.0 or later
Fixed in 78.1591.293.0
Interim mitigation

Update to Firefox 93+, Firefox ESR 91.2+, Thunderbird 91.2+, or Thunderbird 78.15+ to obtain vendor patches.

Recommended fix High confidence

Firefox 93.0+ (or 91.2+); Firefox ESR 78.15+ (or 91.2+); Thunderbird 78.15+ (or 91.2+)

  1. 1. Identify the currently installed Firefox, Firefox ESR, or Thunderbird version by checking Help > About (or running: firefox --version, thunderbird --version)
  2. 2. For Firefox: Upgrade to version 93.0 or later, or version 91.2 or later for the extended support release
  3. 3. For Firefox ESR: Upgrade to version 78.15 or later, or version 91.2 or later for the 91.x ESR line
  4. 4. For Thunderbird: Upgrade to version 78.15 or later, or version 91.2 or later for the 91.x line
  5. 5. On Debian systems: Run 'apt update && apt upgrade' to receive security updates from Debian's repositories
  6. 6. Restart the application after upgrading
  7. 7. Verify the version has been updated to a fixed release (78.15+, 91.2+, or 93.0+)
Caveat Major version upgrades may include feature changes; ESR versions provide stability for enterprise environments

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
4.0 hours of engineering $750
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,200.

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-2021-38496 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

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