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FirefoxWeb browser · Mozilla

CVE-2021-38500

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
Mozilla developers reported memory safety bugs present in Firefox 92 and Firefox ESR 91.1. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. 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

Memory safety bugs in Mozilla Firefox and Thunderbird that could allow arbitrary code execution through memory corruption. The vulnerabilities stem from insufficient bounds checking and improper memory handling in the browser engine.

MitigationUpdate to Firefox >= 93, Firefox ESR >= 91.2 (or 78.15), or Thunderbird >= 91.2 (or 78.15) to obtain the patched versions.

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 Firefox version on Debian
    Run: firefox --version or dpkg -l | grep firefox
    Affected if Version is < 93.0, or is between 91.0 and 91.1.x
  2. Check Firefox ESR version on Debian
    Run: firefox-esr --version or dpkg -l | grep firefox-esr
    Affected if Version is < 78.15
  3. Check Thunderbird version on Debian
    Run: thunderbird --version or dpkg -l | grep thunderbird
    Affected if Version is < 78.15, or is between 91.0 and 91.1.x
  4. Identify Firefox package on Debian
    Run: apt-cache policy firefox and apt-cache policy firefox-esr
    Affected if The available candidate version is lower than 93.0 (Firefox) or lower than 78.15 (ESR)
  5. Verify Thunderbird package on Debian
    Run: apt-cache policy thunderbird
    Affected if The available candidate version is lower than 78.15 or between 91.0 and 91.1.x

A user is affected if they have Firefox < 93.0, Firefox ESR < 78.15, or Thunderbird < 78.15 (or between 91.0-91.1.x) installed on their Debian Linux system.

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 (or 78.15), or Thunderbird >= 91.2 (or 78.15) to obtain the patched versions.

Recommended fix High confidence

Firefox 93.0+, Firefox ESR 91.2+/78.15+, Thunderbird 91.2+/78.15+ (choose the appropriate branch based on current major version)

  1. Identify the currently installed Firefox, Firefox ESR, or Thunderbird version by navigating to Help > About [Product Name]
  2. For Firefox: Upgrade to version 93.0 or later
  3. For Firefox ESR: If running 91.x, upgrade to 91.2 or later; if running 78.x, upgrade to 78.15 or later
  4. For Thunderbird: If running 91.x, upgrade to 91.2 or later; if running 78.x, upgrade to 78.15 or later
  5. On Debian systems, run: apt update && apt upgrade firefox thunderbird (or apt-get)
  6. Restart the application after upgrade completes
  7. Verify the installed version matches or exceeds the fixed release
Caveat Upgrading to newer major releases may introduce minor UI changes or deprecate older extensions; ensure extension compatibility before upgrading

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
  • Testing2.0 h
  • Review / QA1.0 h
5.0 hours of engineering $860
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,376.

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-2021-38500 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