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FirefoxWeb browser · Mozilla

CVE-2021-38493

HIGH · 8.8 CVSS v3.1 Published 2021-11-03
Fix available
A fix is available. Upgrade to 78.14 / 92.0 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
Mozilla developers reported memory safety bugs present in Firefox 91 and Firefox ESR 78.13. 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 Firefox ESR < 78.14, Thunderbird < 78.14, and Firefox < 92.

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 · moderate confidence

Memory safety bugs in Firefox 91 and Firefox ESR 78.13 that could lead to memory corruption. Some of these bugs showed evidence of memory corruption and could potentially be exploited to run arbitrary code.

MitigationUpdate to Firefox 92 or later, Firefox ESR 78.14 or later, or Thunderbird 78.14 or later to remediate this vulnerability.

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:< 92.0
Firefox EsrWeb browser
Affected:< 78.14
ThunderbirdApplication
Affected:< 78.14

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. Identify installed Mozilla product
    Check if Firefox, Firefox ESR, or Thunderbird is installed by looking for the application in /Applications (macOS), Program Files (Windows), or /usr/lib (Linux). Look for executable files like firefox, firefox-esr, or thunderbird.
    Affected if Any of these three products are installed
  2. Determine Firefox version
    Run 'firefox --version' or 'firefox -v' from command line, or check Help > About Firefox within the application. On Windows, right-click the Firefox shortcut and select Properties to view the version.
    Affected if Version is below 92.0 (e.g., 91.x or earlier)
  3. Determine Firefox ESR version
    Run 'firefox-esr --version' or 'firefox-esr -v' from command line, or check Help > About Firefox ( ESR ) within the application.
    Affected if Version is below 78.14 (e.g., 78.13 or earlier)
  4. Determine Thunderbird version
    Run 'thunderbird --version' or 'thunderbird -v' from command line, or check Help > About Thunderbird within the application.
    Affected if Version is below 78.14 (e.g., 78.13 or earlier)

You are affected if any installed version of Firefox is below 92.0, Firefox ESR is below 78.14, or Thunderbird is below 78.14.

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 78.14 / 92.0 or later
Fixed in 78.1492.0
Interim mitigation

Update to Firefox 92 or later, Firefox ESR 78.14 or later, or Thunderbird 78.14 or later to remediate this vulnerability.

Recommended fix High confidence

Firefox 92.0 or later; Firefox ESR 78.14 or later; Thunderbird 78.14 or later

  1. Identify which Mozilla product is in use (Firefox, Firefox ESR, or Thunderbird)
  2. Determine the current installed version of the product
  3. For Firefox: If running a version below 92.0, upgrade to Firefox 92.0 or later
  4. For Firefox ESR: If running a version below 78.14, upgrade to Firefox ESR 78.14 or later
  5. For Thunderbird: If running a version below 78.14, upgrade to Thunderbird 78.14 or later
  6. Restart the application after upgrading to ensure the new version is fully loaded
  7. Verify the installed version matches or exceeds the fixed release
Caveat Major browser updates may affect browser extensions, saved passwords, or settings - backup profile data before upgrading if needed

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

Fix this in Firefox Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing2.0 h
  • Review / QA1.0 h
9.0 hours of engineering $1,600
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,560.

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-2021-38493 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