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FirefoxWeb browser · Mozilla

CVE-2021-38503

CRITICAL · 10.0 CVSS v3.1 Published 2021-12-08
Fix available
A fix is available. Upgrade to 91.3 / 94.0 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

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
The iframe sandbox rules were not correctly applied to XSLT stylesheets, allowing an iframe to bypass restrictions such as executing scripts or navigating the top-level frame. 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

The iframe sandbox attribute was not correctly enforced when processing XSLT stylesheets, allowing embedded iframes to bypass sandbox restrictions. This enabled malicious XSLT content to execute JavaScript or navigate the top-level window, bypassing same-origin policies and frame navigation restrictions.

MitigationUpgrade to Firefox 94, Thunderbird 91.3, or Firefox ESR 91.3 to apply the vendor patch. Alternatively, disable XSLT stylesheet processing if not required.

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
Debian LinuxOperating system
Affected:= 9.0= 10.0= 11.0
ThunderbirdApplication
Affected:< 91.3

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
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/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 and version
    Open the application and navigate to: Firefox: Help > About Firefox; Thunderbird: Help > About Thunderbird; Firefox ESR: Help > About Firefox ESR. Alternatively, check the package manager or executable properties.
    Affected if The product is Firefox < 94.0, Firefox ESR < 91.3, or Thunderbird < 91.3
  2. Confirm application type
    Determine whether you are running standard Firefox, Firefox ESR (Extended Support Release), or Thunderbird by checking the application name in the title bar or About dialog.
    Affected if Running any vulnerable variant (Firefox, ESR, or Thunderbird) below the patched versions
  3. Check for XSLT stylesheet processing in use
    Review your workflows, web content, or applications that load external data. Look for .xsl or .xslt file references, or XSLT processing in add-ons, extensions, or loaded documents.
    Affected if XSLT stylesheets are being processed by the browser - this is the feature that must be active for the vulnerability to be exploitable

You are affected if you run Firefox below 94.0, Firefox ESR below 91.3, or Thunderbird below 91.3 AND you process XSLT stylesheets in the browser.

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 / 94.0 or later
Fixed in 91.394.0
Interim mitigation

Upgrade to Firefox 94, Thunderbird 91.3, or Firefox ESR 91.3 to apply the vendor patch. Alternatively, disable XSLT stylesheet processing if not required.

Recommended fix High confidence

Firefox >= 94.0, Firefox ESR >= 91.3, Thunderbird >= 91.3

  1. 1. Identify the currently installed version of Firefox, Firefox ESR, or Thunderbird by navigating to Help > About [Product Name]
  2. 2. For Debian Linux systems, update the package lists using: sudo apt update
  3. 3. Upgrade Firefox to version 94.0 or later using: sudo apt upgrade firefox
  4. 4. Upgrade Firefox ESR to version 91.3 or later using: sudo apt upgrade firefox-esr
  5. 5. Upgrade Thunderbird to version 91.3 or later using: sudo apt upgrade thunderbird
  6. 6. Restart the application after upgrading
  7. 7. Verify the version by checking Help > About to confirm the installed version meets the minimum fixed release
Caveat Minor version upgrades typically have no breaking changes; review Mozilla release notes for any behavioral changes

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

Fix this in Firefox Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing1.0 h
  • Review / QA1.0 h
5.0 hours of engineering $910
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,456.

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-2021-38503 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