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FirefoxWeb browser · Mozilla

CVE-2021-23973

MEDIUM · 6.5 CVSS v3.1 Published 2021-02-26
Fix available
A fix is available. Upgrade to 78.8 / 86.0 or later.
See remediation →
72/100
Remediation priority · Elevated
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 trying to load a cross-origin resource in an audio/video context a decoding error may have resulted, and the content of that error may have revealed information about the resource. This vulnerability affects Firefox < 86, Thunderbird < 78.8, and Firefox ESR < 78.8.

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

When loading cross-origin audio or video resources in Firefox, Thunderbird, or Firefox ESR, a decoding error could occur and the error message content could reveal information about the cross-origin resource, potentially exposing details like file existence or format. This constitutes an information disclosure vulnerability via error message leakage.

MitigationUpgrade to Firefox 86, Thunderbird 78.8, Firefox ESR 78.8, or later versions to patch 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:< 86.0
Firefox EsrWeb browser
Affected:< 78.8
Debian LinuxOperating system
Affected:= 9.0= 10.0
ThunderbirdApplication
Affected:< 78.8

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
None
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N

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 the installed Mozilla product
    Check if Firefox, Firefox ESR, or Thunderbird is installed. On Linux, run `dpkg -l | grep -E 'firefox|thunderbird'` or check application menus. On Windows, check Program Files for Mozilla folders. On macOS, check /Applications for Firefox or Thunderbird.
    Affected if Any of these products are installed, proceed to version check
  2. Determine the installed Firefox version
    For Firefox: Type `about:support` in the address bar and look at the 'Version' field, or run `firefox --version` from command line.
    Affected if Firefox version is present and is below 86.0 (e.g., 85.x or earlier)
  3. Determine the installed Firefox ESR version
    For Firefox ESR: Type `about:support` in the address bar and look at the 'Version' field, or run `firefox --version` from command line. ESR versions typically end with an 'esr' suffix (e.g., 78.7esr).
    Affected if Firefox ESR version is present and is below 78.8 (e.g., 78.7esr or earlier)
  4. Determine the installed Thunderbird version
    For Thunderbird: Type `about:support` in the address bar and look at the 'Version' field, or run `thunderbird --version` from command line.
    Affected if Thunderbird version is present and is below 78.8 (e.g., 78.7 or earlier)

You are affected if you have Firefox below 86.0, Firefox ESR below 78.8, or Thunderbird below 78.8 installed, and you load cross-origin audio or video resources in the application.

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.

From vendor data
Upgrade available Upgrade to 78.8 / 86.0 or later
Fixed in 78.886.0
Interim mitigation

Upgrade to Firefox 86, Thunderbird 78.8, Firefox ESR 78.8, or later versions to patch this vulnerability.

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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dbcve dependency scanner

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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-2021-23973 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