FirefoxWeb browser · Mozilla
CVE-2021-38491
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 · uneditedMixed-content checks were unable to analyze opaque origins which led to some mixed content being loaded. This vulnerability affects 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 · high confidenceThe Firefox mixed-content security check failed to properly analyze opaque origins, allowing insecure HTTP content to be loaded on HTTPS pages in Firefox versions before 92. This could enable man-in-the-middle attacks or information disclosure via mixed content.
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
< 92.0CVSS 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
- None
- Integrity
- High
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/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 checksWork through these to decide whether this CVE applies to you.
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Find installed Firefox versionOpen Firefox, click the menu button (three horizontal lines) in the top right, select Help, then select About Firefox. The version number is displayed on the About Firefox page.Affected if The version shown is less than 92.0 (for example, 91.x, 90.x, or earlier)
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Verify version via command line (alternative)On Windows, open Command Prompt and run: firefox --version. On macOS, open Terminal and run: /Applications/Firefox.app/Contents/MacOS/firefox --version. On Linux, run: firefox --version in a terminal.Affected if The version output is less than 92.0
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Confirm mixed content protection contextThis vulnerability affects mixed content blocking on HTTPS pages. The flaw allowed HTTP resources to load on HTTPS pages due to improper opaque origin analysis. The primary indicator is the version number - if Firefox is below 92.0, this protection may be insufficient.Affected if Firefox version is below 92.0 and the browser is used to browse HTTPS websites that may contain or load HTTP resources
If Firefox version is 92.0 or higher, the environment is not affected by this CVE. If version is below 92.0, the mixed-content security check vulnerability is present.
Generated from the published advisory. Verify against your own configuration.
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 · scoped92.0
Update Firefox to version 92 or later to receive the security patch that fixes the opaque origin analysis in mixed-content checks.
Firefox 92.0 or later
- Open Firefox and navigate to Help > About Firefox (or go to about:support) to verify the current version
- If the version is below 92.0, download Firefox 92.0 or later from https://www.mozilla.org/firefox/
- Run the installer and follow the prompts to update Firefox
- Restart Firefox to complete the installation
- After restart, verify the version is 92.0 or higher by returning to Help > About Firefox
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
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
Free · runs locallyCheck whether your project pulls in CVE-2021-38491 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-38491 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- 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
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