FirefoxWeb browser · Mozilla
CVE-2020-12390
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 · uneditedIncorrect origin serialization of URLs with IPv6 addresses could lead to incorrect security checks. This vulnerability affects Firefox < 76.
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 confidenceFirefox browsers before version 76 incorrectly serialize origins when URLs contain IPv6 addresses, causing the same-origin security policy checks to fail. This allows an attacker to bypass security boundaries and potentially execute cross-origin attacks.
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
< 76.0CVSS 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
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/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 checksWork through these to decide whether this CVE applies to you.
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Determine installed Firefox versionOpen Firefox, click the menu button (three horizontal lines), select Help, then About Firefox. The version number will displayed on the page that opens.Affected if The displayed version number is less than 76.0 (for example, 75.x, 74.x, etc.)
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Verify Firefox build date if version shows 76In About Firefox, check the build date. Some point releases within 76 may have been patched. Compare the build date against Mozilla's release notes for version 76.Affected if Version shows 76 but build date is before the June 2020 security update release.
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Identify IPv6 usage in the environmentReview any internal applications, services, or configurations that use IPv6 address syntax in URLs (for example, http://[::1]/ or http://[2001:db8::1]/). Check web server access logs for requests containing IPv6 addresses in brackets.Affected if The environment processes or serves URLs with IPv6 address literals, and the Firefox version is below 76.
The environment is affected if Firefox version is below 76.0 and the browser is used to access URLs containing IPv6 addresses, enabling same-origin policy bypass.
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 · scoped76.0
Upgrade Firefox to version 76 or later. In enterprise environments, deploy the updated browser version through standard software distribution mechanisms.
Firefox 76.0
- Open Firefox and navigate to Help > About Firefox to check current version
- If version is below 76.0, click 'Update to the latest version' or use the automatic update feature
- Alternatively, download Firefox 76.0 or later from the official Mozilla website (www.mozilla.org)
- Restart Firefox after the update completes
- Verify the update by checking Help > About Firefox shows version 76.0 or higher
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-2020-12390 — 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-2020-12390 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
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