FirefoxWeb browser · Mozilla
CVE-2026-3846
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 · uneditedSame-origin policy bypass in the CSS Parsing and Computation component. This vulnerability was fixed in Firefox 148.0.2.
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 confidenceA same-origin policy bypass vulnerability exists in Firefox's CSS Parsing and Computation component. The same-origin policy is a critical browser security mechanism that restricts how documents or scripts from one origin can interact with resources from another origin; bypassing this allows potentially malicious web pages to access sensitive data or perform actions across origins that should be restricted.
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
< 148.0.2CVSS 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.
-
Check installed Firefox versionNavigate to Menu > Help > About Firefox, or enter 'about:support' in the address bar, or run 'firefox --version' from the command lineAffected if The displayed version number is less than 148.0.2 (for example, 148.0.1, 148.0, 147.x, or any earlier version)
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Verify CSS parsing component is activeThe CSS Parsing and Computation component is part of the core rendering engine and is active whenever Firefox loads and displays web content with CSS stylingAffected if This is always active in normal browser usage; the vulnerability is present in the browser core itself when the version is vulnerable
A user is affected if their installed Firefox version is below 148.0.2, as the same-origin policy bypass exists in the CSS Parsing component of any earlier version.
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 · scoped148.0.2
Update Firefox to version 148.0.2 or later to remediate this vulnerability.
Firefox 148.0.2
- 1. Open Firefox and navigate to Menu > Help > About Firefox (or go to about:support)
- 2. Click 'Check for Updates' or wait for the automatic check to complete
- 3. If version 148.0.2 or later is available, click 'Update to latest version'
- 4. Wait for the download and installation to complete
- 5. Restart Firefox to apply the update
- 6. Verify the update by checking Menu > Help > About Firefox shows version 148.0.2 or later
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Firefox
Scoped from the published advisory
- Consultation1.0 h
- Implementation1.0 h
- Testing1.0 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 $1,200.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-3846 — 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-2026-3846 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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