FirefoxWeb browser · Mozilla
CVE-2026-2790
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 Networking: JAR component. This vulnerability was fixed in Firefox 148, Firefox ESR 140.8, Thunderbird 148, and Thunderbird 140.8.
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 confidenceA same-origin policy (SOP) bypass in Firefox and Thunderbird's Networking:JAR component allows malicious web content to circumvent browser origin restrictions, potentially enabling cross-origin data theft or unauthorized actions. The JAR protocol handler improperly handles origin calculations, violating the fundamental browser security boundary.
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
< 140.8.0< 148.0< 140.8.0< 148.0CVSS 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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Identify the installed Firefox versionNavigate to Firefox, click the menu (three lines), select Help, then About Firefox. The version number displays on that page.Affected if The version shown is less than 140.8.0 OR is 140.x or any version from 141.0 through 147.x (meaning less than 148.0).
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Identify the installed Thunderbird versionIn Thunderbird, click the menu, select Help, then About Thunderbird. The version number displays there.Affected if The version shown is less than 140.8.0 OR is 140.x or any version from 141.0 through 147.x (meaning less than 148.0).
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Confirm the JAR protocol handler is in useThis vulnerability resides in the JAR protocol handler. Check if any installed extensions, add-ons, or web content actively use jar: URLs in the environment. This can be observed in browser network logs (DevTools > Network tab) looking for URLs starting with jar:.Affected if jar: protocol URLs are being processed by the browser and the version is within the affected range listed above.
You are affected if you are running Firefox or Thunderbird version 140.x or 141.x-147.x (any version below 148.0) and the JAR protocol handler processes web content using jar: URLs.
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 · scoped140.8.0148.0
Update to Firefox 148+, Firefox ESR 140.8+, Thunderbird 148+, or Thunderbird 140.8+ to apply the vendor patch addressing this SOP bypass.
Firefox 148.0 (or Firefox ESR 140.8.0); Thunderbird 148.0 (or Thunderbird 140.8.0)
- Upgrade Firefox to version 148.0 or later, or Firefox ESR 140.8.0 or later
- Upgrade Thunderbird to version 148.0 or later, or Thunderbird 140.8.0 or later
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-2026-2790 — 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-2026-2790 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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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.
- 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