CVE-2026-4700: Critical HTTP Mitigation Bypass in Firefox

CVE-2026-4700 is a mitigation bypass in the Networking: HTTP component in Firefox. The NVD record scores it CVSS 9.8 (critical), while Mozilla rates it Moderate. Mozilla fixed it in Firefox 149, Firefox ESR 140.9, Thunderbird 149 and Thunderbird 140.9. Update to those versions or later, and check bundled copies too.

What's affected

The NVD record lists Mozilla Firefox as affected, and the description says the vulnerability was fixed in the following releases:

• Firefox 149
• Firefox ESR 140.9
• Thunderbird 149
• Thunderbird 140.9

Firefox ESR 115 is not listed for this CVE, so it was not part of the fix set for this entry. Check the advisory for your branch before drawing conclusions.

How to check if you're exposed

1. Find your version. In Firefox open the menu, choose Help, then About Firefox. For Thunderbird use Help, then About Thunderbird.
2. From a terminal, run firefox --version or thunderbird --version. On macOS the binary sits inside the app bundle, for example /Applications/Firefox.app/Contents/MacOS/firefox --version.
3. On Debian or Ubuntu, apt list --installed 2>/dev/null | grep -i -E 'firefox|thunderbird' shows what the package manager installed. On Fedora or RHEL, use rpm -q firefox thunderbird.
4. Compare the result with the fixed versions above. Anything older on the same branch is exposed to CVE-2026-4700.

Bundled copies are easy to miss. Kiosk images, test runners with a pinned Firefox, and container images can carry old builds, and a scan of the image catches those.

What to do now

1. Update Firefox and Thunderbird on every managed device to the fixed version for the branch you run.
2. Restart the browser after the update. Mozilla's updater only protects you once the new build is running.
3. Update images and CI runners that pin a Firefox version, such as browser test containers.
4. Where you cannot update today, limit who can browse untrusted sites from the affected machine and plan the update for the next change window.
5. Record the new version in your inventory so the next review takes minutes.

Why it matters

The record maps this to CWE-288 and CWE-444, which covers authentication bypass using an alternate path or channel, here a mitigation that could be sidestepped, and inconsistent interpretation of HTTP requests, the class behind request smuggling. The affected component is Networking: HTTP. Mozilla's advisory lists the entry as moderate impact and gives no further technical detail, which is normal while most users are still updating.

Mozilla describes this as a bypass of a mitigation in HTTP networking. The CWE list pairs an authentication bypass class with an HTTP request interpretation class. The advisory text does not say more than that, so avoid reading extra detail into it. The record's KEV flag is false and the advisory does not report exploitation.

Quick check

Sources

• Mozilla Foundation Security Advisory 2026-20 (Firefox 149)
• Mozilla Foundation Security Advisory 2026-22 (Firefox ESR 140.9)
• Gold page for this CVE

Last reviewed: 19 October 2026.