CVE-2026-4716: Critical Memory Flaw in Firefox JS Engine

CVE-2026-4716 is incorrect boundary conditions and uninitialized memory in the JavaScript engine in Firefox. The NVD record scores it CVSS 9.1 (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-2026-4716.

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-908, which covers use of an uninitialized resource, where memory is read before it has been set. The affected component is JavaScript Engine. Mozilla's advisory lists the entry as moderate impact and gives no further technical detail, which is normal while most users are still updating.

Browser engine bugs of this kind are generally reached through content a page supplies, which is why the NVD vector assumes network delivery and no privileges. The record does not say whether the bug is exploited, and its KEV flag is false.

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.