CVE-2026-4711 is a use after free in the Widget: Cocoa 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
- Find your version. In Firefox open the menu, choose Help, then About Firefox. For Thunderbird use Help, then About Thunderbird.
- From a terminal, run
firefox --versionorthunderbird --version. On macOS the binary sits inside the app bundle, for example/Applications/Firefox.app/Contents/MacOS/firefox --version. - 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, userpm -q firefox thunderbird. - Compare the result with the fixed versions above. Anything older on the same branch is exposed to CVE-2026-4711.
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
- Update Firefox and Thunderbird on every managed device to the fixed version for the branch you run.
- Restart the browser after the update. Mozilla's updater only protects you once the new build is running.
- Update images and CI runners that pin a Firefox version, such as browser test containers.
- Where you cannot update today, limit who can browse untrusted sites from the affected machine and plan the update for the next change window.
- Record the new version in your inventory so the next review takes minutes.
Why it matters
The record maps this to CWE-416, which covers use after free, where code keeps using memory after it has been released. The affected component is Widget: Cocoa. Mozilla's advisory lists the entry as moderate impact and gives no further technical detail, which is normal while most users are still updating.
This one sits in the Cocoa widget code, which is the native macOS layer of Firefox. Check Mac fleets in particular, since they often update on a slower cycle than Windows desktops. The record's KEV flag is false and the advisory does not report exploitation.
Quick check
Is CVE-2026-4711 exploited in the wild?
The record's KEV flag is false, and the Mozilla advisory I read does not say it is exploited. That can change, so keep watching the advisory and update promptly.
Which version fixes CVE-2026-4711 on ESR?
The record lists Firefox ESR 140.9. If you run a different ESR line, read the Mozilla advisory for it.
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.
