CVE-2026-2777: Privilege Escalation in Firefox Messaging System

CVE-2026-2777 is a critical flaw (CVSS 9.8) in the Messaging System component of Firefox and Thunderbird. Mozilla announced the fix on 24 February 2026 in Firefox 148, Firefox ESR 115.33, Firefox ESR 140.8, Thunderbird 148 and Thunderbird 140.8. Update to those releases or later. The record does not mark it as a CISA Known Exploited Vulnerability.

What's affected

Firefox and Thunderbird builds older than the fixed releases. Mozilla fixed this issue in Firefox 148, Firefox ESR 115.33, Firefox ESR 140.8, Thunderbird 148 and Thunderbird 140.8.

NVD scores it 9.8 (Critical). Mozilla's own advisory rates the issue high. Both numbers are worth knowing: the NVD vector assumes a network attack with no privileges, while the vendor rating reflects how Mozilla weighs it for a browser.

How to check if you're exposed

Check the version you run against the fixed releases.

• Firefox: open the menu, choose Help, then About Firefox. The dialog shows the version and checks for an update.
• Thunderbird: open Help, then About Thunderbird.

From a terminal, on systems where the binaries are on your path:

firefox --version
thunderbird --version

Linux distributions often ship fixes under their own package versions. If you install from a distribution repository, check your distributor's security notice for this CVE instead of comparing version numbers by eye. In a managed fleet, look at your software inventory for Firefox and Thunderbird builds below the fixed releases.

What to do now

1. Update Firefox and Thunderbird to Firefox 148 or later on your release channel, or to the matching ESR release.
2. On managed desktops, push the update through your normal software deployment tool and confirm the installed versions afterwards.
3. Restart both applications. A browser that has downloaded an update keeps running the old code until it restarts.
4. Check the advisory for your exact branch if you run an ESR or a distribution package.

Why it matters

Mozilla describes this only as privilege escalation in a browser component. NVD files it under CWE-269 (Improper Privilege Management), the class where code or content gains rights it was never meant to have. The advisory gives no mechanism, so we do not speculate about one.

Browsers and mail clients process content from outside your organisation all day, so a fix in either is worth rolling out quickly even when the vendor's rating is lower than the NVD score.

Quick check

Sources

• Mozilla Foundation Security Advisory 2026-13 (Firefox 148)
• Mozilla Foundation Security Advisory 2026-14 (Firefox ESR 115.33)
• Mozilla Foundation Security Advisory 2026-15 (Firefox ESR 140.8)
• Mozilla Foundation Security Advisory 2026-16 (Thunderbird 148)

Last reviewed: 19 October 2026.