CVE-2026-2777: Privilege escalation in the Messaging System component

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-02-24 · updated 2026-07-15
Weakness: CWE-269

Description

Privilege escalation in the Messaging System component. This vulnerability was fixed in Firefox 148, Firefox ESR 115.33, Firefox ESR 140.8, Thunderbird 148, and Thunderbird 140.8.

Affected products

Mozilla Firefox: before 115.33.0; before 148.0; from 128.0 before 140.8.0 — fixed in 115.33.0, 140.8.0, 148.0
Mozilla Thunderbird: before 140.8.0; before 148.0 — fixed in 140.8.0, 148.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 115.33.0, 140.8.0, 148.0; Mozilla Thunderbird 140.8.0, 148.0. Check the vendor advisory for mitigations.

References

https://bugzilla.mozilla.org/show_bug.cgi?id=2015305
https://www.mozilla.org/security/advisories/mfsa2026-13/
https://www.mozilla.org/security/advisories/mfsa2026-14/
https://www.mozilla.org/security/advisories/mfsa2026-15/
https://www.mozilla.org/security/advisories/mfsa2026-16/
https://www.mozilla.org/security/advisories/mfsa2026-17/
https://access.redhat.com/errata/RHSA-2026:3338
https://access.redhat.com/errata/RHSA-2026:3339
https://access.redhat.com/errata/RHSA-2026:3361
https://access.redhat.com/errata/RHSA-2026:3491
https://access.redhat.com/errata/RHSA-2026:3492
https://access.redhat.com/errata/RHSA-2026:3493

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