CVE-2026-2771: Undefined behavior in the DOM: Core & HTML component

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.8% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-02-24 · updated 2026-09-21
Weakness: CWE-125

Description

Undefined behavior in the DOM: Core & HTML 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=2014593
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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