CVE-2026-84137: Spoofing issue in the DOM: Core & HTML component

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-01 · updated 2026-09-03
Weakness: CWE-346

Description

Spoofing issue in the DOM: Core & HTML component. This vulnerability was fixed in Firefox 155, Firefox ESR 153.2, Thunderbird 155, and Thunderbird 153.2.

Affected products

Mozilla Firefox: before 153.2.0; from 154.0.0 before 155.0.0 — fixed in 153.2.0, 155.0.0
Mozilla Thunderbird: before 153.2.0; from 154.0 before 155.0 — fixed in 153.2.0, 155.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 153.2.0, 155.0.0; Mozilla Thunderbird 153.2.0, 155.0. Check the vendor advisory for mitigations.

References

https://bugzilla.mozilla.org/show_bug.cgi?id=2051146
https://www.mozilla.org/security/advisories/mfsa2026-82/
https://www.mozilla.org/security/advisories/mfsa2026-85/
https://www.mozilla.org/security/advisories/mfsa2026-86/
https://www.mozilla.org/security/advisories/mfsa2026-88/

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