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CVE-2026-84140: Mozilla Firefox: Origin Validation Error

CVE-2026-84140 is rated critical with a CVSS v3.1 base score of 9.8 and affects Mozilla Firefox. Per its scoring, it can be exploited over the network, without authentication, without any user interaction. The scored impact on confidentiality, integrity and availability is the highest available.

Vulnerability description

Site isolation issue in the DOM: Navigation component. This vulnerability was fixed in Firefox 155, Firefox ESR 153.2, Thunderbird 155, and Thunderbird 153.2.

Severity breakdown

CVSS v3.1 9.8: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

• Attack vector: Network. Exploitable remotely over the network, with no need for local or physical access.
• Attack complexity: Low. No special conditions; the attack can be repeated reliably.
• Privileges required: None. No account or prior access is needed.
• User interaction: None. No action from any user is needed.
• Scope: Unchanged. Impact stays within the vulnerable component.
• Confidentiality impact: High. Total loss of confidentiality; sensitive data can be fully disclosed.
• Integrity impact: High. Total loss of integrity; data or system state can be fully modified.
• Availability impact: High. Total loss of availability; the component can be shut down.

Underlying weakness

CWE-346: Origin Validation Error

The product does not properly verify that the source of data or communication is valid.

Affected products

• Mozilla Firefox: before 153.2.0; from 154.0.0 before 155.0.0
• Mozilla Thunderbird: before 153.2.0; from 154.0 before 155.0

References

• 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/
• https://bugzilla.mozilla.org/show_bug.cgi?id=2063780