Critical severity · published Sep 1, 2026
CVE-2026-84140
Site isolation issue in the DOM: Navigation component
9.8
CVSS
0.2%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
High
Integrity
High
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.2% · higher than 8% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
FI
Firefox · Mozilla
before 153.2.0; 154.0.0 before 155.0.0 · source: NVD
TH
Thunderbird · Mozilla
before 153.2.0; 154.0 before 155.0 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-84140
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-84140
Issue Tracking
https://bugzilla.mozilla.org/show_bug.cgi?id=2063780
Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-82/
Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-85/
Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-86/
Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-88/
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