High severity · published Sep 29, 2026
CVE-2026-100809
Same-origin policy bypass in the DevTools component
8.1
CVSS
0.2%
EPSS
No
On KEV
Description
Same-origin policy bypass in the DevTools component. This vulnerability was fixed in Firefox ESR 153.4, Thunderbird 157, Thunderbird 153.4, and Firefox 157.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
Required
Confidentiality
High
Integrity
High
Availability
None
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.2% · higher than 4% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
FI
Firefox · Mozilla
before 153.4.0; 154.0.0 before 157.0.0 · source: NVD
TH
Thunderbird · Mozilla
before 153.4.0; 154.0 before 157.0 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-100809
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-100809
bugzilla.mozilla.org
https://bugzilla.mozilla.org/show_bug.cgi?id=2063658
www.mozilla.org
https://www.mozilla.org/security/advisories/mfsa2026-100/
www.mozilla.org
https://www.mozilla.org/security/advisories/mfsa2026-101/
www.mozilla.org
https://www.mozilla.org/security/advisories/mfsa2026-103/
www.mozilla.org
https://www.mozilla.org/security/advisories/mfsa2026-97/
More from Firefox
10.0
CVE-2019-11708
Mozilla Firefox and Thunderbird Sandbox Escape Vulnerability
9.8
CVE-2024-9680
Mozilla Firefox Use-After-Free Vulnerability
9.8
CVE-2010-3765
Mozilla Multiple Products Remote Code Execution Vulnerability
9.6
CVE-2022-26486
Mozilla Firefox Use-After-Free Vulnerability
8.8
CVE-2023-5217
Google Chromium libvpx Heap Buffer Overflow Vulnerability