High severity · published Sep 29, 2026
CVE-2026-100803
Same-origin policy bypass in the WebExtensions component
8.1
CVSS
0.2%
EPSS
No
On KEV
Description
Same-origin policy bypass in the WebExtensions component. This vulnerability was fixed in Firefox ESR 153.4, Thunderbird 157, Thunderbird 140.17, Thunderbird 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17.
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 6% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
FI
Firefox · Mozilla
before 115.42.0; 116.0 before 140.17.0; 141.0 before 153.4.0; 154.0.0 before 157.0.0 · source: NVD
TH
Thunderbird · Mozilla
before 140.17.0; 141.0 before 153.4.0; 154.0 before 157.0 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-100803
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-100803
bugzilla.mozilla.org
https://bugzilla.mozilla.org/show_bug.cgi?id=2057988
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/mfsa2026-102/
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