CVE-2026-100803: Mozilla Firefox · CWE-346
Affects Mozilla FirefoxSource CVE Database V5
Not known to be exploited · CVSS 8.1 High.
Data as of 12 Nov 2026, 23:24 UTC
- EPSS
- 0.1% chance of exploitation in 30 days – higher than 6% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:NExploitation evidence
Disclosed: evidence found.
CVE published 29 Sep 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors
A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 13 Nov 2026.
What to do
AI-suggested
Official fixes are available and have been released by Mozilla in Firefox ESR 153.4, Firefox 157, Thunderbird 140.17, 153.4, 157, and Firefox ESR 115.42 and 140.17. Users and administrators should update to these fixed versions or later to mitigate this vulnerability. Since this is not a cloud service, remediation depends on applying these patches.
Description
CVE-2026-100803 is a high-severity vulnerability in Mozilla Firefox involving a same-origin policy bypass in the WebExtensions component. This flaw allows an attacker to bypass security restrictions that normally prevent web content from accessing data from other origins. The issue has been fixed in multiple Mozilla products including Firefox ESR 153.4, Firefox 157, Thunderbird 140.17, 153.4, and 157, as well as Firefox ESR 115.42 and 140.17.
References
- bugzilla.mozilla.org/show_bug.cgi?id=2057988 (opens in a new tab)
- mozilla.org/security/advisories/mfsa2026-100/ (opens in a new tab)
- mozilla.org/security/advisories/mfsa2026-97/ (opens in a new tab)
- mozilla.org/security/advisories/mfsa2026-98/ (opens in a new tab)
- mozilla.org/security/advisories/mfsa2026-99/ (opens in a new tab)
- mozilla.org/security/advisories/mfsa2026-101/ (opens in a new tab)
- mozilla.org/security/advisories/mfsa2026-102/ (opens in a new tab)
- mozilla.org/security/advisories/mfsa2026-103/ (opens in a new tab)