CVE-2026-100821: Mozilla Firefox · CWE-346
Affects Mozilla FirefoxSource CVE Database V5
Not known to be exploited · CVSS 4.7 Medium.
Data as of 12 Nov 2026, 23:24 UTC
- EPSS
- 0.1% chance of exploitation in 30 days – higher than 3% 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:C/C:L/I:N/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
Affected software
| Product | Affected | Fixed in |
|---|---|---|
| Mozilla Firefox | Affected
| Fixed in
|
AI-suggested
Official patches are available and should be applied by upgrading to Firefox ESR 153.4, Firefox 157, Firefox ESR 115.42, Firefox ESR 140.17, Thunderbird 157, Thunderbird 140.17, or Thunderbird 153.4. Users and administrators should update affected products to these fixed versions to mitigate the vulnerability. No additional mitigation steps are indicated by the vendor advisories.
Description
A site isolation vulnerability was identified in the Panning and Zooming component of Mozilla Firefox and Thunderbird. This issue could allow limited information disclosure due to improper site isolation. The vulnerability has a medium severity rating with a CVSS score of 4.7. It affects multiple versions of Firefox and Thunderbird prior to their respective fixed releases. Official fixes have been released in Firefox ESR 153.4, Firefox 157, Firefox ESR 115.42, Firefox ESR 140.17, Thunderbird 157, Thunderbird 140.17, and Thunderbird 153.4.
References
- bugzilla.mozilla.org/show_bug.cgi?id=2071784 (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)