CVE-2026-100803
HIGHCVSS 8.1/10EPSS 0.19%
Last modified
CVE-2026-100803 is a high-severity vulnerability rated 8.1/10 on the CVSS scale. 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.. EPSS estimates a 0.19% chance of exploitation in the next 30 days.
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.
Metrics
Weakness Enumeration
Affected Software
| Vendor | Product | Versions |
|---|---|---|
| Mozilla | Firefox | < 115.42.0 |
| Mozilla | Firefox | >= 116.0, < 140.17.0 |
| Mozilla | Firefox | >= 141.0, < 153.4.0 |
| Mozilla | Firefox | >= 154.0.0, < 157.0.0 |
| Mozilla | Thunderbird | < 140.17.0 |
| Mozilla | Thunderbird | >= 141.0, < 153.4.0 |
| Mozilla | Thunderbird | >= 154.0, < 157.0 |
References
- https://bugzilla.mozilla.org/show_bug.cgi?id=2057988Permissions Required
- https://www.mozilla.org/security/advisories/mfsa2026-100/Vendor Advisory
- https://www.mozilla.org/security/advisories/mfsa2026-101/Vendor Advisory
- https://www.mozilla.org/security/advisories/mfsa2026-102/Vendor Advisory
- https://www.mozilla.org/security/advisories/mfsa2026-103/Vendor Advisory
- https://www.mozilla.org/security/advisories/mfsa2026-97/Vendor Advisory
- https://www.mozilla.org/security/advisories/mfsa2026-98/Vendor Advisory
- https://www.mozilla.org/security/advisories/mfsa2026-99/Vendor Advisory
Timeline
- Published
- Last Modified
- Status
- Analyzed
Frequently Asked Questions
What is CVE-2026-100803?
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.
How severe is CVE-2026-100803?
CVE-2026-100803 has a CVSS score of 8.1/10 (HIGH severity). The EPSS model estimates a 0.19% probability of exploitation in the next 30 days.
How do I fix CVE-2026-100803?
Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.
How Strix Helps
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Are you affected by CVE-2026-100803?
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Scan your code nowSource: NVD / NIST