CVE-2026-100809 Same-origin policy bypass in the DevTools component

What is CVE-2026-100809?

CVE-2026-100809 is a high-severity vulnerability in Mozilla Firefox, published on 29 Sep 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

Official 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.

Weakness: CWE-346

Which versions are affected by CVE-2026-100809?

It affects 2 products.

Products include Mozilla Firefox, Mozilla Thunderbird.

How do I fix CVE-2026-100809?

Upgrade to a fixed version.

Is CVE-2026-100809 being exploited?

There is no known exploitation of CVE-2026-100809 in the wild and no public exploit code.

Other vulnerabilities in Mozilla Firefox

• CVE-2019-11708 Mozilla Firefox and Thunderbird Sandbox Escape Exploited Critical 10.0
• CVE-2010-3765 Mozilla Multiple Products Remote Code Execution Exploited Critical 9.8
• CVE-2022-26486 Mozilla Firefox Use-After-Free Exploited Critical 9.6
• CVE-2022-26485 Mozilla Firefox Use-After-Free Exploited High 8.8
• CVE-2019-11707 Mozilla Firefox and Thunderbird Type Confusion Exploited High 8.8

All Mozilla Firefox vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as DEBIAN-CVE-2026-100809, GHSA-gggx-8wj9-87fx, UBUNTU-CVE-2026-100809.