CVE-2026-100804 Sandbox escape due to use-after-free in the Preferences: Backend component

What is CVE-2026-100804?

CVE-2026-100804 is a critical use after free 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

Sandbox escape due to use-after-free in the Preferences: Backend component. This vulnerability was fixed in Thunderbird 157 and Firefox 157.

Weakness: CWE-416

Which versions are affected by CVE-2026-100804?

It affects 2 products.

Products include Mozilla Firefox, Mozilla Thunderbird.

How do I fix CVE-2026-100804?

Upgrade to a fixed version.

Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.

Is CVE-2026-100804 being exploited?

There is no known exploitation of CVE-2026-100804 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 GHSA-29g5-92cf-qhwc, UBUNTU-CVE-2026-100804.