CVE-2026-100818 Sandbox escape due to use-after-free in the Widget: Gtk component
A use-after-free flaw in the Gtk widget component allows attackers to escape the sandbox and execute arbitrary code.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-100818?
CVE-2026-100818 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 Widget: Gtk component. This vulnerability was fixed in Firefox ESR 153.4, Thunderbird 157, Thunderbird 140.17, Thunderbird 153.4, Firefox 157, and Firefox ESR 140.17.
Weakness: CWE-416
Which versions are affected by CVE-2026-100818?
It affects 2 products.
Products include Mozilla Firefox, Mozilla Thunderbird.
How do I fix CVE-2026-100818?
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-2026-100818 being exploited?
There is no known exploitation of CVE-2026-100818 in the wild and no public exploit code.
Other vulnerabilities in Mozilla Firefox
Sources
Also known as DEBIAN-CVE-2026-100818, GHSA-5pwm-rr4f-fh76, UBUNTU-CVE-2026-100818.