Is CVE-2026-100818 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-100818 being exploited in the wild.

How severe is CVE-2026-100818?

Critical severity, with a CVSS score of 9.6, published 29 September 2026.

Which products and versions are affected?

Mozilla Firefox prior to 157; Mozilla Firefox ESR 140.x prior to 140.17, 153.x prior to 153.4.

Is there a fix for CVE-2026-100818?

A patch was published on 2026-09-29: Firefox ESR 153.4, Firefox 157, Firefox ESR 140.17 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-100818 attacked?

It is attacked over the network, in a web application.

Am I affected by CVE-2026-100818?

Check whether you run Mozilla Firefox or Mozilla Firefox ESR at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-100818: Firefox sandbox escape due to use-after-free in Widget: Gtk

CVE-2026-100818 · Severity: critical · CVSS 9.6 · Published 2026-09-29

Firefox's sandbox, which isolates untrusted web content from the rest of the system, can be escaped through a use-after-free memory vulnerability in the Gtk widget component. An attacker who exploits this flaw could break out of the browser's security isolation and potentially execute arbitrary code with the browser's full privileges. Mozilla fixed this issue in Firefox ESR 153.4, Firefox 157, and Firefox ESR 140.17.

Technologies: Mozilla Firefox, Mozilla Firefox ESR. Vendors: Mozilla.

Quick answers

Is CVE-2026-100818 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-100818 being exploited in the wild.
How severe is CVE-2026-100818?: Critical severity, with a CVSS score of 9.6, published 29 September 2026.
Which products and versions are affected?: Mozilla Firefox prior to 157; Mozilla Firefox ESR 140.x prior to 140.17, 153.x prior to 153.4.
Is there a fix for CVE-2026-100818?: A patch was published on 2026-09-29: Firefox ESR 153.4, Firefox 157, Firefox ESR 140.17 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-100818 attacked?: It is attacked over the network, in a web application.
Am I affected by CVE-2026-100818?: Check whether you run Mozilla Firefox or Mozilla Firefox ESR at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

A use-after-free vulnerability in Firefox's Widget: Gtk component allows sandbox escape. The vulnerability is triggered through normal web browsing, leading to memory safety corruption that permits privilege escalation from the sandboxed content process to the parent browser process. The fix addresses the memory safety issue in the affected component.

Affected products

Mozilla Firefox prior to 157
Mozilla Firefox ESR 140.x prior to 140.17, 153.x prior to 153.4

Timeline

2026-09-29: disclosed
2026-09-29: patched: Firefox ESR 153.4, Firefox 157, Firefox ESR 140.17

References

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