Is CVE-2026-109740 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-109740 being exploited in the wild.

How severe is CVE-2026-109740?

Informational severity, published 13 October 2026.

Which products and versions are affected?

Mozilla Firefox before 158; Mozilla Firefox ESR before 153.5; Mozilla Thunderbird before 158; Mozilla Thunderbird ESR before 153.5.

Is there a fix for CVE-2026-109740?

Yes: fixed in Firefox 158, Firefox ESR 153.5, Thunderbird 158, Thunderbird ESR 153.5. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-109740 attacked?

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

Am I affected by CVE-2026-109740?

Check whether you run Mozilla Firefox, Mozilla Firefox ESR, Mozilla Thunderbird or one of the others listed 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-2026-109740: Firefox sandbox escape in Widget Win32 due to incorrect boundary conditions

CVE-2026-109740 · Severity: info · Published 2026-10-13

Firefox's security sandbox, which isolates web content from the operating system, contains a flaw in its Windows-specific code that allows incorrect memory boundary handling. An attacker could exploit this to break out of the sandbox and gain direct access to the operating system, potentially leading to system compromise.

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

Quick answers

Is CVE-2026-109740 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-109740 being exploited in the wild.
How severe is CVE-2026-109740?: Informational severity, published 13 October 2026.
Which products and versions are affected?: Mozilla Firefox before 158; Mozilla Firefox ESR before 153.5; Mozilla Thunderbird before 158; Mozilla Thunderbird ESR before 153.5.
Is there a fix for CVE-2026-109740?: Yes: fixed in Firefox 158, Firefox ESR 153.5, Thunderbird 158, Thunderbird ESR 153.5. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-109740 attacked?: It is attacked over the network, in a web application.
Am I affected by CVE-2026-109740?: Check whether you run Mozilla Firefox, Mozilla Firefox ESR, Mozilla Thunderbird or one of the others listed 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.

How to fix

Fixed in: Firefox 158, Firefox ESR 153.5, Thunderbird 158, Thunderbird ESR 153.5

Technical details

This is a sandbox escape vulnerability in Firefox's Widget: Win32 component caused by incorrect boundary conditions. The flaw allows an attacker to bypass the sandbox isolation mechanism, gaining the ability to execute arbitrary code with elevated privileges outside the sandboxed environment. A fix was released in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5.

Affected products

Mozilla Firefox before 158
Mozilla Firefox ESR before 153.5
Mozilla Thunderbird before 158
Mozilla Thunderbird ESR before 153.5

Timeline

2026-10-13: disclosed

References

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