Quick answers
- Is CVE-2026-109721 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-109721 being exploited in the wild.
- How severe is CVE-2026-109721?
- 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-2026-109721?
- Yes: fixed in Firefox 158, Firefox ESR 153.5, Thunderbird 158, Thunderbird 153.5. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
- How is CVE-2026-109721 attacked?
- It is attacked over the network, in a web application.
- Am I affected by CVE-2026-109721?
- 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 153.5
Technical details
Incorrect boundary conditions in the JavaScript Engine's JIT (just-in-time) compilation component allow out-of-bounds memory access. The vulnerability is triggered through JavaScript execution and requires no special user interaction beyond visiting a malicious webpage. Exploitation could result in memory corruption, information disclosure, or code execution within the browser sandbox.
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: advisory