# CVE-2026-109708: Firefox sandbox escape due to incorrect boundary conditions in Networking

Source: https://junglewise.ai/threats/cve-2026-109708-sandbox-escape-due-to-incorrect-boundary-conditions-in-the  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-13. Updated: 2026-10-16  
License: CC BY 4.0 with a link back

CVE-2026-109708 · Severity: critical · CVSS 9.6 · Published 2026-10-13

Firefox and Thunderbird contain a flaw in their networking component that allows an attacker to break out of the browser sandbox—the security boundary that prevents malicious websites from accessing the operating system. An exploit of this vulnerability could allow attackers to take full control of a user's computer, steal sensitive data, or install malware.

Technologies: [Mozilla Firefox](https://junglewise.ai/threats/technologies/firefox), [Mozilla Firefox ESR](https://junglewise.ai/threats/technologies/firefox-esr), [Mozilla Thunderbird](https://junglewise.ai/threats/technologies/thunderbird), [Mozilla Thunderbird ESR](https://junglewise.ai/threats/technologies/thunderbird-esr). Vendors: [Mozilla](https://junglewise.ai/threats/vendors/mozilla).

## Quick answers

**Is CVE-2026-109708 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-109708 being exploited in the wild.

**How severe is CVE-2026-109708?** Critical severity, with a CVSS score of 9.6, 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-2026-109708?** 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-2026-109708 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-109708?** 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

Incorrect boundary condition validation in Firefox's Networking component permits sandbox escape. The vulnerability requires network-level attack capability and likely user interaction (visiting a malicious webpage), enabling an attacker to execute arbitrary code with the privileges of the browser process and circumvent the sandbox isolation mechanism.

## 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

- [https://bugzilla.mozilla.org/show_bug.cgi?id=2059151](https://bugzilla.mozilla.org/show_bug.cgi?id=2059151)
- [https://www.mozilla.org/security/advisories/mfsa2026-106/](https://www.mozilla.org/security/advisories/mfsa2026-106/)

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