# CVE-2026-100811: Firefox sandbox escape due to use-after-free in DOM

Source: https://junglewise.ai/threats/cve-2026-100811-sandbox-escape-due-to-use-after-free-in-the-dom-core-html  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-29. Updated: 2026-09-29  
License: CC BY 4.0 with a link back

CVE-2026-100811 · Severity: critical · CVSS 9.6 · Published 2026-09-29

Firefox contains a use-after-free vulnerability in its DOM handling that allows escaping the browser's security sandbox. An attacker could exploit this flaw through a malicious web page to break out of the browser's isolation protections and execute arbitrary code with the privileges of the user browsing the web. This has been patched in recent Firefox releases.

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

## Quick answers

**Is CVE-2026-100811 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-100811 being exploited in the wild.

**How severe is CVE-2026-100811?** Critical severity, with a CVSS score of 9.6, published 29 September 2026.

**Which products and versions are affected?** Mozilla Firefox before 157; Mozilla Firefox ESR before 153.4 and before 140.17.

**Is there a fix for CVE-2026-100811?** A patch was published on 2026-09-29: Firefox 157, Firefox ESR 153.4, and Firefox ESR 140.17 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-100811 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-100811?** 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 the DOM: Core & HTML component allows sandbox escape in Firefox. The flaw is triggered through a network-based attack vector when processing malicious DOM content. An attacker can craft a webpage that triggers the use-after-free condition, leading to arbitrary code execution with user-level privileges and breaking out of the browser sandbox.

## Affected products

- Mozilla Firefox before 157
- Mozilla Firefox ESR before 153.4 and before 140.17

## Timeline

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

## References

- [https://bugzilla.mozilla.org/show_bug.cgi?id=2067973](https://bugzilla.mozilla.org/show_bug.cgi?id=2067973)
- [https://www.mozilla.org/security/advisories/mfsa2026-100/](https://www.mozilla.org/security/advisories/mfsa2026-100/)

## Related threats

- [Firefox ESR use-after-free in Audio/Video component](https://junglewise.ai/threats/cve-2026-124952-use-after-free-in-the-audio-video-component-this-vulnerability)
- [Firefox privilege escalation in Nimbus Desktop Client](https://junglewise.ai/threats/cve-2026-124951-privilege-escalation-in-the-nimbus-desktop-client-component-this)
- [Firefox same-origin policy bypass in CSS Parsing and Computation](https://junglewise.ai/threats/cve-2026-124950-same-origin-policy-bypass-in-the-css-parsing-and-computation)
- [Firefox other issue in SVG component](https://junglewise.ai/threats/cve-2026-124949-other-issue-in-the-svg-component-this-vulnerability-was-fixed-in)
- [Mozilla Firefox use-after-free in Graphics component](https://junglewise.ai/threats/cve-2026-124948-incorrect-boundary-conditions-use-after-free-in-the-graphics)