---
title: "CVE-2026-100787: Mozilla Firefox - Sandbox Escape"
description: "Mozilla Firefox ESR < 153.4 and Firefox < 157 contain a sandbox escape vulnerability caused by a flaw in the XUL component, letting attackers escape sandbox restrictions, exploit requires no special conditions."
url: https://panop.io/threat-heartbeat/CVE-2026-100787/
---

# CVE-2026-100787 Mozilla Firefox - Sandbox Escape

Mozilla Firefox ESR < 153.4 and Firefox < 157 contain a sandbox escape vulnerability caused by a flaw in the XUL component, letting attackers escape sandbox restrictions, exploit requires no special conditions.

CVSS base score 9.6 /10

Critical

CISA KEV

Not listed

Public exploit

None known

Authentication

Not required

EPSS

0.30% chance of exploitation in 30 days

[View the NVD record ↗](https://nvd.nist.gov/vuln/detail/CVE-2026-100787)

## Details

Type

Sandbox Escape

Impact

Privilege Escalation

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H

Published

29 September 2026

Updated

15 October 2026

Weakness

[CWE-20](https://cwe.mitre.org/data/definitions/20.html)

## Vendor fix

Update to Firefox ESR 153.4, Firefox 157 or later.

Patching closes this CVE. It does not tell you which of your systems run the affected version, or what else is exposed alongside them.

## References

- [https://nvd.nist.gov/vuln/detail/CVE-2026-100787](https://nvd.nist.gov/vuln/detail/CVE-2026-100787)
- [https://bugzilla.mozilla.org/show\_bug.cgi?id=2071068](https://bugzilla.mozilla.org/show_bug.cgi?id=2071068)
- [https://www.mozilla.org/security/advisories/mfsa2026-100/](https://www.mozilla.org/security/advisories/mfsa2026-100/)
- [https://www.mozilla.org/security/advisories/mfsa2026-97/](https://www.mozilla.org/security/advisories/mfsa2026-97/)
- [https://www.mozilla.org/security/advisories/mfsa2026-101/](https://www.mozilla.org/security/advisories/mfsa2026-101/)
- [https://www.mozilla.org/security/advisories/mfsa2026-103/](https://www.mozilla.org/security/advisories/mfsa2026-103/)

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## What this record can't tell you

- Whether you run the affected software, and where
- Whether an attacker can actually reach and exploit it
- The misconfigurations that never get a CVE number
- What it would cost the business, ranked into what to fix first

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