---
title: "CVE-2026-100803: Mozilla Firefox - Same-Origin Policy Bypass"
description: "Mozilla Firefox WebExtensions component contains a same-origin policy bypass vulnerability, letting attackers bypass same-origin restrictions, exploit requires crafted web content."
url: https://panop.io/threat-heartbeat/CVE-2026-100803/
---

# CVE-2026-100803 Mozilla Firefox - Same-Origin Policy Bypass

Mozilla Firefox WebExtensions component contains a same-origin policy bypass vulnerability, letting attackers bypass same-origin restrictions, exploit requires crafted web content.

CVSS base score 8.1 /10

High

CISA KEV

Not listed

Public exploit

None known

Authentication

Not required

EPSS

0.19% chance of exploitation in 30 days

[View the NVD record ↗](https://nvd.nist.gov/vuln/detail/CVE-2026-100803)

## Details

Type

Same-Origin Policy Bypass

Impact

Information Disclosure

CVSS vector

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

Published

29 September 2026

Updated

15 October 2026

Weakness

[CWE-346](https://cwe.mitre.org/data/definitions/346.html)

## Vendor fix

Update to Firefox ESR 153.4, Firefox 157, Firefox ESR 115.42, Firefox ESR 140.17 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-2026-100803](https://nvd.nist.gov/vuln/detail/CVE-2026-100803)
- [https://bugzilla.mozilla.org/show\_bug.cgi?id=2057988](https://bugzilla.mozilla.org/show_bug.cgi?id=2057988)
- [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/mfsa2026-98/](https://www.mozilla.org/security/advisories/mfsa2026-98/)
- [https://www.mozilla.org/security/advisories/mfsa2026-99/](https://www.mozilla.org/security/advisories/mfsa2026-99/)
- [https://www.mozilla.org/security/advisories/mfsa2026-101/](https://www.mozilla.org/security/advisories/mfsa2026-101/)
- [https://www.mozilla.org/security/advisories/mfsa2026-102/](https://www.mozilla.org/security/advisories/mfsa2026-102/)
- [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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