CVE-2026-100803
Received Received - Intake

Same-origin policy bypass in Mozilla Firefox

Publication date: 2026-09-29

Last updated on: 2026-09-29

Assigner: Mozilla Corporation

Description
Same-origin policy bypass in the WebExtensions component. This vulnerability was fixed in Firefox ESR 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17.

CVSS Scores

EPSS Scores

Probability:
Percentile:

Meta Information

Published
2026-09-29
Last Modified
2026-09-29
Generated
2026-09-29
AI Q&A
2026-09-29
EPSS Evaluated
N/A
NVD
CVE-2026-100803
EUVD
EUVD-2026-88956

Affected Vendors & Products

Showing 7 associated CPEs
Vendor Product Version / Range
mozilla firefox 153.4
mozilla firefox 157
mozilla firefox 115.42
mozilla firefox 140.17
mozilla firefox to 153.4 (inc)
mozilla firefox to 115.42 (inc)
mozilla firefox to 140.17 (inc)

Helpful Resources

Exploitability

CWE
CWE Icon
KEV
KEV Icon
CWE ID Description
CWE-UNKNOWN

Attack-Flow Graph

AI Quick Actions

Instant insights powered by AI
Executive Summary

This vulnerability is a same-origin policy bypass in the WebExtensions component of Firefox. The same-origin policy is a security feature that restricts how documents or scripts from one origin interact with resources from another origin. A bypass allows unauthorized access between different origins, potentially exposing sensitive data or enabling malicious actions.

Detection Guidance

Detection requires checking Firefox version against patched releases. Compare installed versions with Firefox ESR 153.4, Firefox 157, Firefox ESR 115.42, or Firefox ESR 140.17. Use commands like 'firefox --version' or check update logs for version numbers.

Impact Analysis

If exploited, this vulnerability could allow attackers to access data or perform actions on behalf of a user without proper authorization. This might lead to theft of sensitive information, unauthorized modifications, or installation of malicious extensions. Users could experience privacy breaches or compromised accounts.

Compliance Impact

This vulnerability could lead to unauthorized data access, violating GDPR's data protection principles or HIPAA's requirements for safeguarding protected health information. Organizations may face compliance violations, legal penalties, or reputational damage if exploited.

Mitigation Strategies

Update Firefox to the latest patched version immediately. For ESR users, upgrade to Firefox ESR 153.4, 115.42, or 140.17. For standard releases, upgrade to Firefox 157. Disable WebExtensions if not required until updates are applied.

Chat Assistant

Ask questions about this CVE
Hi! I’m here to help you understand CVE-2026-100803. Ask me anything about the vulnerability, its impact, or mitigation strategies.
0/70

EPSS Chart