CVE-2026-100803

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Same-origin policy bypass in Mozilla Firefox

Publication date: 2026-09-29

Last updated on: 2026-10-01

Assigner: Mozilla Corporation

Description

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

CVSS Scores

EPSS Scores

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Meta Information

Published

2026-09-29

Last Modified

2026-10-01

Generated

2026-10-19

AI Q&A

2026-09-29

EPSS Evaluated

2026-10-18

NVD

CVE-2026-100803

EUVD

EUVD-2026-88956

Affected Vendors & Products

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

KEV

CWE ID	Description
CWE-346	The product does not properly verify that the source of data or communication is valid.

Attack-Flow Graph

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.

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