CVE-2026-74960

Analyzed Analyzed - Analysis Complete

Site Isolation Bypass in Firefox WebExtensions

Publication date: 2026-08-18

Last updated on: 2026-08-24

Assigner: Mozilla Corporation

Description

Site isolation issue in the WebExtensions component. This vulnerability was fixed in Firefox 154, Firefox ESR 140.14, Firefox ESR 153.1, Thunderbird 154, Thunderbird 140.14, and Thunderbird 153.1.

CVSS Scores

EPSS Scores

Probability:
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Meta Information

Published

2026-08-18

Last Modified

2026-08-24

Generated

2026-09-07

AI Q&A

2026-08-18

EPSS Evaluated

2026-09-06

NVD

CVE-2026-74960

EUVD

EUVD-2026-60659

Affected Vendors & Products

Vendor	Product	Version / Range
mozilla	firefox	From 141.0 (inc) to 153.1.0 (exc)
mozilla	thunderbird	From 141.0 (inc) to 153.1.0 (exc)
mozilla	thunderbird	to 140.14.0 (exc)
mozilla	firefox	to 140.14.0 (exc)

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.
CWE-284	The product does not restrict or incorrectly restricts access to a resource from an unauthorized actor.

Attack-Flow Graph

Executive Summary

This is a site isolation issue in Firefox's WebExtensions component. Site isolation is a security feature that separates content from different sites to prevent malicious websites from accessing data from other sites. A flaw in this feature could allow an attacker to bypass these protections and access sensitive information across different sites.

Detection Guidance

Detection requires checking Firefox version against patched releases. Compare installed version with Firefox 154, Firefox ESR 140.14, or Firefox ESR 153.1. Use commands like 'firefox --version' or check 'about:support' in Firefox.

Impact Analysis

If exploited, this vulnerability could allow an attacker to access data from other websites you are logged into, such as cookies, session tokens, or sensitive information. This could lead to account takeovers, data theft, or unauthorized actions on your behalf.

Compliance Impact

This vulnerability could lead to unauthorized access to personal data, which may violate GDPR's data protection requirements or HIPAA's safeguards for protected health information. Organizations using affected Firefox versions may face compliance risks if exploited.

Mitigation Strategies

Update Firefox to version 154 or later, Firefox ESR 140.14 or later, or Firefox ESR 153.1 or later immediately. Disable WebExtensions if updates are not possible.

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