CVE-2020-26958: Mozilla Firefox

Medium severity, CVSS 6.1. EPSS: 1.4% chance of exploitation in the next 30 days.

Firefox did not block execution of scripts with incorrect MIME types when the response was intercepted and cached through a ServiceWorker. This could lead to a cross-site script inclusion vulnerability, or a Content Security Policy bypass. This vulnerability affects Firefox < 83, Firefox ESR < 78.5, and Thunderbird < 78.5.

Affected products

• Mozilla Firefox: before 83.0 (fixed in 83.0)
• Mozilla Firefox ESR: before 78.5 (fixed in 78.5)
• Mozilla Thunderbird: before 78.5 (fixed in 78.5)

Published 2020-12-09. Last modified 2026-06-17.