CVE-2020-26958

Name: CVE-2020-26958
Creator: NVD / NIST
Published: 2020-12-09T01:15:13.08+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.1/10EPSS 1.31%

Last modified Jun 17, 2026

CVE-2020-26958 is a medium-severity vulnerability rated 6.1/10 on the CVSS scale. 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. EPSS estimates a 1.31% chance of exploitation in the next 30 days.

Description

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.

Metrics

CVSS 3.1

6.1/10

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

EPSS Probability

1.31%

67.0th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-79

Affected Software

Vendor	Product	Versions
Mozilla	Firefox	< 83.0
Mozilla	Firefox Esr	< 78.5
Mozilla	Thunderbird	< 78.5

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1669355Issue Tracking, Permissions Required, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2020-50/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2020-51/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2020-52/Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1669355Issue Tracking, Permissions Required, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2020-50/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2020-51/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2020-52/Vendor Advisory

Timeline

Published: Dec 9, 2020
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2020-26958?

How severe is CVE-2020-26958?

CVE-2020-26958 has a CVSS score of 6.1/10 (MEDIUM severity). The EPSS model estimates a 1.31% probability of exploitation in the next 30 days.

How do I fix CVE-2020-26958?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2020-26958?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST