CVE-2020-26961

Name: CVE-2020-26961
Creator: NVD / NIST
Published: 2020-12-09T01:15:13.283+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.5/10EPSS 1.16%

Last modified Jun 17, 2026

CVE-2020-26961 is a medium-severity vulnerability rated 6.5/10 on the CVSS scale. When DNS over HTTPS is in use, it intentionally filters RFC1918 and related IP ranges from the responses as these do not make sense coming from a DoH resolver. However when an IPv4 address was mapped through IPv6, these addresses were erroneously let through, leading to a potential DNS Rebinding attack. EPSS estimates a 1.16% chance of exploitation in the next 30 days.

Description

When DNS over HTTPS is in use, it intentionally filters RFC1918 and related IP ranges from the responses as these do not make sense coming from a DoH resolver. However when an IPv4 address was mapped through IPv6, these addresses were erroneously let through, leading to a potential DNS Rebinding attack. This vulnerability affects Firefox < 83, Firefox ESR < 78.5, and Thunderbird < 78.5.

Metrics

CVSS 3.1

6.5/10

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

EPSS Probability

1.16%

63.1th percentile

Probability of exploitation in the next 30 days. Learn more

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=1672528Issue 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=1672528Issue 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-2020-26961?

How severe is CVE-2020-26961?

CVE-2020-26961 has a CVSS score of 6.5/10 (MEDIUM severity). The EPSS model estimates a 1.16% probability of exploitation in the next 30 days.

How do I fix CVE-2020-26961?

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-2020-26961?

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

Scan your code now

Source: NVD / NIST