CVE-2020-26961: Mozilla Firefox

Medium severity, CVSS 6.5. EPSS: 1.2% chance of exploitation in the next 30 days.

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.

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.