CVE-2020-26978: Mozilla Firefox

Medium severity, CVSS 6.1. EPSS: 1.3% chance of exploitation in the next 30 days.

Using techniques that built on the slipstream research, a malicious webpage could have exposed both an internal network's hosts as well as services running on the user's local machine. This vulnerability affects Firefox < 84, Thunderbird < 78.6, and Firefox ESR < 78.6.

Affected products

• Mozilla Firefox: before 84.0 (fixed in 84.0)
• Mozilla Firefox ESR: before 78.6.0 (fixed in 78.6.0)
• Mozilla Thunderbird: before 78.6.0 (fixed in 78.6.0)

Published 2021-01-07. Last modified 2026-06-17.