CVE-2021-29991: Mozilla Firefox

High severity, CVSS 8.1. EPSS: 0.9% chance of exploitation in the next 30 days.

Firefox incorrectly accepted a newline in a HTTP/3 header, interpretting it as two separate headers. This allowed for a header splitting attack against servers using HTTP/3. This vulnerability affects Firefox < 91.0.1 and Thunderbird < 91.0.1.

Affected products

• Mozilla Firefox: before 91.0.1 (fixed in 91.0.1)
• Mozilla Thunderbird: before 91.0.1 (fixed in 91.0.1)

Published 2021-11-03. Last modified 2026-06-17.