CVE-2021-29946: Mozilla Firefox

High severity, CVSS 8.8. EPSS: 1.3% chance of exploitation in the next 30 days.

Ports that were written as an integer overflow above the bounds of a 16-bit integer could have bypassed port blocking restrictions when used in the Alt-Svc header. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.

Affected products

• Mozilla Firefox: before 88.0 (fixed in 88.0)
• Mozilla Firefox ESR: before 78.10 (fixed in 78.10)
• Mozilla Thunderbird: before 78.10 (fixed in 78.10)

Published 2021-06-24. Last modified 2026-06-17.