CVE-2020-26974: Mozilla Firefox

High severity, CVSS 8.8. EPSS: 1.5% chance of exploitation in the next 30 days.

When flex-basis was used on a table wrapper, a StyleGenericFlexBasis object could have been incorrectly cast to the wrong type. This resulted in a heap user-after-free, memory corruption, and a potentially exploitable crash. 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.