Critical (9.1). EPSS 3.1% chance of exploitation in the next 30 days.
An out-of-bounds read occurs when applying style rules to pseudo-elements, such as ::first-line, using cached style data. This vulnerability affects Thunderbird < 52.3, Firefox ESR < 52.3, and Firefox < 55.
Severity
CVSS 3.0
9.1 critical
CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
EPSS
3.1% chance of exploitation in the next 30 days; higher than 87% of CVEs (scores of 2026-10-31)