CVE-2026-45121: Mybb

Medium severity, CVSS 4.3. EPSS: 0.3% chance of exploitation in the next 30 days.

MyBB is free and open source forum software. Prior to 1.8.40, the calendar module does not check permissions consistently when listing calendars, allowing authenticated users to access titles of calendars that are otherwise inaccessible. The affected calendar-selection paths in calendar.php perform permission checks against an invalid calendar context before returning calendar titles. The uniquely identifying implementation details include titles of inaccessible calendars, and invalid calendar permission context. This issue is fixed in version 1.8.40.

Affected products

• Mybb Mybb: before 1.8.40 (fixed in 1.8.40)

Published 2026-08-18. Last modified 2026-09-08.