CVE-2026-45118: Mybb

Critical severity, CVSS 9.3. EPSS: 0.5% chance of exploitation in the next 30 days.

MyBB is free and open source forum software. Prior to 1.8.40, the Contact module does not validate a redirect URL or protocol correctly, resulting in an open redirect and reflected JavaScript code injection. contact.php accepts the redirect target from the from HTTP parameter in $mybb->input['from'] or the Referer HTTP header in $_SERVER['HTTP_REFERER'] and passes it to redirect() without sufficient verification. A javascript: URI becomes the target of the `Click here if you don't want to wait any longer` link because $force_redirect is true, allowing script execution when a victim selects the link. 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.