A persistent cross‑site scripting flaw exists in MyBB Last User's Threads in Profile Plugin 1.2, allowing an attacker to embed script tags into the subject field of a thread. When other users view the attacker's profile page, the malicious script executes in their browsers, potentially enabling the attacker to steal session tokens, deface content, or redirect users to malicious sites. The weakness is a classic input validation failure that results in unsanitized user‑supplied content being rendered in a browser context.

The vulnerability affects the MyBB Last User's Threads in Profile Plugin version 1.2 from the MyBB community. No other variants or versions are listed as affected.

The CVSS base score is 5.1, indicating a moderate severity. EPSS data is not available, and the flaw is not listed in the CISA KEV catalog, implying no known widespread exploitation at this time. The likely attack vector requires the adversary to create a thread with a malicious subject, which typically needs at least member‑level access or the ability to submit threads. Once a thread is created, attackers can target any visitor to the profile page, making the impact potentially wide across users on the affected forum.