In this version of MyBB is present a problem with HTML on User Title, ok, enjoy this mini tutorial for easy fix that.
Go to your FTP and go to "inc/functions_post.php" edit that and go to line 269
The line 269 present that:
$post['usertitle'] = htmlspecialchars_uni($post['usertitle']);
Replace that with:
// $post['usertitle'] = htmlspecialchars_uni($post['usertitle']);
Now go to ACP and on Title of User make an HTML and in postbit is present an image and star!
PS: Thanks to Dvdxseo for help me on italian forum!
PS2: Don't allow the user to change a User Title is a vulnerable for XSS