MyBB (aka MyBulletinBoard) before 1.4.12 does not properly restrict uid values for group join requests, which allows remote attackers to cause a denial of service (resource consumption) by using guest access to submit join request forms for moderated groups, related to usercp.php and managegroup.php.
Cross-site scripting (XSS) vulnerability in the redirect function in functions.php in MyBB (aka MyBulletinBoard) 1.4.2 allows remote attackers to inject arbitrary web script or HTML via the url parameter in a removesubscriptions action to moderation.php, related to use of the ajax option to request a JavaScript redirect. NOTE: this can be leveraged to execute PHP code and bypass cross-site request forgery (CSRF) protection.
MyBB (aka MyBulletinBoard) 1.4.2 uses insufficient randomness to compose filenames of uploaded files used as attachments, which makes it easier for remote attackers to read these files by guessing filenames.
MyBB (aka MyBulletinBoard) 1.4.2 does not properly handle an uploaded file with a nonstandard file type that contains HTML sequences, which allows remote attackers to cause that file to be processed as HTML by Internet Explorer's content inspection, aka "Incomplete protection against MIME-sniffing." NOTE: this could be leveraged for XSS and other attacks.