MyBB is a free and open source forum software. Custom MyCode (BBCode) for the visual editor (_SCEditor_) doesn't escape input properly when rendering HTML, resulting in a DOM-based XSS vulnerability. This weakness can be exploited by pointing a victim to a page where the visual editor is active (e.g. as a post or Private Message) and operates on a maliciously crafted MyCode message.

This may occur on pages where message content is pre-filled using a GET/POST parameter, or on reply pages where a previously saved malicious message is quoted. The impact is be mitigated when: 1. the visual editor is disabled globally (_Admin CP - Configuration - Settings - Clickable Smilies and BB Code: Clickable MyCode Editor_ is set to _Off_), or 2. the visual editor is disabled for individual user accounts (_User CP - Your Profile - Edit Options_: _Show the MyCode formatting options on the posting pages_ checkbox is not checked). MyBB 1.8.37 resolves this issue with the commit 6dcaf0b4d.

Users are advised to upgrade. Users unable to upgrade may mitigate the impact without upgrading MyBB by changing the following setting (_Admin CP - Configuration - Settings_): - _Clickable Smilies and BB Code - Clickable MyCode Editor_: _Off_. Similarly, individual MyBB forum users are able to disable the visual editor by diabling the account option (_User CP - Your Profile - Edit Options_) _Show the MyCode formatting options on the posting pages_.