XSS in MyBB

Vulnerability Description

In MyBB before version 1.8.24, the custom MyCode (BBCode) for the visual editor doesn't escape input properly when rendering HTML, resulting in a DOM-based XSS vulnerability. The 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. After upgrading MyBB to 1.8.24, make sure to update the version attribute in the `codebuttons` template for non-default themes to serve the latest version of the patched `jscripts/bbcodes_sceditor.js` file.

References

• https://github.com/mybb/mybb/security/advisories/GHSA-37h7-vfv6-f8rj
• https://github.com/mybb/mybb/commit/37ad29dcd25489a37bdd89ebac761f22492558b0
• https://mybb.com/versions/1.8.24/