An issue was discovered in inc/class_feedgeneration.php in MyBB 1.8.17. On the forum RSS Syndication page, one can generate a URL such as http://localhost/syndication.php?fid=&type=atom1.0&limit=15. The thread titles (within title elements of the generated XML documents) aren't sanitized, leading to XSS.
https://www.exploit-db.com/exploits/45393/
https://blog.mybb.com/2018/08/22/mybb-1-8-18-released-security-maintenance-release/