Check_MK before 1.2.8p26 mishandles certain errors within the failed-login save feature because of a race condition, which allows remote attackers to obtain sensitive user information by reading a GUI crash report.
https://www.exploit-db.com/exploits/43021/
https://mathias-kettner.de/check_mk_werks.php?werk_id=5208&HTML=yes
http://mathias-kettner.com/check_mk_werks.php?edition_id=raw&branch=1.2.8