FlatNuke 2.5.6 verifies authentication credentials based on an MD5 checksum of the admin name and the hashed password rather than the plaintext password, which allows attackers to gain privileges by obtaining the password hash (possibly via CVE-2005-2813), then calculating the credentials and including them in the secid cookie.
https://exchange.xforce.ibmcloud.com/vulnerabilities/22159
http://cvs.sourceforge.net/viewcvs.py/flatnuke/flatnuke/Changelog?rev=1.78&view=markup