source: http://www.securityfocus.com/bid/3435/info

PostNuke, successor to PHPNuke, is a content management system written in PHP. PostNuke versions 0.62 to 0.64 suffer from a vulnerability that allows a remote user to log-in as any user with known username and ID without authentication. The problem lies in a failure to filter inappropriate characters from variables that can be passed to the program's components by a remote attacker. This allows the attacker to alter a mysql query to the user database, bypassing password checking and assuming the identity of a specified user.

The component "article.php" calls a routine in "mainfile2.php" to update user information (i.e., log the user on) when the variable "save=1" (and the appropriate user ID and name) is specified in the URL. This routine, getusrinfo(), performs a mysql query to load user information from the database. Since part of this query is taken from insecure input that can be passed (in base64 encoded form) to "article.php" by a remote attacker, this query can be altered with the use of a properly placed single quote character followed by mysql statements.

This allows an attacker to bypass the condition "where user=$user3[1] and pass=$user3[2]" of the affected mysql query, for example by appending "or user=USERNAME" to it. 

The attacker must base64 encode the string containing the malformed
User ID, Username and Password combination. The unencoded string would be in the following format (with USERID and USERNAME appropriately replaced):

USERID:USERNAME:' or uname='USERNAME

This encoded string would then be passsed to the article.php script by requesting a URL of the following form (this could be trivially accomplished from a web browser):

http://targethost/article.php?save=1&sid=20&cookieusrtime=160000&user=USERID:encodedstring

Where encodedstring is the previously described base64 encoded string. Base64 encoding can be trivially accomplished with the use of any of a number of simple utilities. 