Adobe Document Server for Reader Extensions 6.0, during log on, provides different error messages depending on whether the user ID is valid or invalid, which allows remote attackers to more easily identify valid user IDs via brute force attacks.
https://exchange.xforce.ibmcloud.com/vulnerabilities/25772
http://www.vupen.com/english/advisories/2006/1342
http://www.securityfocus.com/bid/17500
http://www.securityfocus.com/archive/1/430869/100/0/threaded
http://www.adobe.com/support/techdocs/331917.html