Alt-N MDaemon WorldClient Username Enumeration

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Alt-N MDaemon's WorldClient Username Enumeration Vulnerability  
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Software: Alt-N MDaemon v13.0.3 and prior versions  
Vendor: http://www.altn.com/  
Vuln Type: Username Enumeration  
Remote: Yes  
Local: No  
Discovered by: QSecure and Demetris Papapetrou  
References: http://www.qsecure.com.cy/advisories/Alt-N_MDaemon_WorldClient_Username_Enumeration.html  
Discovered: 14/09/2012  
Reported: 19/12/2012  
Fixed: 15/01/2013 (http://files.altn.com/MDaemon/Release/RelNotes_en.html)  
Disclosed: 18/02/2013  
  
VULNERABILITY DESCRIPTION:  
==========================  
Alt-N WorldClient is prone to a username-enumeration weakness by  
querying the user's Free-Busy schedule. The DTSTART and DTEND  
parameters in the returned FBData.vfb file, may indicate whether an  
email address/username is valid or not.  
  
Attackers may exploit this weakness to discern valid usernames. This  
may aid them in brute-force password cracking or other attacks.  
  
Alt-N MDaemon v13.0.3 & v12.5.6 were tested and found vulnerable;  
other versions may also be affected.  
  
PoC Exploit:  
============  
http://www.example.com:3000/WorldClient.dll?View=fbinfo&[email protected]  
  
For valid email accounts the DTSTART and DTEND values in the .vfb file  
start with a recent date backwards (e.g. 20120505) whereas for invalid  
ones the date is 19801231.  
`