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Alt-N MDaemon's WorldClient Username Enumeration Vulnerability

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.