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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/
Vulnerability Type:    Username Enumeration
Remote:    Yes
Local:    No
Discovered:    14 September 2012
Reported:    19 December 2012
Disclosed:    18 February 2013
Whitepaper:   Pwning_MDaemon.pdf

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&User=mickey.mouse@qsecure.com.cy


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.