“An old feature of ISA Server/Forefront TMG 2010, which if memory helps created over the time a few sparks within the community, is the forward web proxy’s ability to “not limit the ports to which the Web proxy may forward requests”. 
Personally I’m not a fan of such an “ability”.
Recently, during a security audit, I’ve put to “work” this ability to scan/fingerprint TCP ports/services through Forefront TMG 2010 while TMG provided network separation.
Since we can actually issue “HTTP requests” to “web sites” on virtually almost any ports, little stops us to issue requests to other services like SMTP, IMAP, POP3, FTP, RDP, etc.; all we need is to have allowed HTTP to the required destination…”
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DEBRA LITTLEJOHN SHINDER
MVP (Enterprise Security)