IIS security guy Nazim Lala has just blogged about a IIS 6 security issue, related to the use of Semi-colon (;) in a URL. It's not much of an issue if your IIS 6 server is properly configured. Good example of how Best practices can make your day.
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Read the complete post at http://blogs.iis.net/ma_khan/archive/2009/12/29/iis-security-semi-colons-in-url.aspx