Previous versions of ASP.NET limit accepted URLs to a specific set of characters. The following characters were considered invalid in a URL: <, >, &. You can use the new requestPathInvalidChars property to specify invalid characters. The following example makes a, b, and c invalid in requests (which isn't too useful but demonstrates the feature):
<httpRuntime requestPathInvalidCharacters="a,b,c">
NOTE
The Microsoft documentation states that ASP.NET 4.0 will reject paths with characters in the ASCII range 0x00 to 0x1F (RFC 2396).
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