I think you are attacking it from the wrong angle by trying to encode all posted data.
Note that a "<" could also come from other outside sources, like a database field, a configuration, a file, a feed and so on.
Furthermore, "<" is not inherently dangerous. It's only dangerous in a specific context: when writing strings that haven't been encoded to HTML output (because of XSS).
In other contexts different sub-strings are dangerous, for example, if you write an user-provided URL into a link, the sub-string "javascript:" may be dangerous. The single quote character on the other hand is dangerous when interpolating strings in SQL queries, but perfectly safe if it is a part of a name submitted from a form or read from a database field.
The bottom line is: you can't filter random input for dangerous characters, because any character may be dangerous under the right circumstances. You should encode at the point where some specific characters may become dangerous because they cross into a different sub-language where they have special meaning. When you write a string to HTML, you should encode characters that have special meaning in HTML, using Server.HtmlEncode. If you pass a string to a dynamic SQL statement, you should encode different characters (or better, let the framework do it for you by using prepared statements or the like)..
When you are sure you HTML-encode everywhere you pass strings to HTML, then set ValidateRequest="false" in the <%@ Page ... %> directive in your .aspx file(s).
In .NET 4 you may need to do a little more. Sometimes it's necessary to also add <httpRuntime requestValidationMode="2.0" /> to web.config (reference).
Add a web.config containing
<system.web>
<pages validateRequest="false" />
</system.web>
to the directory with the page that has the form in question.
See http://www.asp.net/learn/whitepapers/request-validation for a complete description.
In case you use asp.net 4.0, you may try
<httpRuntime requestValidationMode="2.0" />
See also
Best Answer
It's intriguing that you can see the full error, but are not capable of accessing the ASP.NET code. Normally, one can only see the full error when in debug mode, because in production, the error-setting is (should be)
RemoteOnlyorOff. This seems a configuration mistake and a potential risk on the side of the ASP.NET site.You say "to http post some xml files". If you were indeed posting files, you wouldn't receive this response. Maybe you can contact the site's owner and ask for him to change the form to allow file-input.
You can change your input such that it doesn't look like XML anymore, but then it isn't XML anymore either. I.e., change all
<in<and you'll be able to get your data through, but it must be unescaped when processed.If this site is supposed to accept XML, it must be changed to accept XML. Either it should accept files, or it should accept HTML/XML input by turning
ValidateRequestto off. If it is not supposed to receive XML, there's little you can do. It's like filling in a bank's payment form by putting letters in the amount-field: it just won't work (unless it was designed to work that way).