internal server error 500 when using http post containing %0A%3C

Posted on 2008-06-19
Last Modified: 2013-12-16

I am trying to perform an HTTP POST to a website. Every time i do the POST i get a response of 500. Internal server error.

I have found out that the reason i am getting the error is because the POST contains the string:


This is a line feed and a < sign.

For some reason i am unable to post this data when it is a requirement for me. If i use %0A%3E (which is line feed and > sign) it works.

Does anyone know why this would happen?

Any help would be appreciated. I have attached the code that i am using.

I am running Microsoft Visual Studio 2005. .Net 2.0 Service Pack 1, Windows XP and IIS 5.1

(i tried it on server 2003 with iis 6.0 and had the same error)

static void Main(string[] args)


           string query = "?%0A%3Chey";

            string a = HTTPCallback("url", query, "POST");




        // simple generic http get/post request function

        static string HTTPCallback(string uri, string query, string method)


            string output = string.Empty;



                // request

                WebRequest request;

                // do a POST if requested, otherwise default to a GET

                if (method.ToUpper() == "POST")


                    request = WebRequest.Create(uri);

                    request.Method = "POST";

                    byte[] buffer = Encoding.UTF8.GetBytes(query);

                    request.ContentType = "application/x-www-form-urlencoded";

                    request.ContentLength = buffer.Length;

                    // add form data to request stream

                    System.IO.Stream requestStream = request.GetRequestStream();

                    requestStream.Write(buffer, 0, buffer.Length);






                    request = WebRequest.Create(uri + query);

                    request.Method = "GET";


                // response

                HttpWebResponse response = (HttpWebResponse)request.GetResponse();

                System.IO.Stream responseStream = response.GetResponseStream();

                StreamReader responseReader = new StreamReader(responseStream);

                // output

                output = responseReader.ReadToEnd();

                // clean-up





            catch (Exception e)




            return output;


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Question by:Apyreal
LVL 25

Expert Comment

ID: 21821725
I'm not sure, but wouldn't you need to encode that like &lt; for <
like in html, to keep the page from thinking "<" is the beginning of a tag..that it subsequently won't find?

Accepted Solution

Apyreal earned 0 total points
ID: 21821787
nope, actually. the it is already encoded. I found the problem about 5 minutes ago. IIS was treating %0A%3C as a possible dangerous request, so it would always fail it.

Fix was to place ValidateRequest="false" in the page to where i am posting. Not a perfect fix but it is the only one that works

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