A potentially dangerous Request.Form

Question

Anyone know why I am getting the following error? I have debugging enabled.

Server Error in '/' Application.
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A potentially dangerous Request.Form value was detected from the client (strContent="<p>
test</p>
"). 
Description: Request Validation has detected a potentially dangerous client input value, and processing of the request has been aborted. This value may indicate an attempt to compromise the security of your application, such as a cross-site scripting attack. You can disable request validation by setting validateRequest=false in the Page directive or in the configuration section. However, it is strongly recommended that your application explicitly check all inputs in this case. 

Exception Details: System.Web.HttpRequestValidationException: A potentially dangerous Request.Form value was detected from the client (strContent="<p>
test</p>
").

Source Error: 

The source code that generated this unhandled exception can only be shown when compiled in debug mode. To enable this, please follow one of the below steps, then request the URL:

1. Add a "Debug=true" directive at the top of the file that generated the error. Example:

  <%@ Page Language="C#" Debug="true" %>

or:

2) Add the following section to the configuration file of your application:

<configuration>
   <system.web>
       <compilation debug="true"/>
   </system.web>
</configuration>

Note that this second technique will cause all files within a given application to be compiled in debug mode. The first technique will cause only that particular file to be compiled in debug mode.

Important: Running applications in debug mode does incur a memory/performance overhead. You should make sure that an application has debugging disabled before deploying into production scenario.  

Stack Trace: 


[HttpRequestValidationException (0x80004005): A potentially dangerous Request.Form value was detected from the client (strContent="<p>
    test</p>
").]
   System.Web.HttpRequest.ValidateString(String s, String valueName, String collectionName) +8725306
   System.Web.HttpRequest.ValidateNameValueCollection(NameValueCollection nvc, String collectionName) +111
   System.Web.HttpRequest.get_Form() +129
   System.Web.HttpRequest.get_HasForm() +8725415
   System.Web.UI.Page.GetCollectionBasedOnMethod(Boolean dontReturnNull) +97
   System.Web.UI.Page.DeterminePostBackMode() +63
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +6785
   System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +242
   System.Web.UI.Page.ProcessRequest() +80
   System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +21
   System.Web.UI.Page.ProcessRequest(HttpContext context) +49
   ASP.ajax_create_new_page_aspx.ProcessRequest(HttpContext context) +37
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +181
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75



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Version Information: Microsoft .NET Framework Version:2.0.50727.3615; ASP.NET Version:2.0.50727.3618

Answer 1

The post contains HTML elements (the <p> tag, in your case) - this can be indication of a cross site scripting attack, which is why asp.net does not allow it by default.

You should either HTML encode before submitting (best practice), or disable the warning and potentially expose yourself to XSS.