C# – A potentially dangerous Request.Path value was detected from the client (?)

asp.netciis

Environment:

IIS 8.5

.NET Framework Version: 4.6.2 (using WebForms)

Windows Server 2012 R2

Problem:

The following exception is being reported:

BASE EXCEPTION: System.Web.HttpException (0x80004005): A potentially dangerous Request.Path value was detected from the client (?).
   at System.Web.HttpRequest.ValidateInputIfRequiredByConfig()
   at System.Web.HttpApplication.PipelineStepManager.ValidateHelper(HttpContext context)

BASE EXCEPTION HRESUT: -2147467259

EXCEPTION: System.Web.HttpException (0x80004005): A potentially dangerous Request.Path value was detected from the client (?).
   at System.Web.HttpRequest.ValidateInputIfRequiredByConfig()
   at System.Web.HttpApplication.PipelineStepManager.ValidateHelper(HttpContext context)

Other information shown in our logs:

PATH_INFO
/cities/index.aspx?locid=4163
----
QUERY_STRING
----
REMOTE_ADDR
66.249.65.204
----
REMOTE_HOST
66.249.65.204
----
REQUEST_METHOD
GET
----
SCRIPT_NAME
/cities/index.aspx?locid=4163
----
URL
/cities/index.aspx?locid=4163
----
HTTP_FROM
googlebot(at)googlebot.com
----
HTTP_USER_AGENT
Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)

What I do not understand is if I cut and paste the path in my browser, the page is rendered just fine and without error.

Questions:

Why does googlebot, when crawling the page produce this error, yet no error is generated when I enter the path in a browser? (I do find it odd that the error log shows no value for the query string, even though it is present).
Why is the "?" character considered potentially dangerous?

Any advice would be appreciated as I am trying to understand how this particular "error" is being raised when the path is in fact valid.

Thanks in advance.

Best Answer

From Asp.net 4.0+ introduced a strict validation, so what ever error you are seeing might be the part of it . there are certain dangerouss characters in the url which might cause XSS attack . so ? is one among them. remaining characters are as follows:

< > * % & : \ ?

Probably there might be two solutions

you can allow these characters in your URL , or atleast certain character ,by configuring the following configuration in web config as follows

<system.web> <httpRuntime requestPathInvalidCharacters="<,>,*,%,&,:,\,?" /> </system.web>
You can roll back to asp.net 2.0 , with the following configuration

<system.web> <httpRuntime requestValidationMode="2.0" /> </system.web>

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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).

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