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A potentially dangerous Request.Form value was detected from the client


Question

Every time a user posts something containing < or > in a page in my web application, I get this exception thrown.

I don't want to go into the discussion about the smartness of throwing an exception or crashing an entire web application because somebody entered a character in a text box, but I am looking for an elegant way to handle this.

Trapping the exception and showing

An error has occurred please go back and re-type your entire form again, but this time please do not use <

doesn't seem professional enough to me.

Disabling post validation (validateRequest="false") will definitely avoid this error, but it will leave the page vulnerable to a number of attacks.

Ideally: When a post back occurs containing HTML restricted characters, that posted value in the Form collection will be automatically HTML encoded. So the .Text property of my text-box will be something & lt; html & gt;

Is there a way I can do this from a handler?

2017/06/16
1
1483
6/16/2017 10:24:33 PM

Accepted Answer

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).

2020/08/30
1090
8/30/2020 2:53:35 PM

There's a different solution to this error if you're using ASP.NET MVC:

C# sample:

[HttpPost, ValidateInput(false)]
public ActionResult Edit(FormCollection collection)
{
    // ...
}

Visual Basic sample:

<AcceptVerbs(HttpVerbs.Post), ValidateInput(False)> _
Function Edit(ByVal collection As FormCollection) As ActionResult
    ...
End Function
2020/06/02

In ASP.NET MVC (starting in version 3), you can add the AllowHtml attribute to a property on your model.

It allows a request to include HTML markup during model binding by skipping request validation for the property.

[AllowHtml]
public string Description { get; set; }
2018/04/17

If you are on .NET 4.0 make sure you add this in your web.config file inside the <system.web> tags:

<httpRuntime requestValidationMode="2.0" />

In .NET 2.0, request validation only applied to aspx requests. In .NET 4.0 this was expanded to include all requests. You can revert to only performing XSS validation when processing .aspx by specifying:

requestValidationMode="2.0"

You can disable request validate entirely by specifying:

validateRequest="false"
2017/06/23

For ASP.NET 4.0, you can allow markup as input for specific pages instead of the whole site by putting it all in a <location> element. This will make sure all your other pages are safe. You do NOT need to put ValidateRequest="false" in your .aspx page.

<configuration>
...
  <location path="MyFolder/.aspx">
    <system.web>
      <pages validateRequest="false" />
      <httpRuntime requestValidationMode="2.0" />
    </system.web>
  </location>
...
</configuration>

It is safer to control this inside your web.config, because you can see at a site level which pages allow markup as input.

You still need to programmatically validate input on pages where request validation is disabled.

2016/01/30

The previous answers are great, but nobody said how to exclude a single field from being validated for HTML/JavaScript injections. I don't know about previous versions, but in MVC3 Beta you can do this:

[HttpPost, ValidateInput(true, Exclude = "YourFieldName")]
public virtual ActionResult Edit(int id, FormCollection collection)
{
    ...
}

This still validates all the fields except for the excluded one. The nice thing about this is that your validation attributes still validate the field, but you just don't get the "A potentially dangerous Request.Form value was detected from the client" exceptions.

I've used this for validating a regular expression. I've made my own ValidationAttribute to see if the regular expression is valid or not. As regular expressions can contain something that looks like a script I applied the above code - the regular expression is still being checked if it's valid or not, but not if it contains scripts or HTML.

2017/06/23

Source: https://stackoverflow.com/questions/81991
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