Understanding Script Injection Attacks
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by SANJIT SIL
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Request Validation

Script injection attacks are a concern for all web developers, whether they are using ASP.NET, ASP, or any other web development technologies. ASP.NET includes a feature designed to automatically combat script injection attacks, known as request validation. Request validation checks the posted form input and raises an error if any potentially malicious tags (such as <script>) are found. In fact, request validation disallows any nonnumeric tags, including HTML tags (such as <b> and <img>), and tags that do not correspond to anything (such as <xyz>).

To test the script validation features, we can create a simple web page like the one shown below.

Figure 1

If we try to enter a block of content with a script tag and then click the button, ASP.NET will detect the potentially dangerous value and generate an error.

Figure 2

Disabling Request Validation

There may be such a situation where users have a genuine need to specify HTML tags (for example, an advertisement purpose) or a block of XML data. In these situations we need to specifically disable script validation using the ValidateRequest Page directive, as shown below.

Listing 1

<%@ Page ValidateRequest="false" Language="C#" AutoEventWireup="true" 
CodeFile="Default.aspx.cs" Inherits="_Default" %>

We can also disable request validation for an entire web application by modifying the web.config file. We need to add or set the validateRequest attribute of the <pages> element, as shown here.

Listing 2

<configuration>
    <appSettings/>
    <connectionStrings/>
    <system.web>
      <pages validateRequest="false"/>
    </system.web>
</configuration>
protected void btnSubmit_Click(object sender, EventArgs e)
{
  Response.Write(txtInput.Text);
}

The following screenshot is showing what will happen when a user clicks on the submit button.

Figure 2

Encode Output

Use the HttpUtility.HtmlEncode method to encode output if it contains input from the user or from other sources such as databases. HtmlEncode replaces characters that have special meaning in HTML-to-HTML variables that represent those characters. For example, < is replaced with &lt; and " is replaced with &quot;. Encoded data does not cause the browser to execute code. Instead, the data is rendered as harmless HTML.

To prevent a script injection attack from happening when request validation is turned off, we need to explicitly encode the content before we display it using the Server object.

Listing 3

protected void btnSubmit_Click(object sender, EventArgs e)
{
  Response.Write("Entered Input is:"+Server.HtmlEncode(txtInput.Text));
}

The following screenshot is showing the output of the above mentioned code.

Figure 3


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