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Testing for HTML Injection



HTML injection is a type of injection vulnerability that occurs when a user is able to control an input point and is able to inject arbitrary HTML code into a vulnerable web page. This vulnerability can have many consequences, like disclosure of a user’s session cookies that could be used to impersonate the victim, or, more generally, it can allow the attacker to modify the page content seen by the victims.

This vulnerability occurs when user input is not correctly sanitized and the output is not encoded. An injection allows the attacker to send a malicious HTML page to a victim. The targeted browser will not be able to distinguish (trust) legitimate parts from malicious parts of the page, and consequently will parse and execute the whole page in the victim’s context.

There is a wide range of methods and attributes that could be used to render HTML content. If these methods are provided with an untrusted input, then there is an high risk of HTML injection vulnerability. For example, malicious HTML code can be injected via the innerHTML JavaScript method, usually used to render user-inserted HTML code. If strings are not correctly sanitized, the method can enable HTML injection. A JavaScript function that can be used for this purpose is document.write().

The following example shows a snippet of vulnerable code that allows an unvalidated input to be used to create dynamic HTML in the page context:

var userposition=location.href.indexOf("user=");
var user=location.href.substring(userposition+5);
document.getElementById("Welcome").innerHTML=" Hello, "+user;

The following example shows vulnerable code using the document.write() function:

var userposition=location.href.indexOf("user=");
var user=location.href.substring(userposition+5);
document.write("<h1>Hello, " + user +"</h1>");

In both examples, this vulnerability can be exploited with an input such as:<img%20src='aaa'%20onerror=alert(1)>

This input will add an image tag to the page that will execute arbitrary JavaScript code inserted by the malicious user in the HTML context.

How to Test

For example, consider at the following URL:

The HTML code contains the following script:

<script src="../js/jquery-1.7.1.js"></script>
function setMessage(){
    var t=location.hash.slice(1);
    $("div[id="+t+"]").text("The DOM is now loaded and can be manipulated.");
$(document).ready(setMessage  );
    <script src="../js/embed.js"></script>
    <span><a href="#message" > Show Here</a><div id="message">Showing Message1</div></span>
    <span><a href="#message1" > Show Here</a><div id="message1">Showing Message2</div>
    <span><a href="#message2" > Show Here</a><div id="message2">Showing Message3</div>

It is possible to inject HTML code.