XSS can be used in a range of ways to cause serious problems. The traditional (and dangerous) uses of XSS is the ability for an attacker to steal session cookies allowing an attacker to impersonate a victim. However, it is not just about stealing cookies, XSS has been used to wreak havoc on social networks, spread malware, website defacements, phish for credentials and even used in conjunction with social engineering techniques to escalate to more damaging attacks.
Cross-site Scripting can be classified into three major categories — Stored XSS, Reflected XSS and DOM-based XSS.
The most damaging type of XSS is Stored (Persistent XSS). Stored XSS attacks involves an attacker injecting a script (referred to as the payload) that is permanently stored (persisted) on the target application (for instance within a database). A classic example is a malicious script inserted by an attacker in a comment field on a blog or in a forum post.
When a victim navigates to the affected web page in a browser, the XSS payload will be served as part of the web page (just like a legitimate comment would). This means that victims will inadvertently end-up executing the malicious script once the page is viewed in a browser.
The second, and by far most common type of XSS is Reflected XSS. Here, the attacker’s payload script has to be part of the request which is sent to the web server and reflected back in such a way that the HTTP response includes the payload from the HTTP request. Using Phishing emails and other social engineering techniques, the attacker lures the victim to inadvertently make a request to the server which contains the XSS payload and ends-up executing the script that gets reflected and executed inside the browser. Since Reflected XSS isn’t a persistent attack, the attacker needs to deliver the payload to each victim – social networks are often conveniently used for the dissemination of these attacks.
DOM-based XSS is an advanced type of XSS attack which is made possible when the web application’s client side scripts write user provided data to the Document Object Model (DOM). The data is subsequently read from the DOM by the web application and outputted to the browser. If the data is incorrectly handled, an attacker can inject a payload, which will be stored as part of the DOM and executed when the data is read back from the DOM. Read more about DOM-based XSS »
The most dangerous part of this attack is often a client-side attack, and the attacker’s payload is never sent to the server. This makes it even more difficult to detect for Web Application Firewalls (WAFs) and security engineers analyzing the server’s logs since they will never even see the attack.
Among various objects that make up the DOM, there are some objects in particular which an attacker can manipulate in order to generate the XSS condition. Such objects include the URL (
document.URL), the part of the URL behind the hash (
location.hash) and the Referrer (