Cross-site scripting (XSS) attacks inject malicious scripts (or code) into websites that are otherwise trustworthy or seemingly harmless. This type of attack takes advantage of the trust users have in web applications while leveraging vulnerabilities found within webpages and browsers themselves.
This makes cross-site scripting a particularly tricky threat. And because the attack method is so "accessible" (with few technical barriers), it's fairly widespread. XSS falls into the injection attack category and now ranks third overall on the latest OWASP Top Ten list.
How does cross-site scripting (XSS) work?
An attacker uses malicious code to access private objects such as cookies, tokens, or other information the browser collects during a session. An XSS attack can also fundamentally change webpage content by directly altering the page's HTML. However, an unsuspecting user—especially during their first visit to a given website—wouldn't notice these changes.
XSS attacks are possible since the browser doesn't validate the script before executing it, and therefore has no way of knowing it's harmful. Simple user inputs can trigger these attacks when two conditions are met:
A web request pulls data from an untrusted source into a web application.
Unverified and malicious data is incorporated within a website's dynamic content before it's sent to a user.
While Flash is no longer the active vulnerability factory it once was, XSS attacks continue to leverage HTML, JavaScript, and other code to perform a variety of actions. Aside from those we've touched on, an attacker might send sensitive data to themselves or redirect users to a malicious site. This latter method can open users up to further, more sophisticated attacks since the attacker fully controls the environment.
There are three main types of cross-site scripting attacks:
Reflected – Attackers reach users by injecting malicious code into a request (such as an unprotected argument for a redirect) which the server includes in its response to the client. The client's browser then executes said code with the permissions of the attacked website. The attacker can get a victim’s browser to make the malicious request by including the link in an email or chat, embedding it in another website (for example, via a frame), or in other ways depending on the specific vulnerability.
Stored – A database, server, or online element (such as a form) permanently retains a copy of malicious code, which it serves to unsuspecting users when they request a page normally. This is sometimes called "persistent" XSS.
DOM – By injecting a script without impacting the visual contents of a webpage, DOM-based scripting attacks can "hijack" legitimate website scripts by altering their behavior. This causes a web application to act differently on the backend to potentially steal sensitive data. Changes to the DOM environment are common and integral to these types of XSS attacks.
XSS attacks pose a serious problem because they're only limited by the attacker's imagination. There are many different ways to implement cross-site scripting. Some are simple, some are advanced, yet all are challenging to address effectively. For example, numerous HTML tags can be used as attack vectors, while a detailed code review is often needed to uncover suspicious HTTP request behaviors.
Can HAProxy help mitigate cross-site scripting (XSS) attacks?
Yes! HAProxy, HAProxy Enterprise, HAProxy ALOHA, and HAProxy Enterprise Kubernetes Ingress Controller include an OWASP CRS compatible WAF mode based on the evolving OWASP Top Ten threats list. Enhanced by our Intelligent WAF Engine, it reliably detects and blocks malicious scripting attempts to keep your systems safe.
To learn more about threat mitigation in HAProxy, check out our Web App Security vs. API Security blog post or our HAProxy Enterprise Web Application Firewall datasheet.
