To run malicious JavaScript code in a victim’s browser, an attacker must first find a way to inject malicious code (payload) into a web page that the victim visits.There are two stages to a typical XSS attack: You can read more about them in an article titled Types of XSS. There are several types of Cross-site Scripting attacks: stored/persistent XSS, reflected/non-persistent XSS, and DOM-based XSS. Cross-site Scripting can also be used in conjunction with other types of attacks, for example, Cross-Site Request Forgery (CSRF). XSS vulnerabilities provide the perfect ground to escalate attacks to more serious ones. The above, in combination with social engineering, allow criminals to pull off advanced attacks including cookie theft, planting trojans, keylogging, phishing, and identity theft. Most of these APIs require user opt-in, but the attacker can use social engineering to go around that limitation. For example, it can gain access to the user’s geolocation, webcam, microphone, and even specific files from the user’s file system. JavaScript in modern browsers can use HTML5 APIs.

JavaScript can use the XMLHttpRequest object to send HTTP requests with arbitrary content to arbitrary destinations.Luckily, this is only possible within the page where JavaScript is running. JavaScript can read the browser DOM and make arbitrary modifications to it.If an attacker can obtain a user’s session cookie, they can impersonate that user, perform actions on behalf of the user, and gain access to the user’s sensitive data. Cookies are often used to store session tokens. This includes access to the user’s cookies. Malicious JavaScript has access to all the objects that the rest of the web page has access to.

However, JavaScript can still be dangerous if misused as part of malicious content: JavaScript has limited access to the user’s operating system and the user’s files. Most web browsers run JavaScript in a very tightly controlled environment. Consequences of the ability to execute JavaScript on a web page may not seem dire at first. XSS vulnerabilities are perceived as less dangerous than for example SQL Injection vulnerabilities. What Can the Attacker Do with JavaScript? The attacker can use injected scripts to change the content of the website or even redirect the browser to another web page, for example, one that contains malicious code. If it is affecting your users, it affects you.Ĭross-site Scripting may also be used to deface a website instead of targeting the user. XSS is not the user’s problem like any other security vulnerability. If an attacker can abuse an XSS vulnerability on a web page to execute arbitrary JavaScript in a user’s browser, the security of that vulnerable website or vulnerable web application and its users has been compromised. “Isn’t Cross-site Scripting the User’s Problem?” However, they are most common in JavaScript, primarily because JavaScript is fundamental to most browsing experiences. XSS attacks are possible in VBScript, ActiveX, Flash, and even CSS.

This user input must then be parsed by the victim’s browser. Vulnerable vehicles that are commonly used for Cross-site Scripting attacks are forums, message boards, and web pages that allow comments.Ī web page or web application is vulnerable to XSS if it uses unsanitized user input in the output that it generates. The web page or web application becomes a vehicle to deliver the malicious script to the user’s browser. The actual attack occurs when the victim visits the web page or web application that executes the malicious code. The attacker aims to execute malicious scripts in a web browser of the victim by including malicious code in a legitimate web page or web application. Cross-site Scripting (XSS) is a client-side code injection attack.

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