Cross-Site Scripting (XSS)

Overview

Cross-Site Scripting (XSS) is a widespread vulnerability that affects many web applications. XSS attacks consist of injecting malicious client-side scripts into a website and using the website as a propagation method. The risks behind XSS is that it allows an attacker to inject content into a website and modify how it is displayed, forcing a victim’s browser to execute the code provided by the attacker while loading the page. XSS is present in about two-thirds of all applications. Generally, XSS vulnerabilities require some type of interaction by the user to be triggered, either via social engineering or via a visit to a specific page. If an XSS vulnerability is not patched, it can be very dangerous to any website.

Description

Cross-Site Scripting (XSS) is a widespread vulnerability that allows attackers to inject malicious client-side scripts into a website. This vulnerability can modify the content displayed on the website and force the victim's browser to execute the attacker's code. XSS is present in about two-thirds of all web applications, making it a major security concern. XSS vulnerabilities can be triggered through user interactions, such as visiting a specific page or falling victim to social engineering tactics. If left unpatched, XSS vulnerabilities can pose significant risks to websites.

How to Prevent ?

To prevent XSS vulnerabilities, it is important to separate untrusted data from active browser content. The following measures can help mitigate the risks of XSS attacks: - Use frameworks that automatically escape XSS, such as Ruby on Rails or React JS. - Escape untrusted HTTP request data based on the context in the HTML output. - Apply context-sensitive encoding when modifying the browser document on the client side to prevent DOM XSS. - Enable a content security policy (CSP) as a defense against XSS. By implementing these preventive measures and following OWASP guidelines, web developers can reduce the chances of XSS attacks.

Example Attack Scenarios:

• WordPress Stored XSS Vulnerability:  Imagine you are on your WordPress wp-admin panel adding a new post. If you are using a plugin with a stored XSS vulnerability that is exploited by a hacker, it can force your browser to create a new admin user while you’re in the wp-admin panel or it can edit a post and perform other similar actions. An XSS vulnerability gives the attacker almost full control of the most important software of computers nowadays: the browsers. Back in 2017, our research team disclosed a stored XSS vulnerability in the core of WordPress websites. Remote attackers could use this vulnerability to deface a random post on a WordPress site and store malicious JavaScript code in it.

• Reflected XSS Attack:  In a reflected XSS attack scenario, the attacker includes unvalidated user input as part of the HTML output. This can be achieved through malicious links that direct victims to attacker-controlled pages. Once the victim interacts with the link, arbitrary HTML and JavaScript can be executed in their browser, leading to potential risks such as session stealing, account takeover, or downloading malicious software.

• DOM XSS Attack:  DOM XSS vulnerabilities occur when JavaScript frameworks, single-page applications, or APIs dynamically include attacker-controllable data in a page. This vulnerability can be exploited to perform actions like replacing or defacing DOM nodes, trojan login panels, and other client-side attacks. Preventive measures should be taken, such as avoiding sending attacker-controllable data to unsafe JavaScript APIs.