Cross Site Scripting (XSS)

How Do XSS Attacks Work?

XSS attacks involve manipulating vulnerable websites to deliver malicious scripts to users. The primary process unfolds as follows:

• Attackers identify vulnerable websites and exploit weaknesses to inject malicious scripts into web pages. Script injection often targets websites lacking proper validation of user input. These malicious scripts are typically written in languages such as JavaScript, Java, VBScript, ActiveX, or Flash.

• Once a user accesses a website, the malicious script is loaded onto their browser and subsequently executed. There are numerous methods to activate these malicious scripts, such as enticing users to click on a link or automatically triggering the script upon webpage loading or when a user's mouse hovers over a specific element on the page.

• Malicious scripts have the capability to access cookies, session tokens, and other sensitive data stored in the browser, enabling them to modify page content as well. Through the theft of user cookies, attackers can impersonate users, carry out authorized actions, and obtain user identity information and files. Moreover, XSS can be exploited by attackers to disseminate malware, manipulate webpage content, and conduct phishing attacks, thereby facilitating more harmful cyber assaults.

Common Types of XSS Attacks

Figure 1 illustrates the typical varieties of XSS attacks.

Reflected XSS

Reflected XSS represents the most straightforward form of XSS attack. It occurs when a malicious script is embedded within a request sent to a vulnerable web server, which subsequently echoes the request back to the browser for execution. Essentially, the malicious script is transmitted to the web server as part of a client request. Upon parsing the request, the server includes the script in the response message, prompting the browser to execute it. Consequently, attackers aim to entice users into clicking on links that generate requests to the web server, often employing tactics such as phishing emails, pop-up links, and other forms of social engineering.

Attackers typically append malicious code to the end of a legitimate URL to craft such links. For instance:

Once the user clicks on this link, the malicious script executes because the browser trusts the website.

In the scenario described, the web server immediately dispatches HTTP GET or POST requests to execute the attack, rather than storing malicious scripts. This characteristic simplifies the implementation of reflected XSS. This type of attack is also referred to as non-persistent XSS since it requires activation by each individual victim.

Stored XSS

Stored XSS, also referred to as persistent XSS, represents the most severe form of XSS attack. In a stored XSS scenario, the injected script is permanently stored on the target web server, whether in a database, memory, or file system. As long as the injected script remains unremoved, it executes whenever a user accesses a web page.

Stored XSS attacks typically occur on platforms allowing content sharing, such as forums, blogs, and message boards. Websites with vulnerabilities in user input validation are susceptible to exploitation by attackers. Attackers leverage functions like posting, commenting, or modifying personal information to submit malicious scripts to the web server for storage. Consequently, all users visiting the compromised page unknowingly encounter the malicious script.

Compared to reflected XSS, stored XSS carries a broader impact, endangering all visitors. However, executing this type of attack is more challenging for attackers as it requires identifying exploitable vulnerabilities.

DOM-based XSS

DOM-based XSS represents a more sophisticated form of XSS, where attackers inject malicious scripts into websites by manipulating the Document Object Model (DOM). The DOM provides a structured representation of documents and connects HTML pages with scripts and programming languages. When a web page loads, the browser constructs the DOM. Client-side JavaScript in web applications can then dynamically alter the page's content by editing the DOM.

Essentially, JavaScript retrieves data from the DOM and executes it locally. If data is improperly processed during this process, attackers can inject malicious scripts into the DOM, triggering attacks when reading the DOM data. Commonly exploited DOM elements include documents.URL, location. hash, and document. referrer.

DOM-based XSS attacks differ significantly from reflected and stored XSS attacks. In a DOM-based attack, the entire attack process unfolds within the user's browser, without involving the web server in parsing or responding to access requests. Consequently, this makes it challenging to detect, as Web Application Firewall (WAF) devices cannot intercept communication traffic.

How Do I Prevent XSS Attacks?

In web program development, adhering to security principles is crucial to mitigate XSS attacks.

Developers should implement the following measures:

• Validate and sanitize user input to ensure it meets expected formats. Filtering out special characters and tags, such as "script", helps prevent malicious script injection.

• Escape user input before delivery to users. This involves encoding characters like ">" to "&gt", preventing interpreted code execution and neutralizing malicious scripts.

• Enhance cookie security by setting the HttpOnly Cookie attribute. This prevents JavaScript from accessing cookies, safeguarding user authentication tokens and sensitive data.

• Utilize HTTP response header Content-Security-Policy (CSP) to limit resources that can be loaded and executed. For instance, restricting JavaScript sources mitigates the risk of malicious script injection.

• Adhere to OWASP's Cross-Site Scripting Prevention Cheat Sheet during web program development.

• Conduct penetration tests on web programs to bolster security.

On the user end, awareness of XSS risks is essential for prevention:

• Disable JavaScript in the browser to mitigate XSS vulnerabilities.

• Exercise caution when clicking on unfamiliar links in emails and forums.

• Keep software and operating systems up to date with timely patches.

• Install antivirus software for additional protection against XSS threats.