What is Cross-Site Scripting?  

Cross-site scripting (XSS) is a vulnerability that enables threat actors to inject malicious scripts into web pages. These scripts can execute within the victim’s browser, which could lead to data theft, session hijacking, defacement, or malwareWhat is Malware? Malware, a portmanteau of “malicious software,” constitutes a broad category of software specifically designed to infiltrate, damage, or disrupt computer systems, networks, and devices without the user’s… More distribution. XSS is one of the most common security flaws in web applications and is listed in the OWASP Top 10 security risks.  

How Does XSS Work?  

XSS exploits vulnerabilities in web applications that fail to validate or escape user input properly. When an application reflects or stores untrusted data and renders it as part of the webpage, an attacker can craft scripts that execute in the context of the victim’s session. These scripts often target sensitive user data, such as cookies, authenticationWhat is Authentication? Authentication is the process by which the identity of a user or system is verified. It ensures that the entity attempting to access a resource is who… More tokens, or session identifiers.  

Types of XSS Attacks  

There are several types of XXS attacks:  

Stored XSS (Persistent XSS)  

• In a Stored XSS attack, the malicious script is permanently stored on the target website’s server, typically in a database, forum post, comment section, or user profile.  

• When a user visits the infected page, the browser executes the malicious injected script.   

Example: An attacker inserts JavaScript into a user comment field, which is then displayed to every visitor without sanitization.  

Reflected XSS (Non-Persistent XSS)  

• In Reflected XSS, the attacker’s payload is included in a URL or a request parameter and is executed when the victim clicks a malicious link or submits a manipulated form.  

• Unlike Stored XSS, the injected script does not persist on the server but is reflected back in the response.  

Example: A phishingWhat is Phishing? Phishing is a type of cyberattack in which attackers send fraudulent communications, or direct people to counterfeit websites in order to trick those individuals into revealing sensitive… More email containing a crafted URL that, when clicked, executes a JavaScript snippet to steal session cookies.  

DOM-Based XSS   

• This type of XSS occurs entirely in the client-side script (JavaScript) rather than the server response. 

• The malicious actor manipulates the Document Object Model (DOM) environment in a way that executes malicious scripts within the browser.  

Example: A web application dynamically updates page content using document.write() or innerHTML based on user input without proper sanitization.  

The Risks and Consequences of XSS  

An XSS vulnerability can have serious consequences, including:  

• Session Hijacking: Bad actors can steal session cookies to impersonate users. 

• Phishing Attacks: Malicious scripts can create fake login forms to steal credentials. 

• Defacement: Malefactors can modify website content to mislead or embarrass the site owner. 

• Malware Distribution: Scripts can redirect users to malicious websites or download harmful files. 

• Data Theft: Sensitive information, including personal data, can be extracted and sent to unauthorized parties.  

Preventing XSS Attacks  

To mitigate the risks that go hand in hand with XSS vulnerabilities, developers and security teams should implement a mixture of proactive defenses and best practices to prevent malicious scripts from executing within web applications.  

Input Validation and Sanitization 

•  Filter user input to ensure only expected data is accepted. 
• Remove or escape special characters (<, >, ‘, “, &, /) to prevent script execution. 

 Use Content Security Policy (CSP) 

• Implement a CSP header to restrict allowed sources of scripts, reducing the risk of XSS execution. 

 Encode Output Properly 

• Use proper encoding functions – for example, htmlspecialchars()in JavaScript – to ensure untrusted data is displayed as text rather than executed as a script. 

 Implement Secure Cookies and HTTP Headers  

• Use HttpOnly and Secure flags for cookies to prevent unauthorized access.  

• Set X-XSS-Protection: 1; mode=block in HTTP headers to enable browser-based XSS protection.  

Use Modern Frameworks and Security Libraries  

• Leverage frameworks that handle input sanitization and secure template rendering, such as React, Angular, and Vue.js, which implement automatic output encoding. 

Regular Security Audits and Penetration Testing 

•  Continuously test web applications using automated security tools like OWASP ZAP or Burp Suite. 
• Conduct code reviews and security assessments to detect and patch vulnerabilities. 

 Cross-site scripting (XSS) remains one of the most common and dangerous web vulnerabilities. Developers and security teams need to implement robust input validation, output encoding, and security policies to mitigate the risk. By following best practices, entities can protect users from data breaches, phishing attacks, and other malicious acts enabled by XSS exploitation.  

For more essential cybersecurity definitions, visit our glossary pages here.