Threats

Cross-Site Scripting (XSS) is a type of cybersecurity attack tactic typically used against in web applications, but may also be found in desktop applications that use JavaScript components such as ReactJS. In XSS attacks, the attacker is able to inject malicious JavaScript code into the Document Object Model (DOM) of an otherwise trusted website.

The vulnerability arises due to improper validation or escaping of user-supplied input, allowing the attacker to execute scripts in the context of a user's browser. When a user visits the website, the attacker supplied JavaScript executes on their computer's browser. 

How Are XSS Attacks Classified?

XSS attacks are primarily classified into three main types based on how the attacker is able to inject the malicious JavaScript into the victim's web-session.

Here are the most common ways that XSS attacks are classified:

• Stored XSS (Persistent XSS): In this form of XSS attack, the attacker is able to get their malicious content permanently stored on the target web-server. They may submit a comment on a blog post, engage in an online chat system, or forum which stores their content in a database. When the stored malicious content is served to a user such as when they visit the blog post, chat, or forum, it gets sent to and executed in their browser.

• Reflected XSS (Non-Persistent XSS): This type of attack occurs when the malicious script is reflected off a web server, such as in a HTTP GET or POST attribute that is converted to a message, search term, or other response and subsequently included in the server's response page. The script is executed in the victim’s browser immediately after being reflected from the server.

• DOM-Based XSS: This XSS variant happens when exploitable vulnerabilities exist in the webpage's legitimate JavaScript code client-side rather than server-side as in a typical reflected XSS weakness. In fact, an XSS weakness could be both reflected and DOM-based. For example, if a webpage's client side JavaScript takes the value from a text input box and directly injects it into the webpage using the .innerHTML() function. 

How Do Attackers Benefit From XSS Attacks?

By exploiting XSS vulnerabilities, attackers can bypass traditional security measures, gain unauthorized access to sensitive data stored in the browser cache, or escape the browser altogether to execute a variety of malicious activities on the victim's computer. Attackers may use XSS attacks to victimize the user's of a website, or even use XSS attacks to compromise the website owner. For example, if they are submitting contact messages that the website owner views in an administrative account built into the website.

Here are some ways that attackers can leverage XSS attacks:

• Escaping the Browser Sandbox: Attackers may be able break out of the browser's security sandbox by exploiting flaws and execute arbitrary code on the victim's system. This allows them to install malware, steal sensitive files, or gain complete control over the victim's computer. Once they have escaped the sandbox, attackers can establish persistent access to the victim's system, enabling long-term monitoring and exploitation.

• Stealing Sensitive Information: Session hijacking involves attackers capturing session cookies used to authenticate users to web applications, allowing them to impersonate victims and gain unauthorized access to their accounts. Other data theft such as logging keystrokes can enable attackers to steal usernames, passwords, and other sensitive information. Additionally, attackers may be able to steal sensitive data stored in the browser, such as saved passwords, autofill information, and browsing history.

• Phishing and Social Engineering: Phishing and social engineering techniques involve attackers modifying the content of a trusted website to deceive users into providing personal information or downloading malicious files, making phishing attacks more convincing and harder to detect. Additionally, attackers can inject fake forms or pop-ups to collect personal and financial information from victims, further exploiting users' trust in legitimate websites.

• Spreading Malware: Malicious JavaScript can initiate downloads of malware without the user's knowledge or consent. If executed by the user, these trojanized files can give the attacker a foothold on the victim's system or network.

• CSRF (Cross-Site Request Forgery): XSS can be used to execute attacks known as cross-site request forgery (CSRF). CSRF are unauthorized actions on behalf of the user by sending crafted requests to other sites where the user is authenticated. This can lead to actions such as changing account settings, making purchases, or initiating fund transfers.

• Network Reconnaissance: Attackers can perform network scans and identify other vulnerable systems within the victim's internal network using techniques such as DNS rebinding attacks.

How to Mitigate XSS Attacks

Protecting web applications from XSS attacks requires effective input validation, proper escaping of content when it is placed into a dynamic webpage, and adhering to other security best practices to mitigate these risks. 

Tactics for mitigating XSS attacks can be viewed from both the client and server perspectives.  The client represents the user who visits the website, while the server represents the website developer and owner. It is important to understand mitigation from both perspectives since the website visitors cannot guarantee a website's integrity, meaning they may receive malicious code from the website owner. At the same time, website developers want to do everything they can to protect their users and their own reputation for having a safe website. 

Let's cover how to mitigate XSS attacks from both perspectives:

Mitigating XSS Attacks From The Server Side

To prevent such attacks, web developers should implement robust security measures including:

• Properly sanitizing user inputs: Before storing or rendering user supplied data, the web application should sanitize the data in order to detect attempts as XSS. Unless javascript code is expected from the user, this may include altering the data to strip tags that enable JavaScript code to be defined and executed.

• Properly escaping data:  As user supplied dynamic content is injected into a webpage, it should be escaped according to HTML encoding standards.  This can ensure that user supplied data is always treated as plain text rather than executable code.  HTML also provides standards for more complex applications such as rendering computer code without executing it. 

• Content Security Policy (CSP): Content Security Policy is an HTTP security header that website administrators can set to control the resources a web page can load and execute. By specifying allowed sources for scripts, styles, and other resources, CSP reduces the risk of executing malicious scripts injected by an attacker.

• SameSite Cookie Attribute: The SameSite cookie is another HTTP security header attribute that helps mitigate cross-site request forgery (CSRF) attacks but also contributes to XSS protection by controlling how cookies are sent with cross-site requests. By setting cookies with the SameSite attribute to Strict or Lax, browsers can prevent cookies from being sent along with cross-site requests, reducing the risk of session hijacking via XSS.

• Other HTTP Security Headers: Various HTTP headers have been introduced to enhance browser security and protect against XSS. The X-Content-Type-Options header prevents browsers from interpreting files as a different MIME type than what is specified, reducing the risk of executing malicious scripts.

• Application Security Auditing: Employing secure coding practices and regularly auditing code for vulnerabilities can help identify exploitable vulnerabilities in a website's source code that could allow XSS attacks. When using a Content Management System (CMS) such as WordPress, auditing installed plugins and themes can help identify potential vulnerabilities.

• JavaScript Frameworks and Libraries: Modern web development frameworks like React and Angular and libraries have built-in protections against XSS by default. They encode and sanitize user inputs to prevent the injection of malicious scripts. Frameworks like React and Angular use template systems that automatically escape HTML content, preventing XSS vulnerabilities in user-generated content.

• Regularly Apply Updates: By regularly updating any CMS and web-development frameworks, JavaScript libraries, CMS plugins and themes, you can benefit from patches to any discovered security vulnerabilities. 

Mitigating XSS Attacks From The Client Side

Over the years, web browsers have significantly advanced their technologies to protect users from Cross-Site Scripting (XSS) attacks. These enhancements play a crucial role in the defense-in-depth strategy against web-based threats. 

Here are some key browser features and technologies designed to mitigate XSS attacks

• XSS Auditor and Filters: Modern browsers incorporate built-in XSS filters that can detect and block some types of XSS attacks by looking for patterns that resemble XSS attacks and take preventive actions. For instance, Google Chrome's XSS Auditor blocks the execution of suspected malicious scripts reflected in the page's URL.

• Secure Contexts and HTTPS: Browsers enforce the use of secure contexts (HTTPS) to ensure that sensitive web pages are always loaded over secure connections. This prevents attackers from intercepting or injecting malicious scripts during transmission.

• Browser Sandboxing: Browser sandboxing is the process of isolating the execution context of web pages and their included scripts from each other and from the underlying operating system. This limits the damage that a successful XSS attack can cause since a script should not be able to access data or commands outside of its own context. Modern browsers use process isolation to run each web page or tab in its own process, to sandbox loaded web pages from each other. Furthermore, some solutions exist such as remote browser isolation (RBI) designed to isolate the part or all of the browser execution from the user's system to another computer. 

Conclusion

Cross-Site Scripting (XSS) attacks exploit vulnerabilities in web applications allowing attackers to inject malicious scripts into trusted websites. These attacks can lead to significant risks, including browser escape and remote code execution, unauthorized access to sensitive data, session hijacking, and the spread of malware. XSS vulnerabilities are classified into Stored, Reflected, and DOM-Based types, depending on how the attacker is able to inject their malicious code into the website's content. 

Effective mitigation strategies include robust input validation, using Content Security Policy (CSP), and leveraging modern browser features such as XSS filters and secure contexts. Both server-side and client-side measures are essential to protect against these attacks and ensure a secure browsing experience. Organizations and developers must prioritize these practices to safeguard their applications and users from the diverse threats posed by XSS attacks.