What Is Cross-Site Scripting (XSS)?
Cross-site scripting (XSS) attacks inject malicious scripts into trusted websites to target visitors and influence future interactions. While XSS attacks have been around for a long time (first identified in 1999), they remain a popular form of cyberattack. In fact, XSS was the most discovered cloud vulnerability in IBM’s 2024 Cloud Threat Landscape Report.
XSS attacks utilize security mismatches within web applications to inject the malicious code, most often a client-side JavaScript code, without the site owner or visitor being aware. Trusted sites with the proper credentials, checked by the victim’s browser, unwittingly act as the delivery mechanism for malicious scripts from untrusted sources.
These scripts then run on the victims’ browsers, compromising how they interact with the web application. This can happen through multiple methods, including defacing the website, hijacking their session, injecting malware, and phishing.
How XSS Attacks Work
A simplified 3-step XSS attack process looks like this:
- The attacker identifies vulnerable websites to target and inject malicious code. For example, if a site doesn’t validate all of its inputs.
- Once the malicious code is on the website, the attacker either passively waits for unsuspecting visitors to execute it or proactively sends the compromised site link to potential victims. This can be done through social media messages or emails, tricking them into visiting the page.
- Finally, once the site visitors execute the code and become victims, the attacker can manipulate future interactions in a number of different ways depending on their goal.
XSS attacks bypass the browser’s origin policy and hide their malicious scripts within a trusted website. Browsers check whether websites are secure and have the proper credentials. However, they cannot verify any additional scripts that may have been injected into the site. Therefore, poor web application security can expose users to malicious scripts with severe consequences.
The Impact of XSS Attacks
Depending on the type of attack and the hacker’s goals, XSS can cause a range of consequences for both the web application and its users. For the people visiting the website or using the web application, XSS attacks can lead to:
- Website Defacement: Manipulating the image of the website or adding new elements through HTML or CSS modifications to change what the visitor sees. Attackers could show them something completely different from the original website to make them do something they wouldn’t normally do.
- Session Hijacking: With their malicious code running on the victim’s browser, the attacker could access cookies to hijack the session. This allows them to take control of their account on the web application, perform unauthorized actions, and access sensitive information stored on the browser.
- Malware Injection: The attacker’s malicious scripts inject malware onto the visitor’s browser, which can then escape to run on their system natively. This malware may have a number of different goals, including keyloggers that capture login credentials, cryptocurrency mining software, or taking control of the system as part of a broader botnet for use in a different DDoS attack.
- Phishing: Redirecting visitors to phishing sites with fake input fields to capture user credentials and take control of their accounts.
While visitors are typically the primary target of XSS attacks, they also impact the web application. Data breaches create compliance issues that bring potential fines, legal consequences, and reputational damage. Website defacement may lead to attackers displaying offensive or malicious content, also causing reputational damage.
Additionally, if the XSS attack compromises victims who have privileged access, it can lead to hackers controlling web application functionality and more significant data breaches.
Types of XSS Attacks
Reflected Cross-Site Scripting
Reflected XSS is when the web application “reflects” malicious code to the user, most often through vulnerable input fields. For example, if a website does not sanitize inputs provided by the URL parameter, attackers can inject malicious scripts by embedding them into the link. Users clicking this link then unwittingly execute the script, exposing their browser to the attacker. Reflected XSS only occurs when the link is clicked. It is a non-persistent form of cross-site scripting.
Stored Cross-Site Scripting
Stored XSS is a persistent form of cross-site scripting that continues to infect new users until the website owner detects and responds to the threat. In these attacks, untrusted sources hide their code on the website using permanently stored input fields such as comment sections. If the site does not validate inputs from the comment section, attackers can embed HTML tags that execute when new visitors load the page.
DOM-based Cross-Site Scripting
DOM-based XSS modifies and injects code into the domain object model (DOM) on the visitor’s browser. Only affecting the client side, the response on the server remains the same. Examples include websites that write unstructured data from the URL into the web page.
Best Practices for Identifying XSS Vulnerabilities and Preventing Attacks
By following web application security best practices, you can keep your website visitors protected and your reputation intact.
Identify Web Application Vulnerabilities
The first step is to check your web application for any vulnerabilities to XSS attacks. You can either do this manually or through dedicated tools:
- Manual code review: Conduct code analysis to identify weak points where bad actors could inject malicious scripts into your site. Additionally, you can use test attack payloads to determine if it is possible to execute benign scripts on browsers visiting your site.
- Automated web vulnerability scanners: Tools are available to scan web applications and identify XSS vulnerabilities. These typically use static and dynamic JavaScript analysis.
Validate Input
Your web application requires robust input validation procedures to sanitize all user input. Examples include:
- Validating URLs have a safe protocol.
- Checking that user-supplied numeric values only contain integers.
- Ensuring inputs have the expected set of characters.
By blocking invalid inputs, you go a long way to preventing untrusted sources from injecting code into your website.
Encode Output
The next step is to encode all user-controlled data written to the website. The encoding required depends on context and may require multiple combinations, such as both Javascript and HTML layers of encoding.
Enforce Content Security Policy
If both these preventative techniques fail, content security policies (CSPs) help mitigate the impact of an XSS attack. They limit whether external scripts load, allowing you to specify they only load from the same origin. With proper CSP enforcement, you can reduce the success of XSS attacks even if the attacker was able to inject the malicious code.
Set HTTP-only Cookies
By flagging HTTP-only cookies, session cookies can only be captured by client-side scripts. This blocks access to JavaScript code, significantly reducing the chances of session hijacking from XSS.
Implement a Web Application Firewall
Finally, implementing a web application firewall (WAF) monitors and filters traffic, enabling you to develop and enforce security policies that target XSS attacks. This includes scanning URLs and other input fields to identify and block malicious code reflected to users. Many WAFs now also include AI and machine learning technology to analyze contextual information and spot attempts to bypass security policies using unknown XSS attack variations.
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