Cross-site scripting

A B C D E F G H I K M P R S T V Z
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Cross-site scripting (XSS) is an attack method that specifically targets web applications in order to inject malicious code (usually JavaScript) into a website. This code is then executed unnoticed in the user’s browser when the infected page is accessed. This allows attackers to access and manipulate confidential information such as passwords, session data or users’ personal data. XSS attacks are particularly dangerous as they exploit the trustworthiness of a website and are often difficult to detect.

Functionality and types of cross-site scripting

XSS attacks exploit vulnerabilities in web applications that allow code to be inserted into a web page that other users unknowingly execute. There are different types of XSS attacks that target different vulnerabilities in a web application:

  1. Stored XSS (persistent XSS): In this attack, the malicious code is permanently stored on the server, for example in databases or user profiles. When a user accesses the affected page, the code is automatically executed. This often happens in forums or comment functions, where the attacker leaves malicious code in a post.
  2. Reflected XSS (non-persistent XSS): Here, the malicious code is injected via a URL parameter or form submission and immediately reflected back to the user. This method is often spread via phishing links, where the attacker tricks the user into opening a prepared URL containing the code.
  3. DOM-based XSS: This variant aims to manipulate the Document Object Model (DOM) of a website so that the malicious code is executed directly in the user’s browser without server interaction. DOM-based XSS is particularly difficult to detect as the manipulation takes place on the client side.

Targets and risks of XSS attacks

XSS attacks aim to steal users’ personal information or perform malicious actions on their behalf. The most common risks and effects include

  • Data theft: The infiltrated code can intercept sensitive data such as passwords, e-mail addresses or bank details.
  • Session hijacking: Attackers can steal session cookies and log in as a legitimate user.
  • Drive-by downloads: XSS can be used to download and execute malware unnoticed.
  • Website manipulation: Attackers can change website content to deceive users or present them with malicious links.

Protective measures against cross-site scripting

To prevent XSS attacks, it is important that developers and website operators take appropriate security precautions. Here are some essential protective measures:

  1. Input validation and filtering: User input should be carefully checked and cleaned to filter out malicious code. Special filters for HTML, JavaScript and other codes can be helpful here.
  2. Encoding of output: User content should always be output in encoded form to ensure that it is not interpreted as executable code. For example, HTML encoding prevents tags such as <script> from being executed as JavaScript code.
  3. Content Security Policy (CSP): A CSP defines which scripts and content a website is allowed to execute. It blocks external scripts and limits the execution of JavaScript to trusted sources.
  4. HTTPOnly and Secure Cookies: Cookies should be marked as HTTPOnly to protect them from JavaScript access. Secure marked cookies are only transmitted via HTTPS and thus increase security.
  5. Web Application Firewalls (WAF): A WAF can recognize and block potentially harmful data traffic before it reaches the web application.
  6. Developer training: Developers should be informed about the risks and techniques for defending against XSS attacks in order to avoid security-relevant errors.

Reaction to XSS attacks

If a website falls victim to an XSS attack, operators should take immediate action to stop the attack and protect affected users:

  1. Close gaps: The vulnerability that enabled the attack should be fixed as soon as possible.
  2. Reset sessions: Users could be at risk from stolen cookies; therefore, it is advisable to reset active sessions and prompt users to log in again.
  3. Inform users: If sensitive data has been compromised, affected users should be informed and made aware of the potential risks.

In summary, cross-site scripting is a dangerous web attack that steals sensitive information or performs malicious actions on behalf of the user by injecting malicious code. Developers can help to significantly reduce the risk of an XSS attack through careful validation and security configurations such as CSP and encoding.

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