Cross-site scripting (XSS)

  • Fabian Sinner
  • December 5, 2023

Table of content

    Cross-site scripting (XSS)

    Cross-site scripting (XSS) describes a security vulnerability that occurs in web applications when an attacker successfully inserts malicious code (usually in the form of scripts) into web pages that other users see and execute.

    XSS attacks have a variety of uses, including stealing sensitive information such as session tokens or other personal data, taking over the identity of a user in a session, or tricking the user into performing unwanted actions on the affected website. To protect themselves from XSS attacks, web developers should validate and sanitize input and ensure that website content is processed securely.

    How does Cross-site scripting work?

    In Cross-site scripting, the attacker finds a vulnerability in a web application that allows them to inject their own code. This can be done, for example, by direct input into forms, by manipulating URL parameters, or by posting comments.

    When another user visits the manipulated website, the embedded malicious code is executed as part of the website in the user’s browser. Since the browser assumes that the code comes from a trusted source (the website itself), the code is executed.

    The malicious code can perform a variety of actions, depending on the attacker’s intentions:

    • Stealing cookies or session tokens, which allows the attacker to take over the user’s identity.
    • Redirecting the user to a malicious website.
    • Accessing and modifying website content, which can lead to phishing attacks or spreading false information.
    • Performing actions on user’s behalf on the website.

    In some cases, the XSS attack can also spread beyond the affected website; for example, by tricking users into sharing links containing the malicious code.

    What types of Cross-site scripting are there?

    A distinction is made between three different types of Cross-site scripting:

    Reflected XSS

    This type of XSS occurs when the attacker tricks a user into opening a specially crafted URL that sends malicious code to the vulnerable website. The server inserts this code into the response that is sent back to the user’s browser. The code is then executed in the user’s browser.

    Stored XSS

    In this form of attack, the malicious code is stored in a database, message forum, comment field or other location on the server. When other users visit the affected page, the stored malicious code is executed.

    DOM-based XSS

    In this case, the malicious code is not executed via the server, but directly in the DOM (Document Object Model) of the website in the user’s browser. This can happen if a website uses JavaScript that inserts the user’s input into the page without validating or sanitizing it appropriately.

    What are the effects of Cross-site scripting?

    Cross-site scripting can have a number of serious consequences, both for the users and the operator of the affected website.

    XSS attacks often allow an attacker to steal cookies or session tokens. This information can be used to take over a user’s identity and gain unauthorized access to their account. Attackers can also alter the appearance or content of a website to carry out fraudulent activities, such as collecting sensitive user information through fake login forms.

    XSS can be used to create phishing pages that mimic legitimate websites in order to trick users into revealing sensitive information. XSS can also be used to spread malware by tricking visitors into downloading malicious software from the compromised website.

    An attacker could use XSS to launch browser-based network attacks against other systems, such as by exploiting the trust that other systems place in the user. Security breaches caused by XSS can seriously affect user trust in a website or brand.

    Organizations affected by XSS attacks can face legal consequences and suffer financial losses, e.g., through claims for damages, fines, and the cost of fixing the security vulnerabilities.

    To minimize these risks, it is important that web developers apply security best practices, such as validating and sanitizing input data, implementing content security policy (CSP), and using secure coding practices.

    How can such an attack be prevented?

    Preventing Cross-site scripting requires a combination of secure coding practices, careful validation and sanitization of user input, and the implementation of security measures on the website.

    • Input validation: All input, especially any coming from users, should be validated. This includes checking for correct data types, length restrictions and filtering potentially dangerous characters.
    • Output escaping: When displaying data that originates from users or comes from an insecure source, it can be “escaped”. This means that special characters that have a meaning in HTML, JavaScript or other scripting languages are converted into a secure form.
    • Use of Content Security Policy (CSP): CSP is a security technique that allows web servers to define the type of content that may be executed in the browser. CSP can be configured to restrict or prevent the execution of JavaScript unless it comes from trusted sources.
    • Secure framework and templates: Many modern web frameworks and template engines have built-in mechanisms to prevent XSS by automatically escaping input. Using these tools can help to avoid many XSS vulnerabilities from the outset.
    • Provide cookies with the HttpOnly attribute: If cookies are given the HttpOnly attribute, client-side scripts can be prevented from accessing them.
    • Regular security checks and code reviews: Regular security checks and code reviews can ensure that security vulnerabilities are identified and fixed before they can be exploited.

    By applying these measures, developers and organizations can significantly reduce the risk of XSS attacks and strengthen the security of their web applications.

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