Cross-Site Request Forgery (CSRF) is a subtle but dangerous web security issue that can compromise user accounts and sensitive data without the victim even realizing it.
Imagine if someone tricked you into clicking a seemingly innocent link, only to discover that your account on a website has been used to perform unwanted actions, like changing your account settings or making unauthorized transactions. That’s exactly what CSRF does.
It takes advantage of the trust between a user’s browser and a website by making unauthorized requests on behalf of the user. This guide will explain how CSRF attacks work, their potential impacts, and the steps you can take to protect your web applications from these hidden threats.
What is CSRF?
Cross-site request Forgery is a type of attack that tricks a website into thinking that a request is coming from a trusted user. An attacker uses social engineering tricks to get you, the user, to click on a seemingly harmless link in an email or chat message. Because you’re already logged into a site, the website thinks the request is from you. This allows the attacker to perform actions on the site using your account without you knowing.
For example, the attacker might use your account to make unauthorized changes, transfer money, or even compromise the entire website if you’re an admin with high-level access.
Cross-site request Forgery attacks can harm both the user and the business running the site. They can lead to stolen money, damaged reputations, and loss of customer trust. Well-known sites like Gmail and Facebook have been targeted by CSRF attacks. Cross-Site Request Forgery is also known by other names like XSRF, session riding, or one-click attacks, and is listed as a serious issue in the OWASP Top 10. What is an Open Web Application Security Project (OWASP)?
History and Real-World Example
CSRF vulnerabilities have been recognized since the early 2000s. Despite their long history, they remain a significant threat due to the evolving complexity of web applications and the continuous development of new exploitation techniques.
Notable case studies and examples include:
Case Study 1: Netflix (2006)
In 2006, a CSRF vulnerability in Netflix’s website allowed attackers to add DVDs to a user’s rental queue and alter account details. The exploit involved crafting a malicious webpage that included a form submission to Netflix’s server. When the victim visited the page while logged into their Netflix account, the browser automatically included the session cookies, allowing the attack to succeed.
Case Study 2: ING Direct
Attackers used CSRF to initiate unauthorized money transfers from accounts at ING Direct. The attack involved tricking users into visiting a malicious webpage that submitted a form to ING Direct’s server, initiating a transfer. The attack succeeded because ING Direct’s website lacked proper CSRF protection mechanisms.
Case Study 3: YouTube (2008)
In 2008, a CSRF vulnerability in YouTube allowed attackers to perform actions on behalf of users, such as adding videos to favourites or subscribing to channels. The attack involved crafting a malicious webpage that included a form submission to YouTube’s server. When users visited the page while logged into YouTube, their browser automatically included the session cookies, making the request appear legitimate.
Case Study 4: Router DNS Settings (2018)
In 2018, CSRF attacks targeted router DNS settings, redirecting users to malicious websites. Attackers crafted malicious web pages that included form submissions to the router’s configuration page. When users visited the page while logged into their router’s web interface, the browser automatically included the session cookies, allowing the attack to succeed. This prompted manufacturers to release security updates to mitigate the vulnerability.
How Does CSRF Work?
When you visit a website, your browser often sends along your login details automatically. This can include things like your session cookie, IP address, or other authentication info to make it easier for you to stay logged in.
However, attackers can exploit this convenience. They can trick a website into thinking a request is coming from you, even if it’s not. Here’s how a CSRF attack usually unfolds:
- Tricking the Victim: The attacker convinces you to click a malicious link, which could be sent through email, a chat message, or another method.
- Sending the Fake Request: This link or a webpage you visit requests a website where you’re already logged in.
- Acting on Your Behalf: Because you’re logged in, the website can’t tell the difference between your legitimate requests and the fake ones. It ends up carrying out actions as if they were from you, without your knowledge.
Overall, CSRF takes advantage of the fact that your browser automatically includes your credentials in requests, allowing attackers to impersonate you and make changes on your behalf.
Example Scenario
Imagine a banking website where users can transfer money using a form that submits a POST request to transfer. If this form does not include proper CSRF protection, an attacker can create a malicious web page with a hidden form that automatically submits a request to transfer money from the victim’s account to the attacker’s account. When the victim visits the malicious page while logged into their banking site, the transfer occurs without their knowledge.
What Are the Characteristics of CSRF?
Cross-Site Request Forgery (CSRF) attacks occur when an attacker tricks a user into unknowingly making a harmful web request to a site to which they have access. The attacker manipulates the request so it appears legitimate to the website, exploiting the fact that the victim is already logged in.
In these attacks, the attacker takes advantage of how web browsers handle cookies. When a user is logged into a site, their browser automatically includes cookies with every request sent to that site. An attacker can craft a malicious request and trick the victim into sending it, often through a hidden link or embedded code in an email or webpage. Since the request includes the victim’s cookies, the site believes it’s coming from a legitimate user and processes it accordingly.
The key to a successful CSRF attack lies in exploiting the site’s trust in the user’s identity. Because the site trusts that the request is coming from an authenticated user, it performs actions based on that request, such as changing account settings or making unauthorized transactions. This type of attack takes advantage of the browser’s automatic inclusion of cookies and the site’s lack of additional authorization steps to verify the request’s legitimacy.
Why CSRF Attacks Happen
When you visit a website, it may pull data from other sites for you. For example, a video on a webpage might come from YouTube or Vimeo, not from the website itself. To help load content faster and use fewer resources, websites often use content delivery networks (CDNs) to store and serve these elements.
While this makes browsing smoother, it can also create security risks. If a website asks your browser to get data from another site without your permission, it can lead to a CSRF attack. This is a serious threat and is listed multiple times in the Open Web Application Security Project (OWASP) Top 10 list.
Common CSRF Issues
Lack of CSRF Protection
Websites without CSRF protection mechanisms are highly vulnerable. Without safeguards like anti-CSRF tokens or SameSite cookies, attackers can easily forge requests.
Predictable Tokens
Weak or predictable CSRF tokens can be brute-forced by attackers. Tokens should be randomly generated and unique for each session.
Misconfigured CORS
Incorrect Cross-Origin Resource Sharing (CORS) configurations can expose websites to CSRF attacks by allowing cross-origin requests that should be blocked.
Insecure Authentication
Transmitting authentication tokens over insecure channels or storing them insecurely can lead to CSRF vulnerabilities. Attackers can intercept or manipulate these tokens to perform unauthorized actions.
Best Practices
The following best practices should be taken into consideration to safeguard your web applications from CSRF attacks:
- Implement CSRF Tokens: Ensure all state-changing requests include anti-CSRF tokens.
- Use SameSite Cookies: Configure cookies with the SameSite attribute to limit their use in cross-site requests.
- Validate Referer Header: Check the Referer header to ensure requests come from trusted sources.
- Secure Authentication Mechanisms: Use HTTPS to encrypt authentication tokens and avoid storing them in insecure locations.
- Frequent Security Audits: To find and fix vulnerabilities, conduct frequent security audits and penetration tests.
- Educate Users and Developers: Ensure that both users and developers are aware of CSRF risks and the importance of adhering to security best practices.
What Are CSRF Tokens?
To protect against Cross-Site Request Forgery (CSRF) attacks, websites use a special tool called a CSRF token. This token helps ensure that requests to change data on a website are coming from a legitimate user.
- Token Creation: The web server generates a unique token and saves it.
- Token Placement: This token is added as a hidden field in forms on the website.
- Form Submission: When you submit the form, the token is included in the request data.
- Token Check: The server checks if the token in the request matches the one it saved.
- Request Validation: If the tokens match, the request is allowed; if not, it’s blocked.
This method is known as the synchronizer token pattern. It makes it hard for attackers to trick you into performing unwanted actions because they would need to guess the token.
The token should be regularly updated and invalidated when you log out. It’s also used in AJAX requests, where it’s sent as part of the request headers or parameters.
For best results, the token should be hard to guess and not follow a predictable pattern. It’s a good idea to use built-in CSRF protection features provided by frameworks like AngularJS rather than creating your own, as this can prevent mistakes and simplify implementation.
Detailed Explanation of CSRF Mitigation Techniques
SameSite Cookies
The SameSite attribute on cookies can be set to control when cookies are sent with requests initiated by third-party websites. There are three possible settings:
- Strict: The browser only sends the cookie if the request originates from the same site that set the cookie. This setting offers the highest level of security but may disrupt some legitimate functionalities, such as linking from an external site.
- Lax: The cookie is sent with top-level navigation requests and with requests originating from the same site. This setting balances security and usability, making it suitable for most applications.
- Note: Every request, including cross-site requests, is sent with a cookie. This setting should only be used when cross-site requests are necessary, and additional security measures are in place.
Anti-CSRF Tokens
Anti-CSRF tokens are a widely used method to protect against CSRF attacks. These tokens are unique, random values generated for each session and included in forms and requests. When a request is received, the server checks the token to ensure it matches the token stored for that session. This verification ensures the request originates from the legitimate user.
Implementing anti-CSRF tokens involves:
- Token Generation: Generate a unique token for each user session. The token should be cryptographically secure and unpredictable.
- Token Inclusion: Include the token in every state-changing request, typically as a hidden form field or a request header.
- Token Verification: On the server side, verify that the token included in the request matches the token stored for the session.
Double Submit Cookies
The double submit cookies technique involves sending the CSRF token in two places: a cookie and a request parameter. The server checks that the token in the cookie matches the token in the request parameter, ensuring the request’s legitimacy. This method adds an extra layer of security without requiring a state on the server side.
Custom Request Headers
Requiring custom headers, such as X-Requested-With, can help prevent CSRF attacks. Browsers restrict setting custom headers via cross-origin requests, making it difficult for attackers to forge requests with the necessary headers. This method is particularly useful for AJAX requests.
Implementing CAPTCHA
CAPTCHAs require users to solve a challenge before submitting a form, which helps prevent automated form submissions and reduces the risk of CSRF attacks. While effective, CAPTCHAs can impact user experience and should be used judiciously.
Frequently Asked Questions (FAQs)
Q 1. What is Cross-Site Request Forgery (CSRF)?
A. CSRF is a web security vulnerability where an attacker tricks an authenticated user into performing unintended actions on a website. The attacker exploits the user’s trust in the website, using malicious links or forms to execute actions without the user’s consent, potentially compromising their account or sensitive data.
Q 2. How does a CSRF attack work?
A. A CSRF attack works by tricking an authenticated user into submitting a malicious request. The user’s browser sends a request to a trustworthy website using their login credentials when they click on an attacker-created link or visit a page, acting on their behalf without the user’s awareness.
Q 3. What are common defences against CSRF attacks?
A. Common defences include using SameSite cookies to restrict cross-site requests, implementing anti-CSRF tokens that verify request authenticity, and employing referer-based validation to check the origin of requests. These methods help ensure that requests come from legitimate sources and not malicious ones.
Q 4. Why are CSRF tokens important?
A. CSRF tokens are important because they ensure that requests to perform actions are legitimate. A unique token is generated for each user session and included in state-changing requests. The server validates the token to confirm that the request is coming from the authenticated user, blocking unauthorized actions.
Q 5. What tools can help detect CSRF vulnerabilities?
Tools like OWASP ZAP, Burp Suite, and CSRFGuard can help detect CSRF vulnerabilities. These tools scan for common issues, test the effectiveness of security measures, and provide guidance on implementing robust defences against CSRF attacks.
Conclusion
Cross-Site Request Forgery (CSRF) is a serious threat to web application security that exploits the trust between users and websites. Understanding how these attacks work and their history helps organizations see why strong security measures are essential. Using strategies like SameSite cookies, anti-CSRF tokens, and custom request headers, along with secure design principles, can greatly reduce the risk of CSRF. These steps ensure that requests come from authenticated users, protecting against unauthorized actions.
