What is Cross-Site Request Forgery?

Cross-Site Request Forgery (CSRF), also known as one-click attack or session riding, is a web security vulnerability that allows attackers to trick users into performing actions they do not intend to perform.

These actions are performed on a web application where the user is authenticated, and the attacker exploits the fact that the web application trusts the user’s browser, which automatically sends session cookies or other credentials along with the request.

This type of attack can lead to serious consequences, such as unauthorized fund transfers, credential changes, or even account takeovers.

In this article, we will explore the mechanics of CSRF, how it works, the potential impact of a successful CSRF attack, examples of vulnerabilities, and the various methods of preventing CSRF.

Additionally, we will look at real-world instances of CSRF attacks to illustrate the dangers of this vulnerability.

What is CSRF?

CSRF attacks take advantage of the fact that modern web applications rely on authentication mechanisms, such as session cookies, to identify users.

When a user is authenticated, the browser automatically includes the session cookie in any subsequent request to the web application.

This is where the vulnerability lies. An attacker can craft a malicious request that is executed by the victim’s browser without their knowledge.

Since the session cookie is automatically included, the web application processes the request as if it had been made by the victim.

Key Characteristics:

1. Exploitation of Trust: CSRF exploits the trust that a web application has in a user’s browser.
2. Unintentional Actions: The victim unknowingly performs actions on the web application, such as changing account details, transferring money, or deleting data.
3. Session Management: This attack often succeeds because modern browsers automatically include session cookies in cross-origin requests, unless mitigations like CSRF tokens or SameSite cookies are in place.

How CSRF Works

To understand how CSRF works, it is essential to break down the conditions that must be met for an attack to succeed. According to a common breakdown, the following conditions make a CSRF attack possible:

Conditions for a CSRF Attack:

1. Relevant Action: There must be an action that the attacker wants the victim to perform. This could be a privileged action (such as modifying user permissions) or any action on user-specific data (such as changing the victim’s password).
2. Cookie-based Session Handling: The web application must rely on cookies for session management, and the browser must automatically include these cookies with any request to the application.
3. No Unpredictable Request Parameters: The attacker must be able to determine all the necessary parameters to execute the request. For example, if the request to change a password requires the current password, the attack will fail unless the attacker knows this password.

Anatomy of a CSRF Attack

1. Constructing the Malicious Request: The attacker creates a malicious request that, when executed, will act on behalf of the victim. This can be done using an HTML form, a link, or even an image tag.

Example:

2. Inducing the Victim to Execute the Request: The attacker needs to trick the victim into visiting a malicious page or clicking on a malicious link. For example, the attacker might send the victim an email with a link to the malicious page or embed the malicious request in a webpage that the victim is likely to visit.
3. The Victim’s Browser Sends the Request: When the victim visits the malicious page, the browser automatically sends the request to the target web application, including the victim’s session cookie.
4. The Web Application Processes the Request: The web application processes the request as if it were made by the victim because the session cookie is automatically included. The action is carried out without the victim’s knowledge.

Example of a CSRF Attack

Let’s assume a web application allows users to change their email address with a simple HTTP request like the following:

POST /email/change HTTP/1.1
Host: vulnerable-website.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 30
Cookie: session=yvthwsztyeQkAPzeQ5gHgTvlyxHfsAfE
[email protected]

An attacker can craft a malicious request that changes the victim’s email address to their own:

When the victim visits the page with the malicious form, their browser will automatically include the session cookie, and the vulnerable web application will change the email address associated with the victim’s account.

Impact of CSRF Attacks

The impact of a CSRF attack depends on the nature of the action that the attacker can induce the victim to perform. Some common consequences include:

1. Account Takeover: If the attacker can change the victim’s email address or reset their password, they can take control of the victim’s account.
2. Fraudulent Transactions: On banking or e-commerce websites, CSRF attacks can result in unauthorized money transfers or purchases.
3. Data Manipulation: Attackers can modify personal information, delete data, or change user settings.
4. Privilege Escalation: If the victim has administrative privileges, the attacker could use CSRF to gain control over the entire application or sensitive data.

Examples of CSRF Vulnerabilities

1. Changing User Information: A common example of a CSRF attack is changing the email address or password of a user’s account. If the web application does not properly validate the origin of requests, an attacker can easily update the victim’s credentials.
2. Submitting Orders or Payments: In e-commerce websites, an attacker can use CSRF to submit an order or initiate a payment on behalf of the victim.
3. Modifying User Permissions: In more complex web applications, if a CSRF vulnerability exists, attackers can modify user roles or permissions, leading to privilege escalation.

How to Prevent CSRF

Several strategies can mitigate the risk of CSRF attacks. These defenses aim to ensure that the application can differentiate between legitimate requests from the user and malicious requests from an attacker.

1. CSRF Tokens

The most effective defense against CSRF is the use of CSRF tokens. A CSRF token is a unique, secret, and unpredictable value that is generated by the server.

This token is included in every form submission or sensitive request. The server validates the token to ensure the request is coming from the intended user.

How it works:

• The server generates a CSRF token and includes it in the HTML form as a hidden field.
• When the form is submitted, the server checks whether the token matches the one stored on the server.
• If the token is missing or incorrect, the request is rejected.

Example: