Implement Cross-Site Request Forgery (CSRF) Protection in Go

Web applications are vulnerable to Cross-Site Request Forgery (CSRF) attacks where an attacker tricks a victim into performing an unwanted action on a web application in which they are currently authenticated. This challenge requires you to implement a robust CSRF protection mechanism in a Go web application to mitigate these attacks.

Problem Description

Your task is to build a simple Go web application that demonstrates and implements CSRF protection for forms that modify data. The application should have at least two routes:

A GET route to display a form for submitting data (e.g., creating a new user, updating a profile). This form should include a hidden CSRF token.
A POST route that handles the form submission. This route must validate the CSRF token before proceeding with the data processing.

You need to use a secure method to generate and validate CSRF tokens. The tokens should be unique per session or request, and invalidated after use (or regenerated for subsequent requests).

Key Requirements:

Token Generation: Implement a mechanism to generate unique and unpredictable CSRF tokens.
Token Inclusion: The generated token must be embedded within the HTML form, typically as a hidden input field.
Token Validation: The POST handler must extract the submitted token and compare it against the token stored on the server (e.g., in the user's session).
Error Handling: If the token is missing or invalid, the application should return an appropriate error response (e.g., HTTP 403 Forbidden).
Security: Tokens should be difficult to guess or tamper with. Consider using a secure random number generator.
Session Management: You will need a basic session management mechanism to store the CSRF token on the server-side, associated with the user's session. For simplicity in this challenge, you can simulate sessions using a global map or a simple in-memory store, but acknowledge that a real application would use more robust session storage.

Expected Behavior:

When a user requests the form page (GET), a unique CSRF token is generated and embedded in the form.
When a user submits the form with a valid token, the action is processed successfully.
When a user submits the form with an invalid, missing, or outdated token, the action is rejected with an error.

Edge Cases to Consider:

A user submitting the form without JavaScript enabled (CSRF token will be in the HTML form).
A user submitting the form with JavaScript (CSRF token can be sent via AJAX, but for this challenge, focus on form submission).
Handling multiple concurrent requests from the same user.
Token expiration (though for this challenge, you can focus on validation against the current session token).

Examples

Example 1: Successful Form Submission

Request 1 (GET /form):

Method: GET
Path: /form

Server Response 1:

Status: 200 OK
Content-Type: text/html

<!DOCTYPE html>
<html>
<head>
    <title>Submit Data</title>
</head>
<body>
    <form action="/submit" method="post">
        <input type="hidden" name="_csrf_token" value="generated_secure_token_abc123">
        <label for="data">Data:</label>
        <input type="text" id="data" name="data"><br><br>
        <input type="submit" value="Submit">
    </form>
</body>
</html>

Explanation: The server generates a unique CSRF token (generated_secure_token_abc123), stores it in the user's session, and embeds it as a hidden field in the HTML form.

Request 2 (POST /submit):

Method: POST
Path: /submit
Content-Type: application/x-www-form-urlencoded

_csrf_token=generated_secure_token_abc123&data=some_value

Server Response 2:

Status: 200 OK
Content-Type: text/plain

Data submitted successfully!

Explanation: The server retrieves the CSRF token from the session, compares it with the submitted token (generated_secure_token_abc123). Since they match, the submission is processed.

Example 2: Failed Form Submission (Invalid Token)

Request 1 (GET /form):

Method: GET
Path: /form

Server Response 1:

Status: 200 OK
Content-Type: text/html

<!DOCTYPE html>
<html>
<head>
    <title>Submit Data</title>
</head>
<body>
    <form action="/submit" method="post">
        <input type="hidden" name="_csrf_token" value="generated_secure_token_xyz789">
        <label for="data">Data:</label>
        <input type="text" id="data" name="data"><br><br>
        <input type="submit" value="Submit">
    </form>
</body>
</html>

Explanation: A CSRF token (generated_secure_token_xyz789) is generated and embedded.

Request 2 (POST /submit):

Method: POST
Path: /submit
Content-Type: application/x-www-form-urlencoded

_csrf_token=tampered_or_old_token_456&data=some_value

Server Response 2:

Status: 403 Forbidden
Content-Type: text/plain

CSRF token validation failed.

Explanation: The server compares the submitted token (tampered_or_old_token_456) with the one stored in the session (generated_secure_token_xyz789). They do not match, so the request is rejected.

Example 3: Failed Form Submission (Missing Token)

Request 1 (GET /form):

Method: GET
Path: /form

Server Response 1:

Status: 200 OK
Content-Type: text/html

<!DOCTYPE html>
<html>
<head>
    <title>Submit Data</title>
</head>
<body>
    <form action="/submit" method="post">
        <input type="hidden" name="_csrf_token" value="generated_secure_token_pqr111">
        <label for="data">Data:</label>
        <input type="text" id="data" name="data"><br><br>
        <input type="submit" value="Submit">
    </form>
</body>
</html>

Explanation: A CSRF token (generated_secure_token_pqr111) is generated and embedded.

Request 2 (POST /submit):

Method: POST
Path: /submit
Content-Type: application/x-www-form-urlencoded

data=some_value

Server Response 2:

Status: 403 Forbidden
Content-Type: text/plain

CSRF token validation failed.

Explanation: The server expects a CSRF token in the form data, but it is missing. The request is rejected.

Constraints

The Go application must be runnable using standard Go tooling.
You are expected to use the standard net/http package for handling HTTP requests and responses.
For token generation, use crypto/rand for cryptographically secure randomness.
Session management can be simplified using an in-memory store (e.g., map[string]string) for this challenge. Avoid using external session libraries if possible to focus on the CSRF logic itself.
The CSRF token should be approximately 32 bytes in length (or an equivalent secure random string).

Notes

Consider using a library like github.com/gorilla/csrf for inspiration on how to structure your CSRF logic, but implement the core token generation and validation yourself for this challenge.
Think about how the CSRF token should be associated with a user's session. For a simple challenge, a cookie could be used to identify the session.
The server-side storage of the CSRF token is crucial. It should be stored securely and be accessible for validation during POST requests.
Ensure your token generation is truly random and not predictable.
When regenerating tokens, consider the trade-offs between security and user experience. For this challenge, regenerating a token for each new form display is sufficient.

Implement Cross-Site Request Forgery (CSRF) Protection in Go

Problem Description

Your task is to build a simple Go web application that demonstrates and implements CSRF protection for forms that modify data. The application should have at least two routes:

A GET route to display a form for submitting data (e.g., creating a new user, updating a profile). This form should include a hidden CSRF token.
A POST route that handles the form submission. This route must validate the CSRF token before proceeding with the data processing.

You need to use a secure method to generate and validate CSRF tokens. The tokens should be unique per session or request, and invalidated after use (or regenerated for subsequent requests).

Key Requirements:

Token Generation: Implement a mechanism to generate unique and unpredictable CSRF tokens.
Token Inclusion: The generated token must be embedded within the HTML form, typically as a hidden input field.
Token Validation: The POST handler must extract the submitted token and compare it against the token stored on the server (e.g., in the user's session).
Error Handling: If the token is missing or invalid, the application should return an appropriate error response (e.g., HTTP 403 Forbidden).
Security: Tokens should be difficult to guess or tamper with. Consider using a secure random number generator.
Session Management: You will need a basic session management mechanism to store the CSRF token on the server-side, associated with the user's session. For simplicity in this challenge, you can simulate sessions using a global map or a simple in-memory store, but acknowledge that a real application would use more robust session storage.

Expected Behavior:

When a user requests the form page (GET), a unique CSRF token is generated and embedded in the form.
When a user submits the form with a valid token, the action is processed successfully.
When a user submits the form with an invalid, missing, or outdated token, the action is rejected with an error.

Edge Cases to Consider:

A user submitting the form without JavaScript enabled (CSRF token will be in the HTML form).
A user submitting the form with JavaScript (CSRF token can be sent via AJAX, but for this challenge, focus on form submission).
Handling multiple concurrent requests from the same user.
Token expiration (though for this challenge, you can focus on validation against the current session token).

Examples

Example 1: Successful Form Submission

Request 1 (GET /form):

Method: GET
Path: /form

Server Response 1:

Status: 200 OK
Content-Type: text/html

<!DOCTYPE html>
<html>
<head>
    <title>Submit Data</title>
</head>
<body>
    <form action="/submit" method="post">
        <input type="hidden" name="_csrf_token" value="generated_secure_token_abc123">
        <label for="data">Data:</label>
        <input type="text" id="data" name="data"><br><br>
        <input type="submit" value="Submit">
    </form>
</body>
</html>

Explanation: The server generates a unique CSRF token (generated_secure_token_abc123), stores it in the user's session, and embeds it as a hidden field in the HTML form.

Request 2 (POST /submit):

Method: POST
Path: /submit
Content-Type: application/x-www-form-urlencoded

_csrf_token=generated_secure_token_abc123&data=some_value

Server Response 2:

Status: 200 OK
Content-Type: text/plain

Data submitted successfully!

Explanation: The server retrieves the CSRF token from the session, compares it with the submitted token (generated_secure_token_abc123). Since they match, the submission is processed.

Example 2: Failed Form Submission (Invalid Token)

Request 1 (GET /form):

Method: GET
Path: /form

Server Response 1:

Status: 200 OK
Content-Type: text/html

<!DOCTYPE html>
<html>
<head>
    <title>Submit Data</title>
</head>
<body>
    <form action="/submit" method="post">
        <input type="hidden" name="_csrf_token" value="generated_secure_token_xyz789">
        <label for="data">Data:</label>
        <input type="text" id="data" name="data"><br><br>
        <input type="submit" value="Submit">
    </form>
</body>
</html>

Explanation: A CSRF token (generated_secure_token_xyz789) is generated and embedded.

Request 2 (POST /submit):

Method: POST
Path: /submit
Content-Type: application/x-www-form-urlencoded

_csrf_token=tampered_or_old_token_456&data=some_value

Server Response 2:

Status: 403 Forbidden
Content-Type: text/plain

CSRF token validation failed.

Example 3: Failed Form Submission (Missing Token)

Request 1 (GET /form):

Method: GET
Path: /form

Server Response 1:

Status: 200 OK
Content-Type: text/html

<!DOCTYPE html>
<html>
<head>
    <title>Submit Data</title>
</head>
<body>
    <form action="/submit" method="post">
        <input type="hidden" name="_csrf_token" value="generated_secure_token_pqr111">
        <label for="data">Data:</label>
        <input type="text" id="data" name="data"><br><br>
        <input type="submit" value="Submit">
    </form>
</body>
</html>

Explanation: A CSRF token (generated_secure_token_pqr111) is generated and embedded.

Request 2 (POST /submit):

Method: POST
Path: /submit
Content-Type: application/x-www-form-urlencoded

data=some_value

Server Response 2:

Status: 403 Forbidden
Content-Type: text/plain

CSRF token validation failed.

Explanation: The server expects a CSRF token in the form data, but it is missing. The request is rejected.

Constraints

The Go application must be runnable using standard Go tooling.
You are expected to use the standard net/http package for handling HTTP requests and responses.
For token generation, use crypto/rand for cryptographically secure randomness.
Session management can be simplified using an in-memory store (e.g., map[string]string) for this challenge. Avoid using external session libraries if possible to focus on the CSRF logic itself.
The CSRF token should be approximately 32 bytes in length (or an equivalent secure random string).

Notes

Consider using a library like github.com/gorilla/csrf for inspiration on how to structure your CSRF logic, but implement the core token generation and validation yourself for this challenge.
Think about how the CSRF token should be associated with a user's session. For a simple challenge, a cookie could be used to identify the session.
The server-side storage of the CSRF token is crucial. It should be stored securely and be accessible for validation during POST requests.
Ensure your token generation is truly random and not predictable.
When regenerating tokens, consider the trade-offs between security and user experience. For this challenge, regenerating a token for each new form display is sufficient.