TL;DR
Your anti-CSRF scanner is still flagging issues even after adding a _csrf token to your login form? This usually means the token isn’t being handled correctly throughout the entire request lifecycle. This guide walks you through common causes and fixes.
Checking Your CSRF Protection
Verify Token Inclusion: Double-check that the _csrf input field is present in your login form’s HTML source code.
<input type=”hidden” name=”_csrf” value=”{{ csrf_token }}” />
Confirm Token Generation: Ensure the _csrf token is being generated on the server-side for each new session or request. The method varies depending on your framework (e.g., Spring Security, Django).
For example, in a Python/Flask application using WTForms:
form = LoginForm()
if form.validate_on_submit():
# … other validation logic…
csrf_token = generate_csrf_token()
# Store csrf_token in session
Session Management: The token *must* be stored securely in the user’s session. Check your session configuration to ensure it’s not being overwritten or expiring prematurely.
Common issues include incorrect cookie settings (e.g., HttpOnly flag missing, short expiry time).
Handling the Token in Your Login Route
Token Validation: The server-side login route *must* validate that the submitted _csrf token matches the one stored in the session.
If they don’t match, reject the request immediately.
Example (simplified Python/Flask):
if form.validate_on_submit():
submitted_token = request.form.get(‘_csrf’)
session_token = session.get(‘csrf_token’)
if submitted_token != session_token:
# Log the attempt!
abort(403) # Or return an error message
Token Regeneration: After successful login, *always* regenerate the _csrf token to prevent session fixation attacks.
This ensures that even if an attacker obtained a valid token before login, it’s no longer usable after authentication.
Example (Python/Flask):
if form.validate_on_submit():
# … validation and authentication…
csrf_token = generate_csrf_token()
session[‘csrf_token’] = csrf_token
Common Pitfalls
AJAX Requests: If your login form uses AJAX, ensure the _csrf token is included in *every* request header or as part of the POST data.
Most JavaScript frameworks provide ways to automatically include CSRF tokens.
Redirects and Postbacks: If your login process involves redirects, make sure the _csrf token is carried over correctly (usually via session).
Multiple Forms: If you have multiple forms on a page, each form should ideally have its own unique CSRF token.
Double Submission Cookies: Some frameworks use double-submit cookies as an additional layer of protection. Ensure these are also correctly configured if used.
These typically require setting a random value in both a cookie and a hidden form field, then verifying they match on the server.
Debugging Tips
Browser Developer Tools: Use your browser’s developer tools to inspect network requests and verify that the _csrf token is being sent with each request.
Server-Side Logging: Add logging statements to your login route to print the submitted token, the session token, and whether they match. This helps pinpoint discrepancies.
Scanner Configuration: Review your scanner’s configuration to ensure it’s correctly identifying CSRF vulnerabilities based on your framework and implementation.
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