Posted 9 months ago. Visible to the public.

Rails: Verify the CSRF token

Rails uses a CSRF token in forms and AJAX requests to verify a user request. Internally it compares the injected CSRF token of the form data with the CSRF token in the encrypted user session. To prevent SSL BREACH attacks, the CSRF token from the form data is masked.

To better debug issues, when these tokens do not match, it is useful to unmask the CSRF token from the form data. Here is a method that works for Rails 5.2.

def decode_csrf_token(csrf_token) decoded_masked_token = Base64.strict_decode64(csrf_token) one_time_pad = decoded_masked_token[0...32] encrypted_csrf_token = decoded_masked_token[32..-1] encrypted_csrf_token_bytes = encrypted_csrf_token.bytes one_time_pad.each_byte.with_index { |c1, i| encrypted_csrf_token_bytes[i] ^= c1 } csrf_token = encrypted_csrf_token_bytes.pack("C*") Base64.strict_encode64(csrf_token) end

Example form:

<input type="hidden" name="authenticity_token" value="af3uqNHd1MDskKeJH7+AiV7o8gozjqCsu78rbndSG/TlE+KPrKe2l3pltzi4O0xtFnUX3cRfsB+MxfP4BXvT6Q==">

Example usage:

decode_csrf_token("af3uqNHd1MDskKeJH7+AiV7o8gozjqCsu78rbndSG/TlE+KPrKe2l3pltzi4O0xtFnUX3cRfsB+MxfP4BXvT6Q==") => jO4MJ316YleW9RCxp4TM5Eid5df30RCzN3rYlnIpyB0=

Rails will only accept this request if jO4MJ316YleW9RCxp4TM5Eid5df30RCzN3rYlnIpyB0= is the same as the value in your cookie:

{ "session_id"=>"8ef662867ab2457717ba74c143c08733", "timestamp"=>1572261371, "_csrf_token"=>"jO4MJ316YleW9RCxp4TM5Eid5df30RCzN3rYlnIpyB0=" }

Note: The session cookie is encrypted by default. You can decrypt it with Rails: Decrypt a session cookie.

makandra has been working exclusively with Ruby on Rails since 2007. Our laser focus on a single technology has made us a leader in this space.

Owner of this card:

Emanuel De
Last edit:
9 months ago
by Emanuel De
About this deck:
We are makandra and do test-driven, agile Ruby on Rails software development.
License for source code
Posted by Emanuel De to makandra dev
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