Phil Strujon REST server - is there really a security vulnerability in Digest Auth or am I misunderstood something?

Question

I recently uploaded Phil Stroujon's REST server for CodeIgniter. I looked at the source code, and when I came to Digest authentication, I saw the following code:

if ($this->input->server('PHP_AUTH_DIGEST')) { $digest_string = $this->input->server('PHP_AUTH_DIGEST'); } elseif ($this->input->server('HTTP_AUTHORIZATION')) { $digest_string = $this->input->server('HTTP_AUTHORIZATION'); } else { $digest_string = ""; } 

And a little later, after some checks for the absence of $ digest_string and the presence of a username:

 // This is the valid response expected $A1 = md5($digest['username'].':'.$this->config->item('rest_realm').':'.$valid_pass); $A2 = md5(strtoupper($this->request->method).':'.$digest['uri']); $valid_response = md5($A1.':'.$digest['nonce'].':'.$digest['nc'].':'.$digest['cnonce'].':'.$digest['qop'].':'.$A2); if ($digest['response'] != $valid_response) { header('HTTP/1.0 401 Unauthorized'); header('HTTP/1.1 401 Unauthorized'); exit; } 

On Wikipedia, I see the following text about HTTP Digest Auth:

For subsequent requests, the hexadecimal request counter (nc) must be greater than the last used value, otherwise the attacker can simply “play” the old request with the same credentials. It depends on the server to ensure that the counter is incremented for each of the nonce values ​​that it issued, accordingly rejecting any bad requests.

The server must remember the nonce values ​​that it recently created. It can also remember when each nonce value was released, expiring them after a certain time. If an expired value is used, the server should respond with the status code "401" and add stale = TRUE in the authentication header, indicating that the client should resend with the new one provided without use, without asking the user for a different username and password.

However, I see nothing about checking cnonce, nc, or nonce in the source code. Does this mean that someone who registered a request from the client to the server that transmitted the authentication can simply “replay” it in the future and get a new resource value?

Is it really vulgar? Or didn’t I understand something?

Answer 1

I noticed this too, looking at codeigniter-restserver. It is vulnerable to repeated attacks because, as you said, it does not use nonce.

Digest authentication requires a handshake:

• the client executes the request with authorization. It will not work because the client does not yet know that nonce
• the server responds with a WWW-Authenticate header that contains the correct nonce value to use
• the client makes the same request using nonce specified in the server response Server
• validates matches and provides the requested URL.

To do this, you need to start a session on the REST server in order to remember nonce. The lightweight nonce provisioning scheme is always unique to base it on the current time using a function like uniqid ()