Token Authentication

Overview

In order to support 389-ds-portal, we need to issue cookies to the client that allow the flask application to rebind as a user. This is so that the flask application holds no state, no persistent connections, and no need to encrypt data etc. We could even have the flask backend behind a load balancer without sticky sessions. This means flask can not hold any state.

However, LDAP is modeled so that a connection is “long lived”. This doesn’t work with the stateless model that we want to achieve with flask. LDAP does a bind (auth), then changes and searches are sent over the authenticated connection as a whole session.

What is required is a way to allow the client session (cookie) to contain the needed data to allow a new connection, bind, operation and unbind to occur.

Current (Insecure) Method

The current proof of concept 389-ds-portal uses fernet tokens in flask. If the bind succeeds we encrypt the dn and password that were used into a fernet token, that is issued in a cookie to the client.

This means the cookie can be re-used on other hosts - modern technologies like same-site policies help to prevent this, but there is still potential for risk as the password contained within contains no expiry. The risk of someone bruteforcing the fernet token is low however, because this uses randomised key material in the poc.

Planned Method

Instead, we should not encrypt user passwords. We should make 389 responsible for issuing the fernet tokens, that contain randomised or nonce-values instead.

To achieve this we would:

simple_bind_s(dn, password)
token = extop_s(cookie_issue)

To then reauth, the token which is a base64 string would be used in simple_bind_s

simple_bind_s(dn, token)
...

Considerations

If you have used a token in the simple_bind_s, you should not be able to perform an extop_s to issue a subsequent token. This is to prevent infinite extension of sessions.

The tokens would be fernet, with a time limit applied - which is effectively the session limit.

The fernet encryption key would be stored in cn=config, and if there are multiple servers must be the same between them all to allow sessions to be used between servers.

Implementation Details

Authenticating the Token

Fernet tokens are an aes128 + hmac token containing a timestamp. If the token doesn’t pass hmac, doesn’t decrypt with the key, or the timestamp has passed currenttime + ttl, then the decryption “fails”. If it fails, we reject the auth. If it passes, we accept it as a token session.

This means in simple bind we:

check if the input is a token
	true:
		if decrypt token:
			auth as tokenand mark conn-auth = token, return
continue simple bind as normal

To issue the token:

if conn-auth != token && dn != anonymous:
	create token and return to the client
Last modified on 5 December 2019