"Oracle native authentication protocols are typical challenge-response protocols. After some negotiation the client sends the username. If the user exists the server sends an encrypted key. The client uses the key to encrypt the user's password and sends it to the server. One of the protocols is documented quite well in [1.]. On reading that description it is quite obvious that the protocol is vulnerable against the off-line brute force attack. Oracle changed the algorithm
in 9i and changed it again in 10g. If we use the OCI driver, our programs will use these newer protocols, but thin drivers use the older version, thus implementing an off-line brute forcer is not
absolutely pointless (if you can sniff the connection you can conduct several other attacks of course). The servers and the clients support the older version of the protocol, thus it is worth a research whether downgrade attack is possible.
This article describes four versions of the Oracle native authentication. These information are based on [3.]. This description is shorter than [3.] and just emphasizes those differences that could
be important in a downgrade attack againts the (SEEMS TO BE stronger) newer authentication protocols.
We do not disclose the details of the downgrading. In the Downgrading chapter you can find screenshots about a successful attack to prove that downgrading is possible."