During extensive investigation of the Remote Desktop Protocol (RDP), the protocol used to connect to Windows Terminal Services, we (Cendio Systems) have found that although the information sent over the network is encrypted, there is no verification of the identity of the server when setting up the encryption keys for the session. This means RDP is vulnerable to Man In The Middle attacks (from here on referred to as MITM attacks). The attack works as follows: 1) The client connects to the server, however by some method (DNS spoofing, arp poisioning, etc.) we've fooled it to connect to the MITM instead. The MITM sends the request further to the server. 2) The server sends it's public key and a random salt, in cleartext, again through the MITM. The MITM sends the packet further to the client, but exchanges the public key to another one for which it knows the private part. 3) The client sends a random salt, encrypted with the server public key, to the MITM. 4) The MITM deencrypts the clients random salt with it's private key, encrypts it with the real servers public key and sends it to the server. 5) The MITM now know both the server and the client salt, which is enough information to construct the session keys used for further packets sent between the client and the server. All information sent between the parts can now be read in cleartext. The vulnerability occurs because the clients by no means try to verify the public key of the server, sent in step 2 above. In other protocols, such as the Secure Shell protocol, most client implementations solve this for example by letting the user answer a question whether a specific serverkey fingerprint is valid. The clients we've seen so far for RDP have no way to preinsert a known server key. There is also no interaction with the user in order to verify a key the first time a connection is made to a new server. We have communicated with Microsoft in this matter, and they confirmed 2003-03-19 that the problem do exist in their current implementation. They are currently "investigating the feasability in adding this functionality". They also point out that they do not claim RDP having the functionality of providing server authentication. We feel that Microsoft is not taking this seriously enough. We know there are sites using Terminal Services to transfer sensitive data, and we feel that they need to be informed about this vulnerability in order to be able protect their networks. This is why we publish this information at this moment. We've tested this vulnerability against Windows 2000 Terminal Server, Windows 2000 Advanced Server and the upcoming Windows Server 2003 using both the clients delivered with Windows 2000 and the latest downloadable RDP client from Microsoft. We have reason to believe that the vulnerability exists when running both RDP version 4 and 5, and regardless of terminal server mode. We have developed software that can be used to exploit this vulnerability, but we choose not to release it. \EF -- Erik Forsberg Telephone: +46-13-21 46 00 Cendio Systems Web: http://www.thinlinc.com
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