APR-RDP enables the capture and the decryption of Remote Desktop Protocol (RDP) traffic between hosts. RDP is the protocol used to connect to Windows Terminal Services of a remote computer.
Microsoft's Windows Terminal Services (built into Windows 2000 Server and Windows Server 2003) and Windows XP's Remote Desktop, provide an easy, convenient way for administrators to implement thin computing within an organization or for users to connect to their XP desktops from a remote computer and run applications or access files.
A Windows 2000 terminal server can be installed in one of two modes: administrative or application server. In administrative mode, only users with administrative accounts can access the terminal server .... this is why these sessions are so interesting.
By default, data that travels between the terminal server and the terminal services client is protected by encryption. The protocol uses the RC4 symmetric encryption algorithm
at one of the following three levels:
RC4 encryption keys are generated after an initial key exchange in which RSA asymmetric encryption is used.
In April 2003 Erik Forsberg released a security advisory to the public ( http://www.securityfocus.com/archive/1/317244 ) explaining that:
"... During extensive investigation of the Remote Desktop Protocol (RDP), the protocol used to connect to Windows Terminal Services, we 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
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. ..."
Microsoft confirmed the above problem and fixed the new versions of Remote Desktop Clients. Recent clients (mstsc.exe), including the one of version XPSP2 5.1.2600.2180, now check the Terminal Server identity verifying its public key. They solved the problem ? No, man-in-the-middle attacks are still possible and can be really invisible for users.
During the initial key-exchange phase, the terminal server sends to the client a server certificate created at the start up of Terminal Server services. This certificate is stored in the registry of the server under the following key:
It contains an RSA public key and its digital signature as illustrated below:
The public key modulus (n) is the same as the one present in the RSA2 key stored in the LSA Secret "L$HYDRAENCKEY" (you can use the Cain's LSA Secret Dumper to check it) of the server; the signature is the information used by the client to verify the server identity.
From a man-in-the-middle attacker's point of view, the public key signature must be modified on the fly to trick the client into verifying the new Mitm public key that will be replaced into the network packet directed to the client. But … what is used to produce this signature ?
Well, a digital signature is noting more nothing less than a hash of something (in this case a server public key) encrypted using a private key and an asymmetric encryption algorithm. This is exactly what is done by the terminal server. At the client-side, this signature is decrypted using a public key and the result is compared with a new hash of the received server public key calculated by the client; if the two hashes match the identity of the server is proven.
Microsoft use another RSA private key to sign the Terminal Server public key and this private key is public ! It could sound strange but this is only the truth, the private key used for the signature creation is hard-coded into mstlsapi.dll and it is dynamically created, used and de-allocated into a subroutine of the "TLSInit" API. Every Windows user has this file ... is this a new kind of public-private key (PPK) ?!?
The Microsoft Windows Terminal Server PPK follows:
public exponent: e
public modulus: n
private exponent: d
secret prime factor: p
secret prime factor: q
d mod (p-1): dmp1
d mod (q-1): dmq1
q^-1 mod p: iqmp
The knowledge of the PPK key lets the attacker calculate a valid signature for the mitm public key generated on the fly during the mitm attack; the client will verify the mitm signature correctly and it will accept the session without informing the users that the server key is changed from the usual one.
The signature is calculated encrypting, with the private part of the PPK key, the MD5 hash of the server public key for a total of 108 bytes hashed.
0) The network packet from the server is hijacked and captured by mean of APR (ARP Poison Routing).
1) The server random and the real server public key are extracted from the packet and stored for future usage.
2) The server public key is replaced in the network packet with a new one generated by Cain (the mitm machine) during the key exchange phase.
3) The MD5 hash of the new mitm public key is calculated.
4) The hash is signed by Cain (encrypted using the private key) using the super secret Microsoft PPK illustrated above.
5) The mitm sign is replaced into the network packet.
6) The packet is routed by APR to the client.
7) The network packet from the client is hijacked and captured by mean of APR (ARP Poison Routing).
8) The client encrypted random is decrypted using the mitm private key.
9) The client random is encrypted using the real server public key and replaced into the network packet for the server.
10) The packet is routed by APR to the server.
11) RC4 symmetric encryption keys are calculated.
12) The key entropy is reduced accordingly with the encryption level used in the session.
13) Packets are decrypted and saved locally to text files.
Cain also try to recognize the keyboard activity at the client-side. This provide some kind of password interception.
This feature needs APR to be enabled and a Man-in-the-Middle condition between the Terminal Server and the victim host.