Detailed information: http://seclab.ce.aut.ac.ir/vreport.htm Summary ======= Microsoft uses SMB Protocol for ?File and Printer sharing service? in all versions of Windows. Upon accessing a network resource, NTLM Authentication is used to authenticate the client on the server. When a logged-in user requests for a network share on the server, Windows automatically sends the encrypted hashed password of the logged-in username to the target SMB server before prompting for password. Although the hashed password is not sent in plaintext format, and it is encrypted by the server challenge, a malicious SMB Server could use this information to authenticate on the client machine and in many cases, gain full control over the shared objects of the client such as C$, etc. Vulnerable Systems ================== All versions of Windows including: Windows XP (confirmed in the lab) Windows 2000 server/professional (confirmed in the lab) Windows .NET server (vulnerable but not confirmed) Windows 9x/Me (vulnerable but not confirmed) Detail ====== SMB, which stands for Server Message Block, is a protocol for sharing files, printers, serial ports, and communications abstractions such as named pipes and mail slots between computers. Microsoft uses this Protocol along with NTLM Authentication protocol to provide a so-called ?User-level file and printer sharing? service in various versions of Windows. When a logged-in user tries to connect to a remote machine network shares, for example \\server\myshare, windows automatically sends the login information of the logged-in user to the SMB server before asking any username or password from the user. At this step, if the authentication fails, Windows pops up a window and asks for a username and password. The ?login information?, which is sent to the SMB server, contains neither the plaintext format of the password, nor the hashed password, which is stored on the SAM. In fact, the client encrypts the hashed password of the user by the challenge it receives from the server and then sends this data to the server. This data is called the NT/LM-Response. Server uses the locally stored hashed password and uses same way to encrypt the hashed password. If the result is equal to the Response sent by the client, then the authentication is successful. This is the way NTLM authentication works. It is obvious that sniffing the Network and extracting the Challenge and the NT-Response could not help us to find the plaintext passwords. Some tools such as L0phtCrack use a brute-force attack to find the plaintext password. But, this way is only suitable for short and not complicated passwords. Also, it?s a time-consuming procedure even for short passwords. As mentioned earlier, windows by default sends the NT/LM-Response of the logged-in username to the SMB server before asking for any username/password. At the first glance, it does not arise any security risk, but by using a slightly tricky method, a malicious SMB server could use this information to authenticate on the client machine. The following procedure illustrates the way an Attacking Server uses to gain access to a Victim Client. Although in this procedure the attack starts by the client request for a network share, it is possible to force the client to initiate this connection. As an example, by sending an HTML email which contains an object with the SRC attribute pointed to a resource on the server, the victim machine will automatically initiates the procedure. This procedure contains 10 steps: 1. The client tries to connect to the Server. It sends a request to the attacking SMB server. 2. Attacking SMB server receives this request, but it does not send its own generated challenge to the client, instead it sends a request for the attacking client. 3. The attacking SMB client sends a request to the victim SMB server. 4. The victim SMB server sends a challenge to the attacking SMB client. 5. Attacking SMB client sends this challenge to the Attacking SMB server, and it sends it back to the victim client. 6. The victim client receives the challenge. It encrypts the password, using the received challenge and sends it back to the Attacking Server. 7. The Attacking server sends this response to the Attacking client. 8. The attacking client sends the received response back to the victim server. 9. The victim server receives the response. 10. A successful authentication occurs on the victim. At this moment, the Attacking client has control over the victim machine. The subtle idea behind this procedure is to request a challenge from the victim server and send it back to victim client whenever the victim requests a connection. By this way, the attacking machine could gain the NT/LM response of a specified user, and it sends it back to the victim to authenticate on victim machine. Although this procedure is a bit complicated and exploitation requires full knowledge about the NTLM and SMB protocols, Open-Source implementations of SMB protocol make exploitation much easier. (Thanks to Samba). Points 1. The attacking machine gains a level of access as equal as the logged-in user privileges on the victim machine. 2. The exploitation will fail, if the victim SMB ports (139/445) are closed, or the victim is behind a firewall. 3. This vulnerability is not limited to ?File sharing service? and is most probably exploitable in various RPC services, which use NT/LM authentication. Solution ======== The best way to fix this vulnerability is that the SMB client refuses the challenges (nonce) which are equal to challenges recently sent by SMB server. Authors ======= This vulnerability has been found and exploited by Salman Niksefat (salman@linux.ce.aut.ac.ir) and Haamed Gheibi (haamed@linux.ce.aut.ac.ir), B.S students of computer engineering at the university of Amirkabir, computer department, network security lab. Exploit ======= We will publish the exploit code after a patch be created by software vendor.
