Baseband Management Controllers (BMC) are a popular feature found on most motherboards targeting the server market. They provide a number of convenience functions for remote management which is great for machines typically located in a cold noisy room. Some of the functions they provide include remote power cycling, keyboard video mouse (KVM), and virtual media emulation. The combination of these functions can allow an administrator to provision a server without ever having to touch it. With that much power over the system they are bound to be a highly valuable target for attack.
This week the security company Eclypsium released a critical vulnerability they found in Supermicro’s BMC implementation. The vulnerability reported is in the virtual media service subsystem. This service allows a remote administrator to attach USB devices, such as DVD drives or keyboards, to the machine remotely as if they were physically plugged into the machine. The feature requires authentication to function properly of course but the researchers found a way to bypass this requirement.
The first weakness is that the BMC would accept authentication requests via plaintext by default. They noted that encryption support is available but based on an old weak Rivest Cipher 4 algorithm. In addition, the key used when using encryption is shared across all Supermicro devices, making man-in-the-middle decryption possible. They also uncovered a complete authentication bypass in the system. This is possible because the BMC does not timeout a valid authorized session in a timely manner. An attacker would be able to re-use the session and gain access if an administrator had recently successfully logged into the system and used the virtual media service. BMC systems are rarely reset due to their nature of being an always online out of band management system, increasing the likelihood of this attack being successful.
Supermicro has issued an update to their BMC software, but it is unlikely that machines will be patched immediately. This is due to the machines needing to be completely powered off in order to apply the update. Until then it is recommended to block the port used by the virtual media service, port 623, until the patch can be applied. Researchers warn that this will likely not be the last BMC vulnerability discovered, so additional measures should be taken when possible. The best defense against these attacks is keeping vulnerable machines on a separate network from other traffic. Ideally management interfaces should be on their own network that is not exposed to public facing traffic.