Microsoft Remote Access Protocol Flaw Affects All Windows Machines (man-in-the-middle attack)

[LASER_oneXM]

Attackers can exploit newly discovered critical crypto bug in CredSSP via a man-in-the-middle attack and then move laterally within a victim network.

A serious vulnerability found in Microsoft's Credential Security Support Provider protocol (CredSSP) could allow a hacker to gain control of a domain server and other systems in the network.

Researchers from Preempt unearthed the previously unknown remote code execution vulnerability, which affects all versions of Windows, and reported it to Microsoft in August of last year. Microsoft today issued a fix (CVE-2018-0886) for the protocol as part of its Patch Tuesday release.

The logical cryptographic vulnerability in CredSSP can be exploited via a man-in-the-middle attack when a client machine and server authenticate to one another over the Remote Desktop Protocol (RDP) and Windows Remote Management (WinRM) connection protocols. CredSSP forwards credentials, encrypted, from the Windows client to the server for authentication.

"We were able to find a classic mistake in the protocol and use that mistake to launch a man-in-the-middle attack," says Yaron Zinar, lead security researcher for Preempt. Zinar and his team will demonstrate the attack next week in Singapore at Black Hat Asia, where they also will release an open source tool that exploits the vulnerability.

Zinar says with CredSSP, the server's certificate doesn't get validated by the client; it's just signed and not hashed. "That allows us to create a malicious [and forged server] certificate that contains" a malicious executable, he says. The client then can be duped with a forged server cert.

Microsoft in its update today said the patch "addresses the vulnerability by correcting how CredSSP validates requests during the authentication process." It recommends also using Group Policy settings or registry-based settings: "We recommend that administrators apply the policy and set it to 'Force updated clients' or 'Mitigated' on client and server computers as soon as possible. These changes will require a reboot of the affected systems," Microsoft said in its update.


To defend against the CredSSP exploit, Preempt recommends patching workstations and servers, but warns that patching alone is not sufficient to stop this attack. Special configuration changes also need to be made, and blocking RDP and DCE/RPC can help. "If you don't use RDP, turn it off on the machine. And if you are not using remote procedure calls, turn them off," Zinar advises. "Also, decrease the use of privileged credentials. An admin should not use privileged credentials from WiFi, and sometimes not even from his personal workstation. From a dedicated workstation, maybe.


"And don't use domain admins at all," he adds.