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'Fascinating' CredSSP Flaw Affects All Versions of Windows

In a flaw described by one non-affiliated security expert as "fascinating," security researchers found a logical flaw in the Credential Security Support Provider (CredSSP) protocol used by Remote Desktop and WinRM and affecting all supported versions of Windows.

Preempt Security reported the flaw to Microsoft last August and Microsoft released a fix this week as part of the March Patch Tuesday release. The flaw, CVE-2018-0886, was rated "important" by Microsoft, which is a middling severity designation in Microsoft's scale, largely because the new flaw is not an initial infection vector.

Instead, an attacker needs to already be inside the network and set up a man-in-the-middle (MITM) attack via methods that could include ARP Poisoning or even the new WPA2 vulnerability known as KRACK.

CredSSP is designed to securely forward a user's full credentials to a target server. The flaw relies in part on the fact that the client trusts the public key provided by the server. In the case of an RDP connection, an attacker would intercept the initial connection request from the client and return a malicious command to the client, which assumes the command is actually a valid public key from the server and signs it. That signed version is passed by the MITM back to the server, which executes the malicious code -- now signed by the client -- on the server.

Preempt positions the flaw as a technique for lateral movement and privilege escalation. One of the most severe scenarios would be if the attacker intercepts an attempt by an administrator to remotely log on to a domain controller.

"This vulnerability is a big deal, and while no attacks have been detected in the wild, there are a few real-world situations where attacks can occur," said Roman Blachman, Preempt CTO and co-founder, in a statement. Preempt also posted a video showing how the attack works and a technical blog post. "Ensuring that your workstations are patched is the logical, first step to preventing this threat. It's important for organizations to use real-time threat response solutions to mitigate these types of threats," Blachman said.

Dustin Childs of the Zero Day Initiative at Trend Micro highlighted the patch in his analysis of Microsoft's Patch Tuesday release, which included 14 updates resolving 78 unique vulnerabilities. "This patch corrects a truly fascinating bug," Childs wrote of the CredSSP flaw. "It's important to understand this is not a constrained delegation. CredSSP passes the user's full credentials to the server without any constraint. That's a key to how an attacker would exploit the bug."

Childs also warned that applying the patch isn't enough to be fully protected. "Sysadmins must also enable Group Policy settings on their systems and update their Remote Desktop clients. While these settings are disabled by default, Microsoft does provide instructions to enable them. Of course, another alternative is to completely disable RDP, but since many enterprises rely on this service, that may not be a practical solution," he wrote.

Microsoft also released a support document that describes the steps required to update Group Policy or Registry settings to protect against the flaw. In a related step, Microsoft plans to update the Remote Desktop Client next month to provide more detail in error messages when an updated client fails to connect to a server that has not been updated.

A team from Preempt will give a presentation on the vulnerability at Black Hat 2018 Asia next week.

Posted by Scott Bekker on 03/14/2018 at 10:10 AM


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