NTLM Relay Attack Exploits Windows RPC Flaws

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Security researchers at SentinelLabs revealed the details of a newly identified NTLM (New Technology LAN Manager) relay attack that exploits a remote procedure call (RPC) flaw to enable elevation of privilege.

This new vulnerability in RPC, which apparently impacts all versions of Windows, enables an attacker to escalate privileges from User to Domain Admin, all without requiring interaction from the user (NTLM relay attacks typically do require user intervention). 

The researchers used a DCOM client that was instructed to connect to an RPC server, operation that involved two NTLM authentications, one without the sign flag being set, and also leveraged the fact that the DCOM activation service can be abused to trigger RPC authentication. 

According to SentinelLabs, the motive behind the attack was that a shell in Session 0, even as a low privileged user, combined with triggering some CLSIDs, could allow the attacker to obtain “an NTLM authentication from the user who is interactively connected.”

Methodology used by cybercriminals 

Threat actors have a shell in Session 0 on the target machine, even with a low privileges account, user with high privileges (such as Domain Admin) logs in interactively, then the attacker triggers the DCOM activation service to impersonate the high-privileged user and then implements a man-in-the-middle to receive an authenticated call, the binding of the RPC under the attacker’s control takes place and then the victim machine makes an authenticated call, authentication is relayed to a privileged resource such as LDAP, SMB, HTTP or other, lastly the authentication is forwarded for privilege escalation.

Researchers at SentinelLabs also published proof-of-concept code to demonstrate how the exploit works, and revealed that, although Microsoft has acknowledged the vulnerability, a patch won’t be released. The researchers, however, did publish a series of mitigations that should help prevent attacks that would trigger an authenticated RPC/DCOM call and then relay the NTLM authentication. 

“This is different from other known techniques such as CVE-2020-1113 and CVE-2021-1678, where relaying happens between a generic ‘client’ protocol vs. an RPC server. In this case, we had an RPC client whose authentication was relayed to other ‘server’ protocols and without ‘victim’ interaction. Therefore, we hope that MS reconsider their decision not to fix this serious vulnerability,” SentinelLabs concludes.

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