One of the vulnerabilities that Microsoft addressed on January 2021 Patch Tuesday could allow an attacker to relay NTLM authentication sessions and then execute code remotely, using a printer spooler MSRPC interface.
Tracked as CVE-2021-1678, the vulnerability has been described by Microsoft as an NT LAN Manager (NTLM) security feature bypass, and is rated important for all affected Windows versions, namely, Windows Server, Server 2012 R2, Server 2008, Server 2016, Server 2019, RT 8.1, 8.1, 7, and 10.
The bug was identified and reported by Yaron Zinar from Preempt, a security firm that was acquired by CrowdStrike in September 2020.
In a blog post detailing the issue, CrowdStrike explains that NTLM relay attack methods are not uncommon, and that relaying NTLM authentications to another protocol (such as SMB, LDAP/S, and MSRPC) is possible when required protections are not present.
CVE-2021-1678, CrowdStrike’s security researchers say, resides in the IRemoteWinSpool MSRPC interface, a Printer Remote Procedure Call (RPC) interface designed for remote printer spooler management.
The researchers explain that MSRPC is inherently vulnerable to relay attacks, because at an authentication level of RPC_C_AUTHN_LEVEL_CONNECT, the user is authenticated on the initial request, but no encryption or signing is enforced on the transferred commands.
Thus, an attacker able to establish an NTLM session with a target machine can bind to the IRemoteWinspool interface and select the weak authentication level, relay the NTLM authentication to the attacker’s machine, and then execute RPC commands.
CrowdStrike’s researchers say they have working proof-of-concept (POC) code, but won’t release it just yet.
Microsoft patched the issue by adding appropriate checks to ensure that the authentication level of the client is not weak, and to reject calls in case it is. However, the check is only performed if a specific registry value is set, otherwise the system remains vulnerable.
“The Windows update addresses this vulnerability by increasing the RPC authentication level and introducing a new policy and registry key to allow customers to disable or enable Enforcement mode on the server-side to increase the authentication level,” Microsoft notes in a support article.
The tech company also explains that, while vulnerable RPC connections are still allowed following this update, an enforcement phase will kick off on June 8, 2021, enforcing “the changes to address CVE-2020-1678 by increasing the authorization level without having to set the registry value.”
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