Restricted Admin Mode – Circumventing MFA On RDP Logons

November 15, 2023 7 mins

Restricted Admin Mode – Circumventing MFA On RDP Logons

This blog post demonstrates the use of Restricted Admin mode to circumvent MFA in RDP as a red team tactic.

Overview

In the last decade, Microsoft introduced a new Remote Desktop security feature called Restricted Admin mode to prevent credential caching and subsequent reuse. When enabled, the Restricted Admin mode allows circumventing multi-factor authentication (MFA) enforced by identity and access management solution providers such as Duo and Okta. While this weakness is known and has been documented by at least one of these providers, this blog post serves to demonstrate leveraging this caveat as a red team tactic.

Context

In recent years, corporate environments have embraced identity and access management solution providers such as Okta and Duo to protect access to a host of applications and resources such as email, office suites, and cloud storage solutions. Those providers offer products to authenticate access to these applications with a number of options including MFA and passwordless authentication using mobile devices.

These technologies found their way into corporate Active Directory domain environments that have for years set the standard for centralized enterprise resource allocation and management. Those environments, comprised of Microsoft Windows servers and workstations, now have implemented MFA for a variety of logon processes ranging from a standard login by an employee at a workstation to a remote desktop login on a server by an administrator.

The addition of these authentication mechanisms in networked enterprise environments has made it increasingly difficult for red team operators and threat actors alike who may rely on Remote Desktop Protocol (RDP) as a means of lateral movement after obtaining an initial foothold. The underlying reason why Remote Desktop is a prime target for adversaries is because it is generally permitted by firewalls and other network security devices since administrators use it to jump around various servers and workstations with ease. While there are other services that may be permitted for smooth functioning of an Active Directory environment such as Microsoft Remote Procedure Call (MSRPC) and Server Message Block (SMB), Remote Desktop remains a critical attack surface to defend, as it is a legitimate way of logging onto a host without executing code that may be detected by antivirus or intrusion detection systems.

The adoption of Remote Desktop also shed light on other security weaknesses, namely caching of credentials on the Windows host being connected to. Specifically, the Windows Local Security Authority Subsystem Service (LSASS) process on the Remote Desktop host stores a copy of the credentials used for interactive logon authentication. These credentials are utilized to service subsequent connection requests to authenticated resources such as file shares. To combat this, Microsoft released an addition to RDP called Restricted Admin mode. This mode allows users that possess local administrative privileges on the Remote Desktop host to complete the authentication process without supplying the password in cleartext. As a result, the password is never cached on the logon host and cannot be reused by a threat actor to escalate privileges across the environment. At the same time, this functionality allows pass-the-hash attacks against RDP and hence puts corporations in a fix whether to prioritize chances of credential compromise versus reuse.

MFA Caveat

However, the specifics of the authentication process are of interest when operating in Restricted Admin mode. The distinction is between an “interactive” versus a “network” logon and since the Restricted Admin mode uses the latter (adopting a token-based method as compared to cleartext credentials), the authentication doesn’t take place on the Remote Desktop server but instead on the client itself. As a result, authentication factors enforced on the destination server such as MFA provisioned via Duo, Okta, or potentially other identity and access management solutions are rendered ineffective.

A caveat that arises from this mode within RDP is that threat actors can completely bypass MFA on servers and/or workstations when attempting to laterally move across the corporate environment if they possess administrative privileges. While the Restricted Admin mode is disabled by default, the following red teaming Tactics, Techniques, and Procedures (TTPs) describe how it could be leveraged to expand coverage and access across the network.

Imagine the following scenario: the adversary has gained an initial foothold within a firm’s internal network infrastructure and has access to a set of administrative credentials that can be used to compromise a domain-connected Windows host. They have attempted to log on via RDP but there’s a multi-factor mechanism that is sending a push notification to the victim user’s phone and awaiting approval. At this point, the adversary could attempt an MFA fatigue attack by spamming the victim device with push requests in hope that they accept the notification, but this approach is noisy and has rate limitations.

Since they have access to credentials, they can interrogate an administrative session over port SMB/445 (which does not enforce MFA as it’s a network logon) and enable the restricted admin mode:


crackmapexec smb <IP> -u <username> -p <password> -x 'reg add HKLM\SYSTEM\CurrentControlSet\Control\Lsa /v DisableRestrictedAdmin /d 0 /t REG_DWORD'

Next, they can generate the NTLM hash for the supplied password and bypass MFA on the Remote Desktop server using FreeRDP on Linux:


xfreerdp /v:<IP> /u:<victim username> /d:<domain> /pth:<victim password-hash>

Or the official Remote Desktop client on Windows (logged-on user’s context):


mstsc.exe /restrictedadmin

On Windows, they can either hijack the logged-on user’s context or request a service ticket using mimikatz:


mimikatz # sekurlsa::pth /user:<victim username> /domain:<domain> /ntlm:<victim password hash> /run:"mstsc.exe /restrictedadmin"

Detection

In order to detect this MFA bypass, EDR systems and/or system administrators should monitor the server registry to ensure that the DisableRestrictedAdmin key is set to 1, which indicates that the mode remains disabled. In the event that this value is modified, a security alert should be triggered which causes the isolation and quarantine of the originating host in order to stop the adversary in their tracks. While these recommendations serve as detective and responsive controls, there is no known remediation at the time of writing for this issue, which has been documented by some identity and access management vendors such as Duo but not all.

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Apurva Goenka

Senior Consultant, Security Testing, Cyber Solutions

Aon’s Cyber Solutions offers holistic cyber risk management, unsurpassed investigative skills, and proprietary technologies to help clients uncover and quantify cyber risks, protect critical assets, and recover from cyber incidents.

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