Understanding the PrintNightmare Vulnerability (CVE-2021-34527)

The PrintNightmare vulnerability (CVE-2021-34527) is a critical security flaw affecting Microsoft Windows systems. It exploits a logic flaw in the RpcAddPrinterDriver function, which is designed for remote printing and driver installation. By leveraging this flaw, an attacker can execute malicious code with SYSTEM privileges, resulting in remote code execution (RCE) or local privilege escalation (LPE). This vulnerability affects multiple Windows versions, including:

• Windows Server 2019
• Windows Server 2016
• Windows Server 2012 and 2012 R2
• Windows Server 2008 and 2008 R2
• Windows 7, 8.1, 10
• Windows Server 2004
• Windows Server 20H2

Notably, the June Patch does not fully mitigate this vulnerability, making it a persistent security concern.

Exploiting PrintNightmare

A proof-of-concept (PoC) exploit released by security researcher Cube0x0 demonstrates how an adversary can remotely or locally execute malicious DLLs. The attack allows a low-privilege domain user to gain code execution on any system with an enabled Print Spooler service.

Step 1: Identifying Vulnerable Systems

Before exploitation, it is essential to verify whether the target system is vulnerable. This can be achieved using the Impacket rpcdump tool, which checks for exposed MS-RPRN protocol services.

Step 2: Hosting the Malicious Payload

To execute the attack, the exploit requires a hosted payload. This is accomplished by creating a shared folder using built-in PowerShell commands to allow anonymous access and store the malicious DLL.

Step 3: Executing the Exploit

Once the malicious DLL is hosted, the exploit is executed by providing:

• A low-privilege domain user
• The path to the malicious DLL
• The target system

Upon execution, the attack results in remote code execution, allowing adversaries to run arbitrary commands with SYSTEM privileges.

Automating Security Testing with Cymulate’s Purple Team Module

The PrintNightmare attack scenario is implemented in the Cymulate Continuous Security Validation platform through the Purple Team module. This allows security teams to evaluate their IT infrastructure’s resilience against this threat and implement necessary remediation steps.

Features of the Cymulate Purple Team Module

The Purple Team module is an open framework designed to automate and execute custom attack scenarios. Security teams of any skill level can utilize it to:

• Perform penetration testing tasks on a daily basis
• Gain visibility into detection and response gaps
• Automate adversary simulation with minimal effort

Execution Approaches

The module supports two execution approaches:

1. Chained Execution: Multiple steps are linked to mimic a real adversary’s attack path, such as credential theft followed by lateral movement and privilege escalation.
2. Atomic Execution: Single-step tests that focus on specific attack techniques.

For the PrintNightmare scenario, a chained execution approach is used. This includes:

• Identifying vulnerable systems
• Creating an SMB share to host the malicious payload
• Executing the exploit to achieve remote code execution

By chaining these steps, the Purple Team module enables security teams to repeatedly assess and refine their defenses against this attack.

Mitigation Strategies for PrintNightmare

Since this vulnerability remains unpatched as of July 5, 2021, organizations should implement the following mitigations:

• Disable the Windows Print Spooler service on domain controllers and systems that do not require printing.
• Restrict remote printing through Group Policy to prevent unauthorized access.
• Prioritize domain controllers when applying security configurations due to their critical role in enterprise environments.

Enhance Security with Cymulate

Testing security controls against real-world threats is essential for maintaining cyber resilience. Cymulate’s Continuous Security Validation platform provides an automated, scalable solution for assessing enterprise security posture against threats like PrintNightmare.