CVE-2021-1733: Microsoft PsExec Privilege Escalation Flaw

CVE-2021-1733 Overview

CVE-2021-1733 is an Elevation of Privilege vulnerability affecting Microsoft Sysinternals PsExec, a widely-used command-line utility for executing processes on remote systems. This vulnerability allows a local attacker with low-level privileges to escalate their access to gain full system-level control over the affected host.

Critical Impact

Successful exploitation enables attackers to elevate privileges locally, potentially gaining complete control over the target system through the trusted PsExec administrative tool.

Affected Products

• Microsoft PsExec (all versions prior to patch)

Discovery Timeline

• 2021-02-25 - CVE CVE-2021-1733 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-1733

Vulnerability Analysis

This vulnerability is classified under CWE-269 (Improper Privilege Management), indicating a fundamental flaw in how PsExec handles privilege separation and access control. PsExec is commonly used by system administrators for remote management and by security professionals for legitimate penetration testing. The tool creates a temporary service on remote systems to execute commands, and this vulnerability allows a local attacker to manipulate this process to gain elevated privileges.

The local attack vector means an attacker must already have some level of access to the target system. However, the low complexity of exploitation and the absence of user interaction requirements make this vulnerability particularly dangerous in enterprise environments where PsExec is frequently deployed.

Root Cause

The root cause stems from improper privilege management (CWE-269) within PsExec's execution flow. The vulnerability exists due to inadequate validation and control of privilege boundaries during the tool's operation, allowing a local user to abuse the trust relationship that PsExec establishes with the operating system's service control manager.

Attack Vector

The attack is performed locally on a system where PsExec is installed or executed. An attacker with low-privilege access can exploit the improper privilege management to elevate their access. The exploitation does not require any user interaction, meaning an attacker can trigger the vulnerability programmatically once they have initial access to the system.

The vulnerability affects confidentiality, integrity, and availability of the target system, as successful exploitation grants the attacker elevated privileges capable of modifying, accessing, or disrupting system resources.

Detection Methods for CVE-2021-1733

Indicators of Compromise

• Unexpected PSEXESVC.exe service installations or modifications on endpoints
• Anomalous privilege escalation events following PsExec execution in Windows Security Event logs
• Suspicious process creation chains where low-privilege processes spawn SYSTEM-level child processes via PsExec

Detection Strategies

• Monitor Windows Event ID 4688 (Process Creation) for PsExec execution patterns combined with subsequent privilege elevation
• Implement behavioral detection rules for service creation events (Event ID 7045) associated with PsExec temporary services
• Deploy endpoint detection capabilities to identify privilege escalation attempts following legitimate PsExec usage

Monitoring Recommendations

• Enable command-line logging and PowerShell script block logging to capture PsExec invocations and parameters
• Configure alerts for unusual local privilege escalation patterns, particularly on systems where PsExec is commonly used
• Implement file integrity monitoring for PsExec binaries to detect tampering or replacement with malicious versions

How to Mitigate CVE-2021-1733

Immediate Actions Required

• Update Microsoft Sysinternals PsExec to the latest patched version immediately
• Audit systems to identify all instances of PsExec deployment across the environment
• Restrict PsExec usage to authorized administrators only through application whitelisting policies
• Review recent security events on systems where PsExec has been executed for signs of exploitation

Patch Information

Microsoft has released a security update to address this vulnerability. Detailed patch information and remediation guidance is available through the Microsoft Security Advisory for CVE-2021-1733. Organizations should download and deploy the latest version of PsExec from the official Microsoft Sysinternals website to ensure the vulnerability is remediated.

Workarounds

• Temporarily disable or remove PsExec from systems until patches can be applied
• Implement strict access controls and network segmentation to limit who can execute PsExec
• Use Windows Defender Application Control (WDAC) or AppLocker to restrict PsExec execution to specific administrative accounts
• Consider alternative remote administration tools that are not affected by this vulnerability during the remediation period

# Restrict PsExec execution using AppLocker (PowerShell example)
# Create a deny rule for non-administrators
New-AppLockerPolicy -RuleType Path -Path "C:\*\psexec.exe" -Action Deny -User "Everyone"