CVE-2021-22907: Citrix Workspace Privilege Escalation

CVE-2021-22907 Overview

An improper access control vulnerability exists in Citrix Workspace App for Windows that potentially allows privilege escalation. This security flaw affects CR versions prior to 2105 and 1912 LTSR prior to CU4. The vulnerability stems from inadequate access control mechanisms within the application, enabling authenticated local attackers to elevate their privileges on affected Windows systems.

Critical Impact

Local privilege escalation vulnerability allowing authenticated users to gain elevated system privileges on Windows workstations running vulnerable versions of Citrix Workspace App.

Affected Products

• Citrix Workspace App for Windows CR versions prior to 2105
• Citrix Workspace App for Windows 1912 LTSR prior to CU4
• Citrix Workspace (Windows platform)

Discovery Timeline

• 2021-05-27 - CVE-2021-22907 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-22907

Vulnerability Analysis

This vulnerability is classified under CWE-284 (Improper Access Control), indicating a fundamental flaw in how the Citrix Workspace App validates and enforces access permissions. The improper access control issue allows a locally authenticated attacker to manipulate application components or system resources in ways that were not intended by the security architecture.

The local attack vector requires an attacker to have initial access to the target system, typically through a standard user account. Once authenticated, the attacker can exploit the access control weakness to escalate their privileges, potentially gaining administrative or SYSTEM-level access. This could lead to complete compromise of the affected workstation, including the ability to access sensitive data, install malware, or use the compromised system as a pivot point for further attacks within the enterprise network.

Root Cause

The root cause of CVE-2021-22907 lies in improper access control implementation within the Citrix Workspace App for Windows. The application fails to properly validate user permissions when accessing certain components or performing privileged operations. This inadequate enforcement of access restrictions creates a pathway for privilege escalation, where standard users can perform actions that should be restricted to administrators.

Attack Vector

The attack vector is local, meaning an adversary must first gain access to the target system with a valid user account. The exploitation requires no user interaction and can be performed with low-privilege credentials. The attacker exploits the improper access control by interacting with vulnerable Citrix Workspace App components that do not adequately verify the caller's authorization level.

The exploitation mechanism involves manipulating the application's access control checks to bypass authorization requirements. This could involve exploiting weaknesses in file permissions, service configurations, or inter-process communication mechanisms used by the Citrix Workspace App.

For detailed technical information regarding exploitation vectors, refer to the Citrix Support Article CTX307794.

Detection Methods for CVE-2021-22907

Indicators of Compromise

• Unexpected privilege changes or new administrative accounts on systems running Citrix Workspace App
• Anomalous process execution with elevated privileges originating from Citrix Workspace App directories
• Suspicious modifications to Citrix Workspace App configuration files or registry keys
• Unusual service behavior or unauthorized service installations related to Citrix components

Detection Strategies

• Monitor Windows Event Logs for privilege escalation indicators, including Event ID 4672 (Special privileges assigned to new logon) associated with Citrix processes
• Deploy endpoint detection rules to identify unauthorized access to protected Citrix Workspace App resources
• Implement application whitelisting to detect unexpected executables running with elevated privileges
• Use SentinelOne Singularity platform to detect behavioral anomalies indicative of privilege escalation attempts

Monitoring Recommendations

• Enable detailed audit logging for the Citrix Workspace App installation directories and related registry keys
• Configure alerts for changes to Citrix service configurations and executable permissions
• Monitor for lateral movement attempts originating from workstations with vulnerable Citrix installations
• Establish baseline behavior for Citrix Workspace App processes and alert on deviations

How to Mitigate CVE-2021-22907

Immediate Actions Required

• Inventory all systems running Citrix Workspace App for Windows to identify vulnerable versions
• Prioritize patching based on asset criticality and exposure level
• Implement network segmentation to limit potential lateral movement from compromised endpoints
• Apply the principle of least privilege to reduce the attack surface

Patch Information

Citrix has released security updates to address this vulnerability. Organizations should update to Citrix Workspace App for Windows CR version 2105 or later, or apply Cumulative Update 4 (CU4) for the 1912 LTSR branch. Detailed patching instructions and download links are available in the Citrix Support Article CTX307794.

Workarounds

• Restrict local access to systems running Citrix Workspace App to trusted users only
• Implement application control policies to limit execution of unauthorized code
• Deploy endpoint detection and response (EDR) solutions like SentinelOne to detect and prevent exploitation attempts
• Consider temporarily uninstalling Citrix Workspace App on non-essential systems until patches can be applied

# Verify Citrix Workspace App version on Windows
# Run in PowerShell to check installed version
Get-WmiObject -Class Win32_Product | Where-Object {$_.Name -like "*Citrix Workspace*"} | Select-Object Name, Version

# Alternative: Check registry for version information
Get-ItemProperty "HKLM:\SOFTWARE\Citrix\Dazzle" -ErrorAction SilentlyContinue | Select-Object CurrentVersion