CVE-2021-1366: Cisco AnyConnect Client RCE Vulnerability

CVE-2021-1366 Overview

A vulnerability exists in the interprocess communication (IPC) channel of Cisco AnyConnect Secure Mobility Client for Windows that could allow an authenticated, local attacker to perform a DLL hijacking attack on an affected device. This vulnerability specifically affects systems where the VPN Posture (HostScan) Module is installed on the AnyConnect client.

The vulnerability stems from insufficient validation of resources that are loaded by the application at run time. An attacker could exploit this vulnerability by sending a crafted IPC message to the AnyConnect process, potentially achieving arbitrary code execution with SYSTEM privileges on the affected machine.

Critical Impact

Successful exploitation enables local privilege escalation to SYSTEM, allowing complete compromise of the affected Windows system through DLL hijacking in a widely-deployed enterprise VPN client.

Affected Products

• Cisco AnyConnect Secure Mobility Client for Windows (with VPN Posture/HostScan Module)
• cisco anyconnect_secure_mobility_client

Discovery Timeline

• 2021-02-17 - CVE CVE-2021-1366 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-1366

Vulnerability Analysis

This vulnerability is classified under CWE-427 (Uncontrolled Search Path Element) and CWE-347 (Improper Verification of Cryptographic Signature). The flaw exists within the IPC channel used by Cisco AnyConnect Secure Mobility Client for Windows, specifically when the VPN Posture (HostScan) Module is installed.

The core issue lies in insufficient validation of resources loaded by the application at runtime. When the AnyConnect process handles IPC messages, it fails to properly verify the legitimacy and integrity of dynamically loaded libraries. This allows an authenticated local attacker to craft malicious IPC messages that direct the application to load attacker-controlled DLL files.

Since the AnyConnect client runs with elevated privileges, successful DLL hijacking results in code execution with SYSTEM-level privileges, representing a significant local privilege escalation attack vector in enterprise environments.

Root Cause

The root cause of this vulnerability is inadequate validation within the IPC message handling mechanism of the Cisco AnyConnect client. Specifically, the application does not properly verify the cryptographic signatures or path integrity of DLL files referenced in IPC communications. This insufficient validation allows an attacker to inject a malicious DLL path that the application subsequently loads and executes with its elevated privileges.

The presence of the VPN Posture (HostScan) Module is a prerequisite for exploitation, as this module introduces the vulnerable IPC handling code path.

Attack Vector

The attack requires local access and valid credentials on the Windows system. The exploitation process involves:

1. The attacker authenticates to the Windows system with a low-privileged user account
2. The attacker places a malicious DLL in a location accessible by the AnyConnect process
3. A crafted IPC message is sent to the running AnyConnect process
4. The vulnerable IPC handler processes the message and loads the attacker-controlled DLL
5. The malicious code executes with SYSTEM privileges, allowing full system compromise

This represents a local privilege escalation scenario where an attacker with limited user access can escalate to full administrative control of the affected system.

Detection Methods for CVE-2021-1366

Indicators of Compromise

• Unexpected DLL files appearing in directories accessible to the AnyConnect process
• Anomalous IPC communications to the vpnagent.exe or related AnyConnect processes
• Unusual process spawning or code execution originating from AnyConnect components with SYSTEM privileges
• Registry modifications associated with DLL search order manipulation

Detection Strategies

• Monitor for unsigned or suspiciously-signed DLLs being loaded by Cisco AnyConnect processes
• Implement application whitelisting to detect unauthorized DLL loading attempts
• Deploy endpoint detection rules to identify privilege escalation patterns from AnyConnect processes
• Audit IPC communications to AnyConnect services for anomalous message patterns

Monitoring Recommendations

• Enable detailed Windows Security Event logging for process creation and DLL loading events
• Configure Sysmon to capture DLL load events (Event ID 7) for AnyConnect-related processes
• Monitor for file creation events in directories within the DLL search path of AnyConnect components
• Implement SentinelOne Singularity platform for real-time behavioral detection of DLL hijacking attempts

How to Mitigate CVE-2021-1366

Immediate Actions Required

• Update Cisco AnyConnect Secure Mobility Client to the latest patched version immediately
• Review systems for any signs of compromise before applying patches
• Restrict user permissions on directories within the DLL search path of AnyConnect
• Consider temporarily disabling the VPN Posture (HostScan) Module if not business-critical until patching is complete

Patch Information

Cisco has released security updates to address this vulnerability. Administrators should consult the Cisco Security Advisory for detailed patching instructions and affected version information.

Organizations should prioritize patching based on the presence of the VPN Posture (HostScan) Module and exposure of affected endpoints.

Workarounds

• Restrict write permissions on directories in the AnyConnect DLL search path to administrators only
• Implement application control policies to prevent unauthorized DLL loading
• Remove the VPN Posture (HostScan) Module if it is not required for your deployment
• Deploy endpoint protection solutions capable of detecting DLL hijacking behavior

# Example: Restrict permissions on AnyConnect installation directory
icacls "C:\Program Files (x86)\Cisco\Cisco AnyConnect Secure Mobility Client" /inheritance:r
icacls "C:\Program Files (x86)\Cisco\Cisco AnyConnect Secure Mobility Client" /grant:r "BUILTIN\Administrators:(OI)(CI)F"
icacls "C:\Program Files (x86)\Cisco\Cisco AnyConnect Secure Mobility Client" /grant:r "NT AUTHORITY\SYSTEM:(OI)(CI)F"
icacls "C:\Program Files (x86)\Cisco\Cisco AnyConnect Secure Mobility Client" /grant:r "BUILTIN\Users:(OI)(CI)RX"