CVE-2022-25165: AWS Client VPN Race Condition Flaw

CVE-2022-25165 Overview

A Time-of-Check Time-of-Use (TOCTOU) race condition vulnerability was discovered in Amazon AWS VPN Client 2.0.0 that could allow a low-privileged local attacker to escalate privileges to SYSTEM level. The flaw exists in the validation mechanism for VPN configuration files, enabling attackers to inject malicious parameters into configuration files after validation but before processing by the AWS VPN Client service.

Critical Impact

Local privilege escalation to SYSTEM through arbitrary file write, enabling complete system compromise or denial of service on affected Windows endpoints.

Affected Products

• Amazon AWS Client VPN 2.0.0
• Windows systems running vulnerable AWS VPN Client versions

Discovery Timeline

• 2022-04-14 - CVE CVE-2022-25165 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2022-25165

Vulnerability Analysis

This vulnerability is classified as CWE-367, a Time-of-Check Time-of-Use (TOCTOU) race condition. The flaw occurs during the validation of VPN configuration files by the AWS VPN Client. The service validates configuration parameters against an allow list, but a timing window exists between this validation check and the actual processing of the configuration file by the AWS VPN Client service running with SYSTEM privileges.

During this timing window, an attacker with low-level user privileges can modify the configuration file to inject parameters that would normally be blocked by the allow list. This race condition is exploitable because the validation and use operations are not performed atomically, creating an opportunity for malicious manipulation.

Root Cause

The root cause is the lack of atomic operations between the validation phase and the file processing phase. The AWS VPN Client service first validates the configuration file contents against an allow list, then subsequently reads and processes the file as SYSTEM. This two-step process without proper file locking or atomic handling creates a vulnerable time window where file contents can be altered between the check and use operations.

Attack Vector

The attack requires local access to the system and involves the following exploitation technique:

1. A low-privileged user prepares a legitimate VPN configuration file that passes the allow list validation
2. The attacker monitors for the validation check to occur
3. During the timing window after validation but before processing, the attacker swaps or modifies the configuration file to include malicious parameters
4. The AWS VPN Client service (running as SYSTEM) processes the modified file without re-validation
5. Dangerous arguments such as log can be injected, allowing arbitrary file destinations for log file writes

The log parameter injection is particularly dangerous as it enables arbitrary file write with SYSTEM privileges. While the attacker has only partial control over the file contents (limited to log data), this can still be abused for privilege escalation through techniques like DLL hijacking or overwriting critical system files, as well as denial of service by corrupting system files.

Detection Methods for CVE-2022-25165

Indicators of Compromise

• Unexpected modification of VPN configuration files in the AWS VPN Client configuration directory
• Log files appearing in unusual system locations or sensitive directories
• Rapid file access patterns indicating race condition exploitation attempts
• Anomalous process behavior from the AWS VPN Client service

Detection Strategies

• Monitor file system activity for rapid read/write cycles targeting AWS VPN Client configuration files
• Implement file integrity monitoring on AWS VPN Client configuration directories
• Alert on VPN configuration file modifications by non-administrative users
• Deploy endpoint detection rules for TOCTOU exploitation patterns

Monitoring Recommendations

• Enable detailed Windows security auditing for file access events in the AWS VPN Client directory
• Monitor for creation of log files in unexpected locations by the AWS VPN Client service process
• Implement process monitoring to detect abnormal SYSTEM-level file write operations originating from the VPN client
• Review endpoint logs for signs of privilege escalation attempts following VPN client activity

How to Mitigate CVE-2022-25165

Immediate Actions Required

• Update Amazon AWS VPN Client to the latest patched version immediately
• Restrict local access to systems running AWS VPN Client where possible
• Implement application whitelisting to prevent unauthorized modifications to VPN configuration files
• Monitor for exploitation attempts using endpoint detection and response solutions

Patch Information

Amazon has addressed this vulnerability in subsequent versions of the AWS VPN Client. Organizations should consult the Rhino Security Labs Analysis for detailed technical information and verify they are running a patched version of the client. Additional technical details and CVE tracking can be found at the GitHub CVE Repository.

Workarounds

• Limit VPN client usage to dedicated, hardened systems with restricted local user access
• Apply strict file system permissions on VPN configuration directories to prevent unauthorized modifications
• Implement Windows AppLocker or similar application control policies to restrict configuration file access
• Consider using alternative VPN solutions until patching is complete in sensitive environments

# Configuration example - Restrict VPN configuration directory access
# Run in elevated PowerShell to limit write access to administrators only
icacls "C:\Users\*\AppData\Local\AWS VPN Client" /inheritance:r
icacls "C:\Users\*\AppData\Local\AWS VPN Client" /grant:r Administrators:(OI)(CI)F
icacls "C:\Users\*\AppData\Local\AWS VPN Client" /grant:r SYSTEM:(OI)(CI)F