CVE-2023-35838 Overview
CVE-2023-35838 is a denial of service vulnerability affecting the WireGuard client version 0.5.3 on Windows. The vulnerability stems from insecure configuration of the operating system and firewall, which causes traffic to local networks using non-RFC1918 IP addresses to be blocked. This flaw allows an adversary to manipulate the victim into inadvertently blocking IP traffic to specific IP addresses and services, even while the VPN connection remains active.
This vulnerability is part of a broader class of attacks known as "LocalNet attacks" as documented on the TunnelCrack research site, which can result in traffic blocking when VPN clients improperly handle local network configurations.
Critical Impact
An adjacent network attacker can exploit this vulnerability to cause denial of service by tricking users into blocking legitimate network traffic to selected IP addresses and services while the VPN appears to be functioning normally.
Affected Products
- WireGuard 0.5.3 for Windows
- Microsoft Windows (as the underlying operating system)
Discovery Timeline
- 2023-08-09 - CVE-2023-35838 published to NVD
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2023-35838
Vulnerability Analysis
The vulnerability is classified under CWE-610 (Externally Controlled Reference to a Resource in Another Sphere), indicating that the WireGuard client allows external influence over how network resources are referenced and handled. When the WireGuard VPN client configures the Windows firewall and networking stack, it fails to properly account for local networks that use IP address ranges outside the standard RFC1918 private address spaces (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16).
This misconfiguration creates a situation where an adversary on an adjacent network can craft network conditions that cause the VPN client to block traffic to legitimate destinations. The attack does not require any privileges and only requires user interaction to be successful.
Root Cause
The root cause lies in the WireGuard client's firewall configuration logic on Windows systems. When establishing a VPN tunnel, the client creates firewall rules that are intended to ensure all traffic flows through the secure VPN connection. However, the implementation incorrectly handles non-RFC1918 IP address ranges, treating them as potentially hostile and blocking traffic to these addresses.
This design flaw means that organizations using non-standard private IP ranges, or legitimate public IP addresses within local network segments, will experience traffic blocking when users connect via WireGuard VPN.
Attack Vector
The attack requires adjacent network access, meaning the attacker must be on the same local network segment as the victim. The attack scenario typically involves:
The adversary configures or manipulates the local network environment to use non-RFC1918 IP addresses. When the victim connects their WireGuard VPN client, the client's firewall configuration blocks traffic to these non-RFC1918 addresses on the local network. This results in denial of service to specific network services or hosts, despite the VPN connection appearing to function normally.
For detailed technical information about this class of LocalNet attacks, refer to the TunnelCrack Vulnerability Details research page.
Detection Methods for CVE-2023-35838
Indicators of Compromise
- Unexpected firewall rule additions by the WireGuard client that block non-RFC1918 IP ranges
- Network connectivity failures to specific IP addresses when VPN is active
- Windows Firewall logs showing blocked outbound connections during active VPN sessions
- User reports of intermittent connectivity to local network resources while VPN is enabled
Detection Strategies
- Monitor Windows Firewall rule changes when WireGuard client establishes connections
- Implement network monitoring to detect traffic blocking patterns affecting non-RFC1918 addresses
- Review VPN client logs for configuration anomalies during tunnel establishment
- Deploy endpoint detection to identify unexpected network isolation behaviors
Monitoring Recommendations
- Enable Windows Firewall audit logging to track rule modifications
- Configure SentinelOne endpoint agents to monitor network configuration changes
- Implement alerts for WireGuard client firewall policy modifications
- Monitor for user-reported connectivity issues correlating with VPN connection events
How to Mitigate CVE-2023-35838
Immediate Actions Required
- Assess your network environment to determine if non-RFC1918 IP addresses are in use on local network segments
- Audit WireGuard client configurations on Windows endpoints running version 0.5.3
- Consider temporarily using alternative VPN solutions if non-RFC1918 local addressing is required
- Educate users about the potential for unexpected traffic blocking while using VPN connections
Patch Information
Organizations should monitor the WireGuard Official Website for updated client versions that address this vulnerability. Check vendor security advisories including the SonicWall Vulnerability Advisory SNWLID-2023-0015 for guidance on affected products and remediation steps.
Upgrade WireGuard clients to versions newer than 0.5.3 once patches addressing this firewall configuration issue are released.
Workarounds
- Reconfigure local network segments to use RFC1918 private IP address ranges where possible
- Manually adjust Windows Firewall rules after VPN connection to allow traffic to necessary non-RFC1918 addresses
- Implement network segmentation to isolate VPN users from networks using non-standard IP addressing
- Use alternative VPN solutions that properly handle non-RFC1918 local network configurations
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
