CVE-2016-5673 in Repeater
Summary
by MITRE
UltraVNC Repeater before 1300 does not restrict destination IP addresses or TCP ports, which allows remote attackers to obtain open-proxy functionality by using a :: substring in between the IP address and port number.
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Analysis
by VulDB Data Team • 11/02/2024
The vulnerability identified as CVE-2016-5673 affects the UltraVNC Repeater software version 1300 and earlier, presenting a critical security flaw that enables unauthorized proxy functionality. This issue stems from insufficient input validation within the repeater component that handles connection requests between VNC clients and servers. The vulnerability specifically manifests when the software processes destination IP addresses and TCP port specifications, failing to properly validate or sanitize the input format.
The technical flaw exploits a parsing weakness in how the UltraVNC Repeater handles connection strings containing IPv6 addresses. When an attacker crafts a malicious connection request with a double colon substring :: placed between an IP address and port number, the software incorrectly interprets this format and allows the connection to proceed without proper destination restrictions. This parsing error effectively bypasses the intended access controls that should prevent connections to arbitrary destinations, transforming the repeater into an open proxy service.
The operational impact of this vulnerability is severe as it allows remote attackers to leverage the compromised repeater as a conduit for unauthorized network access. Attackers can use this functionality to route traffic through the vulnerable system to target external networks, potentially bypassing firewalls and network security controls. The open-proxy capability enables various malicious activities including data exfiltration, command and control communications, and reconnaissance operations against internal networks that the repeater system might be connected to. This vulnerability essentially turns the legitimate VNC repeater into an unauthorized network tunneling device.
From a cybersecurity perspective, this vulnerability aligns with CWE-20, which describes improper input validation, and represents a classic case of insecure data handling in network services. The flaw also maps to several ATT&CK techniques including T1090 for proxy usage and T1071 for application layer protocols, as the vulnerability enables attackers to establish unauthorized communication channels through the VNC infrastructure. Organizations using UltraVNC Repeater software are particularly at risk since the vulnerability allows for persistent network access without proper authentication or authorization checks.
The recommended mitigations include immediate upgrading to UltraVNC Repeater version 1300 or later, which contains the necessary patches to address the input validation issue. Administrators should also implement network-level restrictions such as firewall rules that limit access to the repeater service to trusted IP addresses only, and disable unnecessary features that might expose the system to external connections. Additionally, monitoring network traffic for unusual proxy-like patterns and implementing intrusion detection systems can help identify potential exploitation attempts. Regular security assessments of VNC infrastructure components and maintaining up-to-date software versions remain essential practices to prevent similar vulnerabilities from being exploited in the future.