CVE-2006-2450 in LibVNCServer
Summary
by MITRE
auth.c in LibVNCServer 0.7.1 allows remote attackers to bypass authentication via a request in which the client specifies an insecure security type such as "Type 1 - None", which is accepted even if it is not offered by the server, a different issue than CVE-2006-2369.
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Analysis
by VulDB Data Team • 06/24/2019
The vulnerability described in CVE-2006-2450 represents a critical authentication bypass flaw within LibVNCServer version 0.7.1, a widely used open source library for implementing virtual network computing protocols. This security weakness allows remote attackers to circumvent the authentication mechanism by exploiting how the server processes client security type requests. The flaw specifically manifests when clients attempt to establish connections using insecure security types such as "Type 1 - None" without proper validation of server capabilities. The vulnerability falls under the category of improper authentication handling as defined by CWE-287, which addresses issues where systems fail to properly verify the identity of users or systems attempting to access protected resources. This authentication bypass represents a significant risk to remote desktop environments that rely on LibVNCServer for their VNC server implementations, potentially allowing unauthorized access to systems with minimal effort from attackers.
The technical implementation of this vulnerability stems from the server's acceptance of client-specified security types regardless of whether those types were actually offered or supported by the server configuration. In proper authentication protocols, servers should only accept security types that they have explicitly advertised or made available during the initial negotiation phase of the connection establishment process. The flaw in LibVNCServer 0.7.1 occurs because the authentication.c module fails to validate that the client's requested security type matches the server's actual supported security types. This behavior creates a window where attackers can manipulate the security negotiation process by sending malicious requests that specify insecure authentication methods, effectively bypassing the intended security controls. The vulnerability is particularly concerning because it operates at the protocol level during the initial handshake phase, meaning that authentication bypass can occur before any meaningful session establishment takes place. This type of flaw is categorized under ATT&CK technique T1078.004 which covers Valid Accounts - Default Accounts, as it allows unauthorized access through manipulation of authentication parameters rather than through credential theft or brute force attacks.
The operational impact of this vulnerability extends beyond simple unauthorized access to encompass potential system compromise and data exposure across various network environments. When exploited, this vulnerability enables attackers to gain access to systems running vulnerable LibVNCServer implementations without requiring valid credentials, making it particularly dangerous in environments where VNC servers are exposed to untrusted networks or the internet. The consequences can range from unauthorized system monitoring and data exfiltration to full system compromise, depending on the privileges associated with the VNC sessions and the underlying operating systems. Organizations using VNC-based remote access solutions that rely on LibVNCServer 0.7.1 are particularly at risk, as attackers can exploit this vulnerability to establish unauthorized connections and potentially escalate privileges within the compromised systems. The vulnerability affects not only individual desktop systems but also enterprise environments where VNC servers may be used for remote administration and support, potentially exposing entire network infrastructures to unauthorized access. This authentication bypass can be particularly devastating in environments where VNC servers are configured with weak or default passwords, as the vulnerability allows attackers to bypass authentication entirely without needing to overcome password-based security measures.
Mitigation strategies for this vulnerability should focus on immediate patching of affected LibVNCServer installations, ensuring that all systems running vulnerable versions are updated to patched releases that properly validate client security type requests. Organizations should implement network segmentation and access controls to limit exposure of VNC servers to trusted networks, while also considering the deployment of additional authentication layers such as VPNs or bastion hosts for remote access. Security monitoring should be enhanced to detect unusual authentication patterns or attempts to establish connections using insecure security types, as these may indicate exploitation attempts. The implementation of proper network access controls including firewall rules that restrict VNC server access to known trusted IP addresses can significantly reduce the attack surface. Additionally, organizations should conduct comprehensive vulnerability assessments to identify all systems running vulnerable versions of LibVNCServer and ensure that all remote desktop services are properly configured with strong authentication mechanisms. System administrators should also consider implementing intrusion detection systems that can identify and alert on suspicious VNC protocol traffic patterns, particularly those involving insecure security type negotiations. Regular security audits and penetration testing should be conducted to verify that authentication mechanisms are properly functioning and that no similar vulnerabilities exist in related systems or components. The vulnerability serves as a reminder of the critical importance of proper protocol validation and the potential consequences of accepting client parameters without adequate server-side verification.