CVE-2006-2369 in RealVNC
Summary
by MITRE
RealVNC 4.1.1, and other products that use RealVNC such as AdderLink IP and Cisco CallManager, 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, as originally demonstrated using a long password.
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Analysis
by VulDB Data Team • 06/22/2025
The vulnerability described in CVE-2006-2369 represents a critical authentication bypass flaw in RealVNC 4.1.1 and related products that implement the RealVNC protocol. This issue stems from improper validation of security negotiation mechanisms during the remote desktop connection establishment process. The vulnerability specifically affects the security type selection phase where clients can manipulate the authentication flow by specifying insecure security types that should not be accepted under normal circumstances. The flaw allows remote attackers to circumvent the intended authentication mechanisms by exploiting a weakness in the server's security type validation logic, making it particularly dangerous for remote access scenarios where unauthorized access could lead to complete system compromise.
The technical implementation of this vulnerability occurs during the initial handshake process of the VNC protocol where the client and server negotiate security types. When a client connects to a RealVNC server, it sends a security type selection message indicating which authentication method it wishes to use. The vulnerability exists because the server accepts insecure security types such as "Type 1 - None" even when these types are not explicitly offered by the server during the initial negotiation. This behavior violates fundamental security principles and creates a pathway for attackers to establish connections without proper authentication credentials. The demonstration of this vulnerability using a long password suggests that attackers can leverage this flaw to bypass authentication entirely, potentially gaining administrative access to systems protected by VNC services.
The operational impact of CVE-2006-2369 extends far beyond simple unauthorized access, as it fundamentally undermines the security model of VNC-based remote access solutions. Systems running affected RealVNC versions or products that incorporate this vulnerable code become susceptible to remote exploitation without requiring valid credentials, making them attractive targets for malicious actors. The vulnerability affects not only standalone RealVNC installations but also integrated solutions such as AdderLink IP and Cisco CallManager, which increases the attack surface significantly. Organizations relying on these products for remote access and management operations face substantial risk of unauthorized system compromise, data theft, and potential lateral movement within their networks. The impact is particularly severe in enterprise environments where VNC services are commonly used for remote system administration and support operations.
Mitigation strategies for this vulnerability should focus on immediate patching of affected RealVNC installations and related products to ensure proper security type validation. Organizations should implement network segmentation to limit access to VNC services and ensure that only trusted networks can reach these systems. The use of strong authentication mechanisms such as VNC passwords with sufficient complexity and regular rotation should be enforced, though this alone does not address the core vulnerability. Network monitoring should be enhanced to detect anomalous connection patterns and authentication attempts that might indicate exploitation attempts. Additionally, organizations should consider implementing alternative remote access solutions that do not exhibit this type of authentication bypass vulnerability, particularly for critical systems where the risk of exploitation is unacceptable. The vulnerability aligns with CWE-287 which addresses improper authentication issues, and represents a clear violation of the principle of least privilege in security protocol implementations. This flaw also maps to ATT&CK technique T1075 which covers the use of legitimate credentials for lateral movement, as attackers can leverage the bypass to gain unauthorized access to systems and potentially move laterally within networks.