CVE-2016-9111 in Receiver Desktop Lock
Summary
by MITRE
Incorrect access control mechanisms in Citrix Receiver Desktop Lock 4.5 allow an attacker to bypass the authentication requirement by leveraging physical access to a VDI for temporary disconnection of a LAN cable. NOTE: as of 20161208, the vendor could not reproduce the issue, stating "the researcher was unable to provide us with information that would allow us to confirm the behaviour and, despite extensive investigation on test deployments of supported products, we were unable to reproduce the behaviour as he described. The researcher has also, despite additional requests for information, ceased to respond to us."
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Analysis
by VulDB Data Team • 01/14/2025
The vulnerability described in CVE-2016-9111 represents a critical access control flaw within Citrix Receiver Desktop Lock version 4.5, specifically targeting virtual desktop infrastructure environments. This issue demonstrates how physical security measures can be exploited to circumvent digital authentication protocols, creating a dangerous intersection between hardware and software security boundaries. The vulnerability exploits a fundamental weakness in the session management architecture of Citrix virtual desktop environments, where the system fails to properly validate authentication status during temporary network disconnections.
The technical flaw manifests when an attacker with physical access to a virtual desktop infrastructure endpoint can temporarily disconnect the LAN cable, effectively creating a network interruption that allows bypass of the authentication mechanism. This exploit leverages the fact that Citrix Receiver Desktop Lock 4.5 does not properly maintain session state validation during network disruptions, enabling unauthorized access to locked desktop sessions. The vulnerability specifically affects the desktop lock functionality, where the system should enforce authentication requirements but instead permits access when network connectivity is temporarily lost. This behavior aligns with CWE-284, which addresses improper access control mechanisms, and demonstrates how physical access can be weaponized to compromise digital security controls.
The operational impact of this vulnerability extends beyond simple unauthorized access, as it undermines the entire security model of virtual desktop environments. Organizations relying on Citrix VDI solutions for secure remote access and desktop management face significant risks when this vulnerability is exploited. The attack scenario is particularly concerning because it requires minimal technical expertise beyond physical access to the endpoint, making it accessible to threat actors with basic hardware manipulation capabilities. This vulnerability essentially creates a backdoor that allows attackers to access locked desktop sessions without proper authentication, potentially leading to data exfiltration, privilege escalation, and persistent access within the network environment.
The remediation approach for this vulnerability should focus on implementing proper session state validation during network interruptions, ensuring that authentication requirements remain enforced regardless of network connectivity status. Organizations should consider implementing additional security controls such as mandatory session timeouts, enhanced network monitoring, and proper physical security measures to prevent unauthorized access to endpoints. The vulnerability highlights the importance of considering all attack vectors including physical access points when designing security architectures, as emphasized by the ATT&CK framework's consideration of physical access as a potential initial compromise vector. Security teams should also implement network segmentation and monitoring to detect anomalous behavior patterns that might indicate exploitation attempts.
The vendor's inability to reproduce the issue demonstrates the complexity and environmental dependencies that can affect vulnerability validation. This situation is not uncommon in security research where specific configurations, timing, or environmental factors are required to trigger certain vulnerabilities. The lack of reproducibility does not diminish the potential impact of the vulnerability, but rather emphasizes the need for organizations to implement defensive measures proactively. The vulnerability serves as a reminder that access control mechanisms must be robust against various attack scenarios, including those that exploit edge cases in session management and network interruption handling. Organizations should also consider implementing layered security approaches that do not rely solely on a single point of failure within their authentication systems.