CVE-2017-4941 in ESXi
Summary
by MITRE
VMware ESXi (6.0 before ESXi600-201711101-SG, 5.5 ESXi550-201709101-SG), Workstation (12.x before 12.5.8), and Fusion (8.x before 8.5.9) contain a vulnerability that could allow an authenticated VNC session to cause a stack overflow via a specific set of VNC packets. Successful exploitation of this issue could result in remote code execution in a virtual machine via the authenticated VNC session. Note: In order for exploitation to be possible in ESXi, VNC must be manually enabled in a virtual machine's .vmx configuration file. In addition, ESXi must be configured to allow VNC traffic through the built-in firewall.
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Analysis
by VulDB Data Team • 01/27/2021
The vulnerability identified as CVE-2017-4941 represents a critical stack overflow flaw affecting multiple VMware virtualization products including ESXi, Workstation, and Fusion. This vulnerability specifically targets the VNC (Virtual Network Computing) implementation within these platforms, creating a pathway for authenticated remote code execution within virtual machines. The flaw stems from inadequate input validation in the VNC packet processing mechanism, where maliciously crafted VNC packets can trigger buffer overflow conditions in the stack memory region. The vulnerability's severity is amplified by its requirement for authentication, as it necessitates an existing VNC session to exploit the issue, though this requirement does not mitigate the potential impact of successful exploitation.
The technical implementation of this vulnerability involves the manipulation of VNC packet structures that are processed by the virtualization software's VNC server component. When an authenticated user establishes a VNC session and sends specially crafted packets, the system fails to properly validate the packet boundaries and length parameters before processing them in memory. This lack of proper bounds checking allows attackers to overflow the allocated stack buffer, potentially overwriting adjacent memory locations including return addresses and function pointers. The stack overflow occurs during the parsing of VNC messages, particularly when handling certain encoding formats or extended message types that exceed expected buffer sizes. The vulnerability's exploitation requires precise control over the packet structure to ensure the overflow corrupts memory in a way that allows code execution, making it a sophisticated target for attackers who understand the memory layout of the affected systems.
The operational impact of CVE-2017-4941 extends beyond simple privilege escalation, as successful exploitation can result in complete compromise of virtual machine environments. Attackers who gain access through this vulnerability can execute arbitrary code within the context of the virtual machine, potentially escalating privileges to the host system level depending on the virtualization configuration and access controls in place. The vulnerability's presence in ESXi requires specific configuration conditions to be exploitable, including manual enabling of VNC in the .vmx configuration file and firewall rule configuration allowing VNC traffic. This configuration dependency makes the vulnerability less likely to be exploited in default installations but increases the risk for organizations with relaxed security policies or misconfigured virtual environments. The impact is particularly concerning in multi-tenant cloud environments where virtual machines from different users share the same physical host, as exploitation could lead to cross-tenant data compromise or lateral movement within the virtualized infrastructure.
Organizations should implement immediate mitigations including applying the vendor patches released for ESXi 6.0 build ESXi600-201711101-SG and ESXi 5.5 build ESXi550-201709101-SG, as well as updating VMware Workstation 12.x to version 12.5.8 and Fusion 8.x to version 8.5.9. The recommended approach involves disabling VNC functionality in virtual machine configurations unless absolutely required for administrative purposes, and implementing strict firewall rules to prevent unauthorized VNC traffic. Network segmentation should be enforced to limit VNC access to trusted administrative networks, while regular security audits should verify that VNC is not enabled in production virtual machines without proper authorization. The vulnerability aligns with CWE-121 Stack-based Buffer Overflow, which is classified as a common weakness in software security practices, and maps to ATT&CK technique T1059.007 for Command and Scripting Interpreter: PowerShell, as attackers could use the compromised virtual machines to establish persistent access and execute further malicious commands. Security monitoring should include detection of unusual VNC traffic patterns and authentication attempts, while incident response procedures should account for potential virtual machine compromise scenarios that could affect the broader virtualized infrastructure.