CVE-2023-6683 in QEMU
Summary
by MITRE • 01/12/2024
A flaw was found in the QEMU built-in VNC server while processing ClientCutText messages. The qemu_clipboard_request() function can be reached before vnc_server_cut_text_caps() was called and had the chance to initialize the clipboard peer, leading to a NULL pointer dereference. This could allow a malicious authenticated VNC client to crash QEMU and trigger a denial of service.
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Analysis
by VulDB Data Team • 08/02/2025
The vulnerability identified as CVE-2023-6683 resides within the QEMU virtualization platform's built-in Virtual Network Computing server implementation. This flaw manifests during the processing of ClientCutText messages, which are standard VNC protocol commands used for clipboard data transfer between client and server. The issue represents a classic null pointer dereference vulnerability that occurs due to improper state management within the clipboard handling subsystem. The QEMU VNC server maintains a complex state machine for managing clipboard operations, and this particular flaw exploits a race condition or initialization sequence error that allows malicious actors to manipulate the server's execution flow.
The technical root cause stems from the improper ordering of function calls within the VNC server's clipboard processing logic. Specifically, the qemu_clipboard_request() function can be invoked before the vnc_server_cut_text_caps() function has completed its initialization sequence for the clipboard peer structure. This creates a scenario where the clipboard peer reference remains uninitialized or NULL, yet the code attempts to dereference it during clipboard request processing. The vulnerability is particularly dangerous because it requires only authentication to the VNC server, making it accessible to any user who can establish a valid VNC connection to the affected QEMU instance. This authentication requirement significantly reduces the attack surface compared to unauthenticated vulnerabilities, though it still represents a critical security issue in virtualized environments where VNC access is permitted.
From an operational impact perspective, this vulnerability enables a malicious authenticated VNC client to induce a denial of service condition that completely crashes the QEMU process. The crash occurs because the server attempts to access memory through a NULL pointer reference, causing an immediate termination of the virtual machine's VNC service. In production environments, this could lead to significant service disruption, especially in scenarios where multiple virtual machines are managed through a single QEMU instance or where automated failover mechanisms depend on stable VNC connections. The vulnerability affects any QEMU deployment that has VNC server functionality enabled, which includes virtually all QEMU installations that support graphical console access to virtual machines. The impact extends beyond simple service disruption as it can potentially affect the stability of entire virtualization infrastructures where multiple VMs share the same hypervisor instance.
The vulnerability aligns with CWE-476, which specifically addresses NULL pointer dereference conditions in software implementations. From an adversarial perspective, this flaw maps to ATT&CK technique T1499.004, which covers network denial of service attacks through exploitation of software vulnerabilities. The attack vector requires a legitimate VNC client connection, making it difficult to exploit in environments with proper network segmentation and access controls, but it remains a serious concern for cloud providers and virtualization administrators who must ensure robust protection of their hypervisor infrastructure. Mitigation strategies should focus on immediate patch application from the QEMU maintainers, along with network-level controls that restrict VNC access to trusted networks and implement proper access controls. Additionally, administrators should consider implementing monitoring for abnormal VNC activity patterns and ensure that VNC services are not exposed to untrusted networks without proper authentication and encryption mechanisms in place.