CVE-2016-6833 in QEMU
Summary
by MITRE
Use-after-free vulnerability in the vmxnet3_io_bar0_write function in hw/net/vmxnet3.c in QEMU (aka Quick Emulator) allows local guest OS administrators to cause a denial of service (QEMU instance crash) by leveraging failure to check if the device is active.
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Analysis
by VulDB Data Team • 10/05/2022
The CVE-2016-6833 vulnerability represents a critical use-after-free flaw in the QEMU virtualization platform that specifically targets the vmxnet3 network driver implementation. This vulnerability exists within the hw/net/vmxnet3.c file where the vmxnet3_io_bar0_write function fails to properly validate device state before performing memory operations. The issue manifests when a guest operating system administrator exploits the lack of proper device activity checking, creating a scenario where freed memory regions are accessed after the device has been deactivated or removed from the virtual machine's address space.
The technical exploitation of this vulnerability occurs through the improper handling of memory references within the virtualized network interface driver. When the vmxnet3_io_bar0_write function processes write operations to the device's memory-mapped I/O registers, it does not first verify whether the underlying virtual device remains active and properly initialized. This oversight creates a race condition where memory that has been freed during device deactivation can still be accessed during subsequent write operations, leading to undefined behavior and system instability. The vulnerability is classified as a use-after-free condition under CWE-416, which specifically addresses the reuse of memory after it has been freed, making it particularly dangerous in virtualized environments where memory management is complex and multi-layered.
From an operational perspective, this vulnerability allows local guest OS administrators to execute a denial of service attack against the QEMU hypervisor instance hosting their virtual machine. The attack requires only that the malicious user have administrative privileges within the guest operating system, making it particularly concerning as it can be exploited from within the virtualized environment itself. When successfully exploited, the vulnerability causes the QEMU instance to crash and terminate unexpectedly, resulting in service disruption for all virtual machines running on that hypervisor. This type of attack falls under the ATT&CK framework's privilege escalation and denial of service techniques, specifically targeting the hypervisor layer and potentially affecting multiple virtual machines sharing the same host infrastructure.
The impact of this vulnerability extends beyond simple service disruption as it represents a fundamental flaw in the virtualization security model where guest administrators can compromise host stability. The lack of proper device state validation creates a potential attack vector that could be leveraged in more sophisticated multi-stage attacks, particularly in environments where multiple virtual machines share the same physical host. Organizations using QEMU-based virtualization platforms should prioritize immediate patching of this vulnerability, as the use-after-free condition could potentially be exploited to escalate privileges or achieve arbitrary code execution under certain circumstances. The vulnerability demonstrates the critical importance of proper memory management and state validation in virtualized environments where guest and host components interact through complex memory-mapped I/O interfaces. Security practitioners should implement monitoring for unusual QEMU crash patterns and ensure that all virtualization environments are updated with the latest security patches addressing this specific use-after-free condition in the vmxnet3 network driver implementation.