CVE-2016-7995 in QEMU
Summary
by MITRE
Memory leak in the ehci_process_itd function in hw/usb/hcd-ehci.c in QEMU (aka Quick Emulator) allows local guest OS administrators to cause a denial of service (memory consumption) via a large number of crafted buffer page select (PG) indexes.
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Analysis
by VulDB Data Team • 10/05/2022
The vulnerability identified as CVE-2016-7995 represents a critical memory management flaw within QEMU's USB emulation subsystem, specifically affecting the ehci_process_itd function in the hw/usb/hcd-ehci.c file. This issue arises from inadequate input validation and memory handling when processing USB transactions, creating a condition where malicious actors can exploit the system's resource allocation mechanisms. The vulnerability manifests when a local guest OS administrator crafts a specific sequence of buffer page select (PG) indexes that trigger improper memory deallocation, leading to progressive memory consumption over time. This memory leak occurs within the Enhanced Host Controller Interface (EHCI) implementation, which governs USB 2.0 communication protocols in virtualized environments, making it particularly dangerous in multi-tenant cloud computing scenarios where resource exhaustion could affect other virtual machines sharing the same host infrastructure.
The technical exploitation of this vulnerability involves the manipulation of USB transaction descriptors through crafted buffer page select indexes that bypass normal memory management protocols. When the ehci_process_itd function processes these malformed descriptors, it fails to properly release allocated memory resources, causing a gradual accumulation of unreleased memory blocks within the QEMU process. This memory leak operates at the hypervisor level, meaning that even though the attack originates from within the guest operating system, the impact extends to the host system's resource pool. The vulnerability is classified under CWE-401, which specifically addresses improper release of memory, and demonstrates how insufficient input validation in virtualization components can create persistent resource consumption issues. The flaw is particularly concerning because it requires minimal privileges to exploit, as local guest administrators already possess the necessary access to manipulate USB controller configurations and submit crafted transactions that trigger the memory leak condition.
The operational impact of CVE-2016-7995 extends beyond simple denial of service, as the progressive memory consumption can lead to system instability and potential system crashes when memory resources become exhausted. In virtualized environments, this vulnerability can cause cascading failures affecting multiple virtual machines hosted on the same physical hardware, as memory allocated to one VM gradually consumes resources needed by others. The vulnerability is particularly dangerous in cloud computing environments where resource limits are enforced at the hypervisor level, as attackers can consume memory resources without triggering immediate alerts or resource throttling mechanisms. According to ATT&CK framework category T1499, this vulnerability aligns with resource exhaustion techniques that target system memory, potentially leading to broader system compromise when combined with other attack vectors. The memory leak can also be leveraged as a precursor to more sophisticated attacks, as it may be used to create conditions that facilitate privilege escalation or information disclosure through memory corruption.
Mitigation strategies for CVE-2016-7995 should focus on both immediate patching and operational monitoring. Organizations should prioritize applying the official QEMU patches that address the memory leak in the ehci_process_itd function, which typically involve implementing proper memory deallocation routines and input validation for buffer page select indexes. Additionally, system administrators should implement memory monitoring and alerting mechanisms to detect unusual memory consumption patterns that may indicate exploitation attempts. The vulnerability highlights the importance of implementing strict resource limits and quotas for virtual machines, particularly for USB controller access, to prevent single VMs from consuming excessive host resources. Network segmentation and access controls should also be enforced to limit guest OS administrators' ability to submit potentially malicious USB transactions. Regular security assessments of virtualization environments should include testing for similar memory management vulnerabilities, and organizations should consider implementing automated memory leak detection tools that can identify and alert on abnormal memory consumption patterns in running virtual machines. The incident underscores the critical need for maintaining up-to-date virtualization software and implementing comprehensive monitoring strategies that can detect and respond to resource exhaustion attacks in real-time.