CVE-2016-9915 in QEMU
Summary
by MITRE
Memory leak in hw/9pfs/9p-handle.c in QEMU (aka Quick Emulator) allows local privileged guest OS users to cause a denial of service (host memory consumption and possibly QEMU process crash) by leveraging a missing cleanup operation in the handle backend.
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Analysis
by VulDB Data Team • 10/27/2022
The vulnerability identified as CVE-2016-9915 represents a critical memory management flaw within the QEMU virtualization platform that specifically affects the 9P file system implementation. This issue resides in the hw/9pfs/9p-handle.c component of QEMU's codebase, where a memory leak occurs due to inadequate resource cleanup operations during the handling of file system requests. The vulnerability is particularly concerning because it can be exploited by local privileged users within a guest operating system to consume excessive host memory resources, potentially leading to system instability or complete service disruption.
The technical root cause of this vulnerability stems from a missing cleanup operation within the 9P file system backend handler. When processing file system operations, the code fails to properly release allocated memory resources after completing operations, creating a gradual accumulation of memory consumption over time. This memory leak manifests when guest operating systems with elevated privileges perform file system operations through the 9P protocol interface, which is commonly used for sharing directories between host and guest systems in virtualized environments. The flaw specifically impacts the handling of file system requests in the 9P implementation, where allocated buffers and data structures are not properly deallocated upon completion of operations, resulting in memory fragmentation and progressive resource exhaustion.
The operational impact of this vulnerability extends beyond simple resource consumption, as it can lead to complete denial of service conditions for the affected QEMU host system. Local privileged users within a guest OS can exploit this vulnerability by repeatedly performing file system operations through the 9P interface, causing progressive memory consumption that may eventually exhaust available host memory resources. In severe cases, this memory exhaustion can trigger system instability, application crashes, or even complete system hangs that require manual intervention to restore normal operations. The vulnerability is particularly dangerous in multi-tenant virtualization environments where guest operating systems may be compromised or where users have legitimate but elevated privileges within their virtual machines.
This vulnerability maps directly to CWE-401: "Improper Release of Memory Before Removing Last Reference" and aligns with several ATT&CK techniques including T1499.004: "Endpoint Denial of Service" and T1070.004: "File Deletion." The attack surface is primarily limited to environments where QEMU is used for virtualization and where guest operating systems have local privileged access, typically through legitimate administrative accounts or compromised user sessions. Organizations should consider implementing monitoring for unusual memory consumption patterns in virtualized environments as an early detection mechanism for potential exploitation of this vulnerability.
Mitigation strategies for CVE-2016-9915 should prioritize immediate patching of affected QEMU versions, as the vulnerability has been addressed in subsequent releases through proper memory cleanup implementation. System administrators should also implement memory monitoring and alerting mechanisms to detect abnormal resource consumption patterns that may indicate exploitation attempts. Additionally, virtualization administrators should consider limiting guest privileges where possible and implementing network segmentation to reduce the attack surface. The vulnerability underscores the importance of proper memory management practices in virtualization software and highlights the need for comprehensive security testing of virtualization components, particularly those handling inter-process communication and shared resource access. Organizations should also conduct regular vulnerability assessments of their virtualization infrastructure to identify and remediate similar memory management issues across their entire virtualization stack.