CVE-2016-9916 in QEMU
Summary
by MITRE
Memory leak in hw/9pfs/9p-proxy.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 proxy backend.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 10/27/2022
The vulnerability identified as CVE-2016-9916 represents a critical memory management flaw within QEMU's 9pfs proxy backend implementation. This issue resides in the hw/9pfs/9p-proxy.c file where a memory leak occurs due to inadequate cleanup operations during the proxy backend processing. The vulnerability specifically affects local privileged guest operating system users who can exploit this weakness to consume excessive host memory resources. The flaw manifests when the proxy backend fails to properly release allocated memory structures after processing 9p filesystem operations, leading to gradual memory consumption that can eventually exhaust available host resources.
The technical root cause of this vulnerability aligns with CWE-401, which catalogs memory leaks as a fundamental weakness in software design. In QEMU's context, the proxy backend handles communication between guest and host filesystem operations using the 9p protocol, a standard for accessing filesystems over a network. When guest operating systems make 9p filesystem requests through the proxy mechanism, the backend allocates memory for processing these requests but fails to properly deallocate memory when requests are completed or when errors occur. This memory allocation without corresponding deallocation creates a persistent resource drain that can be exploited systematically by malicious or compromised guest users.
The operational impact of CVE-2016-9916 extends beyond simple resource exhaustion to potentially compromise the entire virtualization environment. Local privileged guest users can leverage this vulnerability to cause sustained denial of service conditions on the host system, consuming memory resources at a rate that may eventually lead to system instability or complete QEMU process crashes. The attack vector requires the guest user to possess sufficient privileges within the guest operating system to make 9p filesystem requests, which is typically achievable in shared or multi-tenant virtualized environments where guest users have elevated access rights. This vulnerability particularly affects virtualization deployments where multiple guests share the same host resources, as the memory leak can accumulate across multiple guest instances, creating cascading effects that impact overall system performance and availability.
From a threat modeling perspective, this vulnerability maps to ATT&CK technique T1499.001 which covers resource exhaustion attacks. The flaw enables attackers to perform sustained memory consumption attacks that can be difficult to detect due to their gradual nature. The impact is significant as it can be exploited to create persistent denial of service conditions that may require manual intervention to resolve, including host system reboots or QEMU process restarts. Mitigation strategies should focus on implementing proper memory management practices within the proxy backend, including comprehensive cleanup operations for all allocated memory structures. The vulnerability also highlights the importance of input validation and proper resource management in virtualization components, particularly those handling guest-to-host communication protocols. Additionally, implementing memory monitoring and alerting mechanisms can help detect unusual memory consumption patterns that may indicate exploitation of this vulnerability, providing early warning capabilities for system administrators to respond before complete service disruption occurs.