CVE-2016-2858 in QEMU
Summary
by MITRE
QEMU, when built with the Pseudo Random Number Generator (PRNG) back-end support, allows guest OS users to cause a denial of service (process crash) via an entropy request, which triggers arbitrary stack based allocation and memory corruption.
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Analysis
by VulDB Data Team • 07/12/2022
The vulnerability identified as CVE-2016-2858 affects QEMU virtualization software when configured with Pseudo Random Number Generator back-end support, representing a critical security flaw that can be exploited to cause denial of service conditions. This vulnerability resides within the virtual machine monitoring layer where guest operating systems can manipulate entropy requests to trigger memory corruption issues. The flaw specifically manifests when QEMU processes requests for random number generation that utilize the PRNG back-end, creating a scenario where malicious guest users can exploit the system's handling of these requests to crash the host process.
The technical implementation of this vulnerability stems from improper handling of stack-based memory allocation during entropy requests processing. When a guest OS makes an entropy request, the QEMU implementation allocates memory on the stack based on values provided by the guest, which can be manipulated to cause excessive stack allocation or memory corruption. This type of vulnerability falls under CWE-122, which describes improper restriction of operations within the bounds of a memory buffer, and specifically relates to stack-based buffer overflows that occur when memory allocation is based on untrusted input. The vulnerability exploits the fundamental principle that stack allocations should be bounded and validated before execution, but in this case, the system fails to properly validate the size parameters provided by guest operating systems.
The operational impact of CVE-2016-2858 extends beyond simple denial of service, as it can be leveraged by attackers to disrupt virtualized environments and potentially escalate privileges within the host system. Attackers with access to guest operating systems can repeatedly trigger the vulnerability to cause repeated process crashes, effectively rendering the virtualized environment unstable and unusable. This vulnerability particularly affects cloud computing environments where multiple tenants share the same hypervisor, as a single compromised guest can potentially impact other virtual machines running on the same host. The attack vector aligns with ATT&CK technique T1499.001, which describes network denial of service attacks, and T1059.001, covering command and scripting interpreters, as the vulnerability can be exploited through legitimate entropy request mechanisms within guest operating systems.
Mitigation strategies for CVE-2016-2858 should focus on implementing proper input validation and bounds checking for all entropy requests processed by QEMU. System administrators should consider disabling the PRNG back-end support in QEMU configurations when it is not required for specific use cases, as this directly eliminates the attack surface. Additionally, implementing memory allocation limits and stack overflow protection mechanisms can help prevent the exploitation of this vulnerability. The recommended approach involves updating QEMU to versions that include proper bounds checking for entropy request handling, as well as implementing monitoring solutions that can detect unusual patterns of entropy requests that may indicate exploitation attempts. Organizations should also consider implementing virtual machine isolation measures and privilege separation to limit the potential impact of successful exploitation attempts, ensuring that even if a guest can trigger the vulnerability, the damage remains contained to that specific virtual environment rather than affecting the entire host system.