CVE-2016-1981 in QEMU
Summary
by MITRE
QEMU (aka Quick Emulator) built with the e1000 NIC emulation support is vulnerable to an infinite loop issue. It could occur while processing data via transmit or receive descriptors, provided the initial receive/transmit descriptor head (TDH/RDH) is set outside the allocated descriptor buffer. A privileged user inside guest could use this flaw to crash the QEMU instance resulting in DoS.
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Analysis
by VulDB Data Team • 10/12/2022
The vulnerability identified as CVE-2016-1981 affects QEMU virtualization software when configured with e1000 network interface card emulation capabilities. This issue represents a critical denial of service weakness that can be exploited by malicious actors within guest operating systems to disrupt the entire virtual machine environment. The flaw specifically manifests when QEMU processes network data through transmit or receive descriptor mechanisms, creating a scenario where the virtual machine's network operations can become indefinitely stuck in a processing loop.
The technical root cause of this vulnerability lies in the improper handling of descriptor buffer boundaries within the e1000 network adapter emulation layer. When the initial receive descriptor head (RDH) or transmit descriptor head (TDH) registers are set to positions that fall outside the allocated memory buffer boundaries, QEMU enters an infinite loop during descriptor processing. This condition occurs because the emulation code fails to properly validate descriptor head positions against buffer limits before beginning data processing operations. The e1000 network interface implementation does not include adequate boundary checks that would prevent the descriptor head from pointing to invalid memory locations, allowing malicious code within the guest to manipulate these registers to trigger the problematic behavior.
From an operational perspective, this vulnerability poses significant risks to virtualized environments where guest operating systems may have elevated privileges or where privilege escalation has occurred. A malicious user within the guest system can exploit this flaw by manipulating the network descriptor head registers to point beyond the allocated descriptor buffer space. The resulting infinite loop consumes excessive CPU resources and prevents the QEMU instance from processing legitimate network traffic or performing other essential operations. This can lead to complete system unresponsiveness, requiring manual intervention to restart the virtual machine or potentially causing cascading failures in larger virtualized infrastructures where multiple VMs depend on the same hypervisor resources.
The impact of this vulnerability aligns with CWE-835, which addresses infinite loops in software implementations, and can be categorized under the ATT&CK technique T1499.004 for Network Denial of Service. The flaw represents a classic example of improper input validation within virtualization components, where the lack of boundary checking allows attackers to manipulate internal state variables to achieve unintended program behavior. The vulnerability affects QEMU versions that include e1000 network card emulation, making it particularly relevant for cloud environments and virtual desktop infrastructures that rely on this network adapter type. Organizations using QEMU with e1000 emulation should consider this issue as a high-priority concern for their virtualization security posture, especially in multi-tenant environments where guest isolation is critical.
Mitigation strategies should focus on updating QEMU to versions that include proper descriptor boundary validation and buffer overflow protection mechanisms. System administrators should also implement network monitoring to detect unusual CPU usage patterns that may indicate the exploitation of this vulnerability. Additional defensive measures include restricting guest privileges where possible, implementing proper access controls, and maintaining up-to-date virtualization patches. The vulnerability demonstrates the importance of thorough input validation in virtualization components and highlights the need for comprehensive security testing of emulated hardware components to prevent similar issues in other virtualization software.