CVE-2016-6834 in QEMU
Summary
by MITRE
The net_tx_pkt_do_sw_fragmentation function in hw/net/net_tx_pkt.c in QEMU (aka Quick Emulator) allows local guest OS administrators to cause a denial of service (infinite loop and QEMU process crash) via a zero length for the current fragment length.
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Analysis
by VulDB Data Team • 10/05/2022
The vulnerability identified as CVE-2016-6834 resides within the QEMU virtualization platform's network subsystem, specifically in the net_tx_pkt_do_sw_fragmentation function located in hw/net/net_tx_pkt.c. This flaw represents a critical security issue that can be exploited by local administrators within a guest operating system to trigger a denial of service condition. The vulnerability manifests when a zero length is provided for the current fragment length parameter, creating a scenario where the network packet processing logic becomes trapped in an infinite loop. This infinite loop ultimately leads to the complete crash of the QEMU process, effectively terminating the virtual machine and rendering it unavailable to users. The flaw demonstrates a classic buffer handling error where insufficient input validation allows malformed data to disrupt normal program execution flow, making it particularly dangerous in virtualized environments where guest administrators may have elevated privileges.
The technical implementation of this vulnerability stems from improper validation of packet fragment lengths within QEMU's software network packet processing pipeline. When a guest operating system sends network packets with zero-length fragments, the net_tx_pkt_do_sw_fragmentation function fails to properly handle this edge case, causing the loop condition to never terminate. This type of vulnerability aligns with CWE-691, which specifically addresses insufficient control flow management in software systems. The flaw operates at the network layer of virtualized environments where QEMU acts as the hypervisor, processing packets between guest operating systems and the host network infrastructure. The infinite loop occurs because the function does not properly check for zero-length fragments before entering the processing loop, allowing malicious or malformed packet data to cause the virtual machine's network processing to become unresponsive.
The operational impact of this vulnerability extends beyond simple denial of service, as it can compromise entire virtualized environments where QEMU serves as the primary virtualization platform. In cloud computing and server virtualization deployments, this vulnerability could allow an authenticated guest administrator to crash multiple virtual machines simultaneously, leading to significant service disruption. The vulnerability affects QEMU versions prior to 2.7.0, making it particularly concerning for organizations that maintain older virtualization infrastructure. Attackers could exploit this weakness to disrupt services in multi-tenant environments, potentially causing cascading failures that impact multiple users or applications. The vulnerability also represents a potential vector for privilege escalation attacks, as guest administrators could use this technique to gain control over host resources or cause system-wide outages. From an ATT&CK framework perspective, this vulnerability maps to T1499.004 (Endpoint Denial of Service) and T1068 (Exploitation for Privilege Escalation).
Mitigation strategies for CVE-2016-6834 involve immediate patching of QEMU installations to versions 2.7.0 or later, where the vulnerability has been addressed through proper input validation and loop termination conditions. Organizations should also implement network monitoring to detect unusual packet fragmentation patterns that might indicate exploitation attempts. Additional defensive measures include restricting guest administrator privileges where possible, implementing network segmentation to limit potential attack surfaces, and conducting regular vulnerability assessments of virtualization environments. System administrators should also consider implementing process monitoring and automatic restart mechanisms for QEMU processes to minimize downtime in case of successful exploitation. The fix implemented in QEMU 2.7.0 specifically addresses the validation of fragment lengths before processing, ensuring that zero-length fragments are properly handled without causing infinite loops. This vulnerability serves as a reminder of the critical importance of input validation in virtualization platforms and the potential for guest-level exploits to cause host-level disruptions.