CVE-2026-101309 in bhyve
Summary
by MITRE • 10/15/2026
The xHCI (USB 3) device model did not validate or save the event ring size provided by the guest. The size was re-read from guest memory on each use, and Transfer Request Block (TRB) fields were loaded without protection against concurrent modification by the guest. This race condition could be triggered by a guest running malicious, privileged code.
A malicious guest can crash the bhyve process or potentially execute arbitrary code in the host userspace context running the virtual machine, subject to bhyve's Capsicum sandbox.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability identified within the xHCI USB 3 device model of the bhyve hypervisor represents a critical failure in input validation and memory safety during the handling of guest-initiated hardware emulation requests. Specifically, the flaw stems from an improper race condition where the event ring size provided by the virtual machine is not validated or persisted at initialization time. Instead, the system re-reads this value directly from guest-controlled memory every time it is accessed for processing USB events. This design choice introduces a significant window of opportunity for exploitation because the data source remains mutable and under the direct control of the untrusted guest environment throughout the lifecycle of the emulation process.
The core technical flaw lies in the lack of atomicity and protection against concurrent modification when loading Transfer Request Block fields from guest memory. In virtualization environments, hardware models must treat all inputs originating from guests as potentially hostile until proven otherwise through strict validation and copying into isolated host structures. By failing to lock or copy these critical data structures before use, bhyve allows a malicious guest operating with privileged code to alter the contents of its own memory pages while the hypervisor is in the middle of processing them. This race condition enables an attacker to manipulate pointers, lengths, or control fields within the TRB structure dynamically during execution, leading to undefined behavior in the host userspace context.
From a security impact perspective, this vulnerability allows for severe consequences ranging from denial of service to arbitrary code execution on the host system. A malicious guest can trigger a crash of the bhyve process by causing memory access violations or logical errors through crafted race conditions, effectively resulting in a virtual machine escape via hypervisor instability. More critically, if the timing and manipulation are precise enough, an attacker could achieve arbitrary read-write primitives within the host userspace running the VM. This capability is particularly dangerous as it bypasses standard isolation boundaries, potentially allowing access to sensitive data or further compromise of the physical host infrastructure, although the presence of Capsicum sandboxing may mitigate some aspects by restricting system call capabilities and memory permissions for the bhyve process itself.
This vulnerability aligns with CWE-362, which describes concurrent execution race conditions that lead to unintended state changes, as well as CWE-119, relating to improper restriction of operations within the bounds of a memory buffer due to unsafe access patterns. In terms of adversary tactics, this exploit vector corresponds to ATT&CK technique T1055, specifically Process Injection or Memory Manipulation techniques used by attackers to escalate privileges and escape containment boundaries in virtualized environments. The failure to properly sanitize and isolate guest inputs before processing is a classic example of how hypervisor-side vulnerabilities can undermine the fundamental security guarantees provided by virtualization technologies.
To mitigate this risk, immediate patching of the bhyve software to enforce strict validation of all device model parameters upon initialization is required. Developers must ensure that event ring sizes and other critical configuration values are copied into host-managed memory structures with appropriate locking mechanisms or atomic operations to prevent race conditions during subsequent accesses. Furthermore, implementing stricter Capsicum capabilities for the bhyve process can limit the blast radius in case of exploitation by restricting access to sensitive system resources and network interfaces. Regular security audits focusing on virtualization device models should prioritize checking for similar patterns where guest memory is accessed directly without prior validation or isolation, ensuring that all hardware emulation logic adheres to zero-trust principles regarding inputs from untrusted guests.