CVE-2026-101311 in bhyve
Summary
by MITRE • 10/15/2026
The framebuffer device model's maximum resolution constants were duplicated between the framebuffer and RFB modules and had gotten out of sync. Additionally, writes to the framebuffer BAR could set arbitrary width and height values without validation, and the framebuffer BAR I/O and RFB rendering threads were not synchronized.
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 involves a critical synchronization failure within the framebuffer device model of the bhyve hypervisor, specifically concerning the interaction between the framebuffer and Remote Framebuffer (RFB) modules. The core technical flaw stems from duplicated maximum resolution constants that were maintained independently in both modules rather than being centralized or synchronized through a shared mechanism. Over time, these duplicate definitions drifted out of sync, creating an inconsistent state regarding what constitutes valid display dimensions within the virtual machine environment. This inconsistency is compounded by insufficient input validation when handling writes to the framebuffer Base Address Register (BAR). The system allows guest operating systems to specify arbitrary width and height values without adequate boundary checks or range validations against the actual hardware capabilities or allocated memory buffers.
This lack of validation permits a malicious guest to manipulate internal state variables in ways that exceed expected limits, leading to potential buffer overflows or out-of-bounds memory accesses within the host userspace process running bhyve. The situation is further exacerbated by a concurrency issue where the framebuffer BAR input/output operations and the RFB rendering threads operate without proper synchronization primitives such as mutexes or locks. This race condition allows for complex exploitation scenarios where timing differences between thread execution can be leveraged to corrupt memory structures or trigger undefined behavior in the host kernel userspace context. The combination of unsynchronized state updates and unchecked dimension parameters creates a high-severity attack surface that compromises the integrity of the virtualization layer.
From an operational impact perspective, this vulnerability allows for severe security breaches including denial of service through process crashes and potential remote code execution on the host system. Since bhyve operates within a Capsicum sandbox to limit privilege escalation, the actual exploitability depends heavily on whether the specific memory corruption or logic error can bypass these sandbox restrictions. If successful, an attacker could execute arbitrary code with the privileges of the bhyve process, potentially leading to full compromise of the host machine and escape from the virtual machine isolation boundary. This represents a significant risk in multi-tenant environments where guest trust boundaries are critical for overall infrastructure security.
To mitigate this vulnerability, immediate patching is required to synchronize the maximum resolution constants across all relevant modules, ensuring that a single source of truth governs display dimension limits. Developers must implement strict validation logic when processing writes to the framebuffer BAR, enforcing bounds checking against allocated buffer sizes and hardware constraints before accepting any width or height parameters from guest requests. Furthermore, concurrency controls such as mutexes or spinlocks must be introduced to synchronize access between the framebuffer I/O threads and RFB rendering threads, preventing race conditions that could lead to state corruption. These changes should align with industry standards for secure coding practices, specifically addressing CWE-20 Improper Input Validation and CWE-362 Concurrent Execution using Shared Resource with Insufficient Synchronization. In terms of attack vector classification, this vulnerability relates to ATT&CK techniques involving privilege escalation via virtualization escape mechanisms, highlighting the need for rigorous testing of hypervisor interfaces against malicious guest behaviors.