CVE-2025-29833 Overview
CVE-2025-29833 is a Time-of-Check Time-of-Use (TOCTOU) race condition in the Windows Virtual Machine Bus (VMBus). The flaw allows an unauthorized local attacker to execute code on a vulnerable host. VMBus is the high-speed communication channel between Hyper-V guests and the parent partition, so successful exploitation crosses a virtualization trust boundary. Microsoft tracks the issue under [CWE-367] and published the advisory in May 2025. The vulnerability affects a broad range of Windows client and server releases, including Windows 10, Windows 11, and Windows Server 2012 through Windows Server 2025.
Critical Impact
A successful race window exploit grants code execution with scope change, potentially allowing a guest-side actor to compromise the Hyper-V host with high confidentiality, integrity, and availability impact.
Affected Products
- Microsoft Windows 10 (1507, 1607, 1809, 21H2, 22H2) across x86, x64, and ARM64
- Microsoft Windows 11 (22H2, 23H2, 24H2) across x64 and ARM64
- Microsoft Windows Server 2012, 2012 R2, 2016, 2019, 2022, 2022 23H2, and 2025
Discovery Timeline
- 2025-05-13 - CVE-2025-29833 published to NVD
- 2025-05-13 - Microsoft releases security advisory and patch
- 2025-05-19 - Last updated in NVD database
Technical Details for CVE-2025-29833
Vulnerability Analysis
The vulnerability resides in the Windows Virtual Machine Bus, the inter-partition communication channel used by Hyper-V. VMBus relies on shared memory ring buffers and channel objects to broker requests between guest virtual machines and the root partition. A race condition exists between the validation of a shared resource and its subsequent use by privileged kernel code. An attacker who wins the race can substitute the validated state with attacker-controlled data before it is consumed. Because the issue spans the virtualization boundary, exploitation results in a scope change with high impact to confidentiality, integrity, and availability.
Root Cause
The defect is classified as [CWE-367] Time-of-Check Time-of-Use. VMBus checks the validity or identity of a guest-supplied object, then re-reads the same object during processing without atomicity guarantees. An attacker controlling concurrent threads in the guest can mutate the shared structure between the check and the use, causing privileged code in the host to operate on unverified input.
Attack Vector
Exploitation requires local access and user interaction, and the attack complexity is high because the attacker must reliably win a narrow timing window. The attacker triggers VMBus operations from a guest context, races the validation logic with rapid concurrent writes to shared memory, and steers the host into executing attacker-influenced code paths. No verified public proof-of-concept code is currently available for CVE-2025-29833.
// No verified public exploit code is available for CVE-2025-29833.
// See the Microsoft Security Response Center advisory for technical details.
Detection Methods for CVE-2025-29833
Indicators of Compromise
- Unexpected crashes or bug checks referencing vmbus.sys or vmbkmcl.sys on Hyper-V hosts
- Hyper-V Worker Process (vmwp.exe) terminations correlated with guest activity spikes
- New or unsigned kernel modules loaded shortly after VMBus channel events in the System event log
Detection Strategies
- Hunt for anomalous child processes spawned by vmwp.exe or System on virtualization hosts
- Correlate Hyper-V-Worker and Hyper-V-VMMS event logs with kernel-mode crashes to surface race-window exploitation attempts
- Monitor for guest workloads issuing high-frequency, low-latency VMBus operations consistent with a race-condition trigger
Monitoring Recommendations
- Enable and forward Microsoft-Windows-Hyper-V-Worker, Hyper-V-VMMS, and Hyper-V-Hypervisor channels to a central log store
- Track driver load events (Sysmon Event ID 6) on Hyper-V hosts and alert on unsigned or unexpected drivers
- Baseline normal VMBus activity per tenant and alert on deviations that coincide with privileged process behavior
How to Mitigate CVE-2025-29833
Immediate Actions Required
- Apply the May 2025 Microsoft security update referenced in the Microsoft Vulnerability Advisory CVE-2025-29833 to every affected Hyper-V host
- Inventory Hyper-V hosts across Windows 10, Windows 11, and Windows Server 2012 through 2025 and prioritize patching of multi-tenant or production virtualization clusters
- Restrict who can create or run virtual machines on shared Hyper-V infrastructure until patches are deployed
Patch Information
Microsoft released fixes for CVE-2025-29833 on 2025-05-13. Patch identifiers and download links for each affected build are listed in the Microsoft Vulnerability Advisory CVE-2025-29833. Administrators should validate the update against their build via Windows Update, WSUS, or the Microsoft Update Catalog.
Workarounds
- No vendor-supplied workaround exists; patching is the required remediation
- Limit guest VM access to trusted users and enforce least privilege on guest operating systems to reduce the local attack surface
- Where feasible, isolate sensitive workloads to dedicated, fully patched Hyper-V hosts until all systems are updated
# Verify the latest cumulative update is installed on a Hyper-V host
Get-HotFix | Sort-Object -Property InstalledOn -Descending | Select-Object -First 10
# Confirm Hyper-V role status and host build
Get-WindowsFeature -Name Hyper-V
[System.Environment]::OSVersion.Version
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
