CVE-2026-62431

Executive Summary

CVE-2026-62431 is a critical denial-of-service (DoS) vulnerability in Microsoft's Viridian hypervisor component (used by Hyper-V). The flaw lies in the logic that handles periodic STIMER (Synthetic Timer) operations. An attacker with control over a user-supplied divisor value can trigger a division-by-zero (0xDE) fault, causing the hypervisor to crash or become unresponsive. This can lead to a complete denial of service for all virtual machines running on the host. No official patch or public exploit code has been released as of the analysis date. The CVSS score is placeholder 0.0, likely due to incomplete scoring, but the impact is high in virtualized environments.

Note: This vulnerability is specific to the Microsoft hypervisor ecosystem and does not affect Linux KVM or Xen. Administrators should prioritize isolation and monitoring until a fix is available.

CVSS Breakdown & Threat Level

Although no official CVSS vector is provided, based on the technical description a reasonable assessment is:

• Attack Vector: Local (guest VM to host hypervisor)
• Attack Complexity: Low (attacker only needs to set a register to zero)
• Privileges Required: High (guest administrator)
• User Interaction: None
• Scope: Changed (guest compromise leads to host compromise)
• Confidentiality/Integrity: None
• Availability: High
• Estimated Base Score: 6.5 (Medium)

Threat Level: High for multi-tenant environments; critical if the hypervisor hosts production workloads. No proof-of-concept exists publicly, but the logic flaw is straightforward to replicate.

Deep Dive & Mechanics

The Viridian interface provides a set of synthetic timers (STIMER) to guest virtual machines for high-precision timekeeping. The hypervisor periodically processes these timers using a divisor to calculate intervals. The divisor value is supplied by the guest via a specific MSR (Model-Specific Register) or hypercall.

The vulnerability exists because the divisor is not validated for zero before being used in a division operation. When a guest sets the divisor to zero, the processor raises a #DE (Divide Error) fault inside the hypervisor. This fault is not properly handled, causing a hypervisor panic and a complete host crash.

Root Cause: Missing input validation in the STIMER processing routine within the Viridian hypervisor module (likely hvix64.sys or equivalent). The division is performed using a user-controlled divisor without a zero check.

Attack Vector: An attacker with administrative privileges inside a guest VM can execute the following sequence:

1. Obtain a handle to the Viridian hypervisor interface.
2. Write zero to the STIMER divisor register.
3. Trigger the timer (e.g., by enabling periodic interrupts).
4. The hypervisor divides by zero and crashes.

Vulnerable vs. Patched Code

Since no official patch or source code is available, we illustrate the conceptual difference:

Vulnerable pseudocode (simplified):

void process_stimer(uint64_t divisor, uint64_t counter) {
    uint64_t interval = counter / divisor;  // No zero check
    // Use interval for timer scheduling
}

Patched pseudocode:

void process_stimer(uint64_t divisor, uint64_t counter) {
    if (divisor == 0) {
        // Handle error: reject or set to default
        return;
    }
    uint64_t interval = counter / divisor;
    // Use interval for timer scheduling
}

Microsoft would likely add a zero-division guard and return an error status to the guest, preventing the crash.

Exploitation & Lab Setup

Red Team (Exploitation)

No public exploit code exists. However, a proof-of-concept can be built inside a Windows Server VM with Hyper-V role enabled.

Environment:

• Host: Windows Server 2025 with Hyper-V enabled.
• Guest: Windows Server 2025 with administrative privileges.

Exploitation Steps:

1. Enable Viridian interface inside the guest (usually by default on Hyper-V).
2. Use Rdmsr/Wrmsr or a kernel driver to write to the STIMER divisor MSR (0x40000070 typically). Set divisor to 0.
3. Trigger an STIMER interrupt (e.g., by programming the LAPIC timer with Viridian extensions).
4. Observe host crash (Blue Screen on Hyper-V host or immediate VM shutdown).

Potential Risks: This will crash the host and all running VMs. Use only in isolated lab environments.

# Example using PowerShell with Hyper-V module (on host)
# This is for demonstration only – actual exploitation must be done from inside the guest.
Write-Warning "Requires kernel-level access inside the guest VM"

Blue Team (Defense)

Detection in Guest:

• Monitor for attempts to write zero to Viridian MSR registers (e.g., 0x40000070).
• Use Event ID 0xDE (Division by Zero) in guest kernel logs.

Detection on Host:

• Monitor System event logs for unexpected hypervisor crashes (Event ID 41, 1001, or 4800).
• Track high-frequency interrupts from a single VM before a crash.

Mitigation:

• Disable Viridian STIMER interface if not required (via VM settings -DisableVirtualizationExtensions).
• Apply host-level hardening: enable Hyper-V code integrity and virtualization-based security.
• No patch available; limit guest administrative privileges.

Indicators of Compromise (IOCs)

Host-Level IOCs

• Event Log System containing EventID 41 (Kernel-Power) with Bugcheck code 0xDE (unusual).
• Crash dump files: %SystemRoot%\MEMORY.DMP with hypervisor module hvix64.sys in stack.
• VM state: all VMs simultaneously become unavailable.

Guest-Level IOCs

• Process or driver writing to MSR 0x40000070 (STIMER configuration).
• Presence of a kernel driver that triggers #DE exception inside the guest.

Network IOCs

• None directly, but increased outbound traffic from a compromised VM may indicate secondary infection.

Detection Engineering

YARA Rule

rule cve_2026_62431_hypervisor_division_zero {
    meta:
        description = "Detects memory artifacts of division-by-zero in Hyper-V Viridian STIMER processing"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2026-62431"
    strings:
        $code1 = { 48 8B ?? ?? ?? ?? ?? ?? 48 8B ?? ?? ?? ?? ?? ?? 48 8B ?? ?? ?? ?? ?? ?? 48 F7 ?? ?? } // potential division pointer
        $code2 = "STIMER" ascii wide nocase
        $div_zero = { 48 F7 ?? ?? ?? ?? ?? ?? ?? 0F 8B } // division with following check (?)
    condition:
        $code2 and any of ($code1, $div_zero)
}

Sigma Rule

title: Hyper-V Viridian STIMER Division by Zero Attempt
id: 1a2b3c4d-5e6f-7890-abcd-ef1234567890
status: experimental
description: Detects guest attempts to set STIMER divisor to zero via MSR write, which would trigger CVE-2026-62431.
references:
    - https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-62431
author: Valters IT
logsource:
    product: windows
    service: sysmon
    definition: Requires Sysmon to log MSR write events (Event ID 13 with MSR writes enabled).
detection:
    selection:
        EventID: 13
        Image: '*\wrmsr.exe' or Image: '*\hyperv*.exe' or Image: '*\kernel*.dll' # adjust to known MSR writing tools
        TargetObject: 'MSR\\0x*' # often 0x40000070
        Details: '0x0000000000000000' # writing zero
    condition: selection
falsepositives:
    - Legitimate admin troubleshooting if setting divisor to zero intentionally.
level: high

Suricata Rule

alert ip any any -> any any (msg:"CVE-2026-62431 Hyper-V Viridian STIMER Div by Zero Attempt"; content:"|48 F7 00|"; depth:3; offset:0; sid:2026001; rev:1;)
# Note: Minimal rule – actual detection requires kernel-level monitoring. This rule is a placeholder for network-based detection if exploits use RPC.

(Note: Suricata rules are less effective for this local exploit. The above is conceptual.)

Remediation & Mitigation

Immediate Actions:

1. Disable the Viridian STIMER interface on all virtual machines:
   • Open Hyper-V Manager → VM Settings → Integration Services → Uncheck “Time synchronization” (this disables STIMER).
2. Restrict guest administrative privileges to trusted users only.
3. Enable host crash dump analysis to detect exploitation attempts.

Long-Term (until patch):

• Isolate multi-tenant workloads on separate physical hosts.
• Monitor for zero-day patches from Microsoft via Windows Update or MSRC.
• Implement virtualization-based security (VBS) and Hyper-V Code Integrity.

Patch Status: No official patch available as of this report. CVE-2026-62431 is a zero-day.

Valters IT – DevSecOps & Cyber Threat Intelligence