CVE-2026-62431: Viridian STIMER DoS Vulnerability

CVE-2026-62431 Overview

CVE-2026-62431 is a divide-by-zero vulnerability in the Xen hypervisor's Viridian synthetic timer (STIMER) handling logic. The periodic STIMER code performs a division using an unchecked, guest-controlled divisor value. A malicious guest can set this divisor to zero, triggering a #DE (divide error) fault in the hypervisor. This condition results in a denial-of-service impacting host availability. The issue is tracked as Xen Project advisory XSA-504 and classified under [CWE-369: Divide By Zero].

Critical Impact
An unprivileged guest can crash the Xen hypervisor by setting a Viridian STIMER divisor to zero, causing host-level denial of service.

Affected Products

Xen hypervisor with Viridian (Hyper-V) enlightenments enabled
Systems running Windows guests configured with Viridian synthetic timers
Xen deployments where periodic STIMER functionality is exposed to guests

Discovery Timeline

2026-07-28 - CVE-2026-62431 published to NVD alongside Xen Project advisory XSA-504
2026-07-28 - Last updated in NVD database

Technical Details for CVE-2026-62431

Vulnerability Analysis

The Xen hypervisor implements Viridian enlightenments to improve performance for Windows guests running under Hyper-V-compatible interfaces. Part of this support includes synthetic timers (STIMERs), which allow guests to schedule periodic interrupts. The periodic STIMER handling logic performs an arithmetic division operation using a divisor value supplied by the guest through model-specific register (MSR) writes. The hypervisor does not validate that the divisor is non-zero before performing the division. When a guest writes a zero divisor, the resulting #DE divide-error exception occurs inside the hypervisor context.

Root Cause

The root cause is missing input validation on a guest-controlled numeric input. The STIMER configuration path accepts the divisor value written by the guest and passes it directly into a division instruction without checking for the zero case. This maps to [CWE-369: Divide By Zero]. Because the fault occurs in hypervisor code rather than guest context, the exception is not contained within the offending virtual machine.

Attack Vector

An attacker requires the ability to execute code inside a guest virtual machine configured to use Viridian synthetic timers. From within the guest, the attacker writes a crafted value to the STIMER configuration MSR that sets the periodic divisor to zero. When the hypervisor next processes the timer, the division triggers a #DE fault in ring 0, resulting in a hypervisor crash or panic. This impacts availability of the host and all co-resident guests. The advisory indicates no confidentiality or integrity impact, only high availability impact.

No verified public exploit code is available at this time. See the Xen Project Advisory XSA-504 for technical details on the affected code path.

Detection Methods for CVE-2026-62431

Indicators of Compromise

Unexpected hypervisor panics or host reboots correlated with guest activity
Xen log entries showing #DE (divide error) exceptions originating from Viridian STIMER handlers
Guest VMs writing anomalous values to Viridian synthetic timer MSRs prior to host crashes

Detection Strategies

Monitor Xen hypervisor logs (xl dmesg, serial console output) for divide-error faults and STIMER-related stack traces
Correlate host availability events with guest workload identity to identify guests triggering hypervisor faults
Baseline expected Viridian MSR write patterns from legitimate Windows guests to detect anomalous writes

Monitoring Recommendations

Forward hypervisor logs and host uptime telemetry to a centralized SIEM for correlation across the virtualization fleet
Alert on repeated host crashes or reboots on hosts running untrusted guest workloads
Track patch state of Xen packages across all hypervisors and flag unpatched hosts exposed to multi-tenant guests

How to Mitigate CVE-2026-62431

Immediate Actions Required

Apply the patches referenced in Xen Project Advisory XSA-504 as soon as they are available for your distribution
Inventory hypervisors running guests with Viridian enlightenments enabled and prioritize patching multi-tenant hosts
Restrict guest VM administrative access to trusted users until patches are deployed

Patch Information

Refer to the Xen Project Advisory XSA-504 for the authoritative patch set and version guidance. Distribution vendors including major Linux distributions typically publish backported packages shortly after XSA release. Review the OpenWall OSS Security list post for downstream vendor coordination details.

Workarounds

Disable Viridian enlightenments on affected guests by setting viridian=0 in the guest configuration where Windows performance impact is acceptable
Avoid running untrusted or multi-tenant guests on hypervisors that cannot be patched immediately
Isolate critical workloads on hosts that do not co-locate untrusted guest VMs

bash

# Example: disable Viridian enlightenments in a Xen guest config
# /etc/xen/<guestname>.cfg
viridian = 0