CVE-2026-79602 in Xen
Summary
by MITRE • 09/08/2026
A guest with a PCI device assigned that has at least a BAR on the IO port space can trigger a BUG() in Xen.
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Analysis
by VulDB Data Team • 09/08/2026
The vulnerability described involves a critical flaw within the Xen hypervisor's handling of Input/Output Memory Management Unit (IOMMU) operations and peripheral component interconnect (PCI) device assignment to virtual machines, specifically when devices expose Base Address Registers (BARs) mapped into the IO port space. In typical virtualization environments, PCI passthrough allows a guest operating system direct access to hardware resources by mapping physical memory regions or I/O ports from the host kernel into the guest's address space. This process relies heavily on strict isolation boundaries enforced by the hypervisor and the underlying hardware MMU to prevent unauthorized memory accesses or side-channel attacks. The specific issue arises when a maliciously crafted or misconfigured guest attempts to interact with these assigned PCI devices, particularly focusing on BARs that are configured for IO port access rather than standard memory-mapped I/O.
The technical root cause lies in the hypervisor's failure to adequately validate or sanitize requests originating from the guest domain before they are processed by the virtualized device emulation logic or passed through directly to the physical hardware. When a guest issues an IO instruction targeting a BAR located in the IO port space, Xen may execute code paths that assume certain safety constraints which do not hold under adversarial conditions. This can lead to a state corruption within the hypervisor's internal data structures, triggering a BUG() assertion failure. In Linux-based kernels and hypervisors like Xen, a BUG() call is designed to halt execution immediately upon detecting an unrecoverable error or invariant violation, effectively causing a local denial of service for all domains running on that physical host. This mechanism prevents further potential exploitation but results in a complete system crash requiring a manual reboot.
From the perspective of industry standards, this vulnerability aligns with CWE-20 Improper Input Validation and CWE-787 Out-of-bounds Write if the assertion failure is preceded by memory corruption attempts. It also relates to CWE-400 Uncontrolled Resource Consumption due to the resulting system halt. In terms of attack vectors, this falls under MITRE ATT&CK technique T1499 Endpoint Denial of Service, where an attacker leverages a software vulnerability to disrupt service availability. The specific context involves virtualization escape or hypervisor-level compromise attempts that degrade into denial-of-service scenarios due to the fragility in handling PCI IO transactions.
The operational impact is severe for any infrastructure relying on Xen for hardware passthrough capabilities, such as high-performance computing clusters, gaming VMs with GPU assignment, or specialized industrial control systems requiring low-latency direct device access. An attacker who gains code execution within a guest domain can exploit this flaw to crash the entire physical host machine. This leads to significant downtime and potential data loss if critical services are abruptly terminated without graceful shutdown procedures. Furthermore, in multi-tenant environments, such an attack could be used as a distraction or resource exhaustion tactic against other tenants sharing the same hardware resources, undermining the fundamental security promise of isolation between virtual machines.
Mitigation strategies primarily involve updating to patched versions of Xen that address this specific validation logic for PCI BARs in IO space. Administrators should ensure that their hypervisor software is kept up-to-date with the latest security patches released by upstream maintainers. Additionally, implementing strict access controls and monitoring tools can help detect anomalous patterns of I/O port accesses from guest domains before they trigger such critical failures. Where possible, limiting PCI passthrough to devices that do not require complex IO space mappings or utilizing virtualized device emulation instead of direct assignment for less performance-critical hardware can reduce the attack surface. Security teams should also review their VM configurations to ensure that only trusted and necessary drivers are loaded within guest operating systems assigned physical devices, minimizing the risk of accidental or malicious misconfiguration leading to hypervisor instability.