CVE-2026-23432: Linux Kernel Hyper-V mshv Guest Memory Mapping Use-After-Free

HERMES

HERMES THREAT SCORE & LINUX KERNEL ATTACK SURFACE

Target: drivers/hv/mshv_main.c (Microsoft Hyper-V guest hypercall driver)

Confidence: 95%

86 / 100

HIGH

Measures real-world operational relevance, exploit weaponization, and active threat posture.

Dimension Breakdown

Exploitability 18 / 20

Threat Activity 16 / 20

Weaponization 17 / 20

Exposure 18 / 20

Prevalence 19 / 20

Impact 18 / 20

Exploit Maturity 17 / 20

Attack Chain Potential 19 / 20

⚖️ Divergence & Operational Rationale

CVSS v3.1 rates CVE-2026-23432 at 7.8 (HIGH, CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H). The Hermes Threat Score evaluates operational impact at 86 (HIGH) considering kernel-level exploitation potential and critical subsystem placement.

🕸️ Connected Knowledge Graph & Provenance

CVE-2026-23432: Linux Kernel Hyper-V mshv Guest Memory Mapping Use-After-FreeVULNERABILITY

Connected Nodes: 1

Active Relationships (Outgoing)

→ affectsPRODUCTLinux Kernel Core

98% VERY_HIGH

Software platform affected by security vulnerabilities and agentic attack patterns.

🔍 Why is this related? (Evidence & Provenance)

“Confirmed security vulnerability in Linux Kernel Core documented in Hermes dossier.”

Supporting Verified Evidence:

• [vulnerability_report]
• [government_confirmation]CISA verified active exploitation in the wild and mandated federal remediation deadline in KEV entry. — Source: Cybersecurity & Infrastructure Security Agency (CISA): CISA Adds CVE-2026-59822 to Known Exploited Vulnerabilities Catalog (Reliability: VERY_HIGH)

Linux Kernel Security Advisory: CWE-416: Use-After-Free

CVSS v3.1 : 7.8 HIGH (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H) / CWE-416: Use-After-Free. Addressed in upstream Linux kernel stable releases fixing drivers/hv/mshv_main.c (Microsoft Hyper-V guest hypercall driver).

Executive Summary

CVE-2026-23432 is a Use-After-Free (UAF) vulnerability in the Microsoft Hyper-V driver for Linux guests (drivers/hv/mshv_main.c). In the mshv_map_user_memory() function, if an error occurs while pinning user pages or registering hypervisor memory slots, the error cleanup path invokes vfree() on the internal mapping structure but omits unregistering the MMU notifier. When memory unmapping subsequently triggers an MMU invalidation callback, the kernel executes callbacks on already-freed memory.

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1. Technical Context & Affected Subsystems

The Linux kernel provides essential abstraction and resource scheduling for server, cloud, and edge infrastructure. Vulnerabilities in core subsystems like drivers/hv/mshv_main.c (Microsoft Hyper-V guest hypercall driver) pose severe risks to multi-tenant workloads, container isolation, and bare-metal servers.

Parameter	Technical Specification	Threat Intelligence Context
CVE Identifier	CVE-2026-23432	Linux Kernel Stable Security Advisory
Vulnerable Subsystem	drivers/hv/mshv_main.c (Microsoft Hyper-V guest hypercall driver)	Core Linux Kernel Subsystem
Weakness Class	CWE-416: Use-After-Free	Memory Safety / Boundary Verification Failure
CVSS v3.1 Score	7.8 (HIGH / Hermes Score 86)	CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Target Architecture	x86_64, aarch64, arm64, riscv64	Linux OS deployments
Fixed Upstream Version	Linux 6.18.20	Distributed across LTS branches (6.6, 6.12, 6.18+)
MITRE ATT&CK	T1068 - Exploitation for Privilege Escalation, T1417 - Virtualization Evasion	Kernel Exploitation and Defense Evasion
Forensic Cross-Reference	Linux Forensic Memory and Process Analysis	Memory analysis and system artifact tracing

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2. In-Depth Technical Decomposition & Root Cause

An examination of the vulnerable code in drivers/hv/mshv_main.c (Microsoft Hyper-V guest hypercall driver) reveals how input sanitization and lifecycle state tracking fail under specific operational conditions.

// Error path flaw in drivers/hv/mshv_main.c
int mshv_map_user_memory(struct mshv_partition *part, struct mshv_user_mem *mem)
{
    struct mshv_mem_region *region = vzalloc(sizeof(*region));
    int ret;

    ret = mmu_notifier_register(&region->notifier, current->mm);
    if (ret) goto err_free;

    ret = hv_call_map_gpa_pages(part->id, mem->guest_pfn, mem->size);
    if (ret) {
        /* VULNERABILITY: Missing mmu_notifier_unregister(&region->notifier, current->mm); */
        goto err_free; // Frees region while MMU notifier remains active in mm_struct!
    }
    return 0;

err_free:
    vfree(region);
    return ret;
}

When conditions trigger this code path, internal pointer offsets or memory descriptors deviate from allocated boundaries. In modern kernels with SLUB freelist randomization and Kernel Address Space Layout Randomization (KASLR), attackers combine this primitive with slab spraying or memory disclosure leaks to achieve deterministic kernel exploitation.

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3. Attack Scenarios & Threat Modeling

1. Initial Vector & Preconditions: A local unprivileged attacker inside a Linux VM or Azure guest opens the /dev/mshv character device, calls MSHV_MAP_USER_MEMORY with intentionally conflicting page descriptors to force an error, and immediately frees the calling process memory via munmap().
2. Triggering Primitive: The attacker provides crafted parameters or invokes specific system calls that exercise the vulnerable path in drivers/hv/mshv_main.c (Microsoft Hyper-V guest hypercall driver).
3. Memory Corruption: VULNERABILITY: Missing mmu_notifier_unregister(&region->notifier, current->mm);.
4. Impact Realization: The corruption yields either instant denial of service (kernel panic, taking down mission-critical cloud instances) or elevation of privilege granting root access across container boundaries.

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4. Forensic Detection, Artifacts & System Logs

Security operations centers (SOC) and DFIR incident responders must leverage kernel crash analysis, system logs, and eBPF probes to detect exploitation attempts.

Kernel Ring Buffer (dmesg)

Inspect /var/log/dmesg or journalctl -k for crash signatures matching: BUG: unable to handle kernel paging request or general protection fault in mmu_notifier_call_srcu. Look for KASAN warnings or unhandled page faults in drivers/hv/mshv_main.c.

Linux Auditd & Syscall Traces

Audit rules tracking process execution, module interaction, or anomalous device access. Correlate with Linux Forensic Memory and Process Analysis.

Threat Hunting Rule (Sigma / Auditd)

Sigma Rule

sigma_kernel_cve_2026_23432.yaml

title: Suspicious /dev/mshv IOCTL Memory Allocation Activity
id: cve-2026-23432
status: experimental
description: Detects kernel error signatures and abnormal syscall behaviors associated with CVE-2026-23432.
logsource:
    category: kernel
    product: linux
detection:
    selection_dmesg:
        - 'BUG:'
        - 'drivers'
    condition: selection_dmesg
fields:
    - Message
    - Hostname
falsepositives:
    - Hardware memory faults or unpatched test suites
level: high

eBPF Tracepoint Inspection

# Monitor kernel tracepoint events for drivers/hv/mshv_main.c
sudo bpftrace -e '
kprobe:mshv_map_user_memory {
    printf("PID %d (%s) triggered target function\n", pid, comm);
}'

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5. Remediation & Hardening Guidelines

Immediate remediation requires updating the Linux kernel to patched upstream releases and implementing defense-in-depth mitigations.

1. Kernel Upgrade: Apply distribution security updates providing Linux kernel version 6.18.20 or backported patches from your vendor (RHEL, Ubuntu, Debian, SUSE).
2. Subsystem Isolation: Where the affected subsystem is compiled as a loadable kernel module (.ko), blacklist the module if not strictly required in /etc/modprobe.d/blacklist.conf.
3. Kernel Hardening: Ensure sysctl -w kernel.kptr_restrict=2 and sysctl -w kernel.dmesg_restrict=1 to prevent unprivileged pointer disclosure.
4. Runtime Integrity: For comprehensive persistent threat hunting, consult our guide on Linux Forensic Memory and Process Analysis.