What is the CVSS score of CVE-2026-29646?

CVE-2026-29646 has a CVSS 3.1 base score of 9.8 (Critical). CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H. EPSS exploitation probability: 0.0%.

Which versions of OpenXiangShan NEMU are affected by CVE-2026-29646?

CVE-2026-29646 is a critical privilege isolation bypass in OpenXiangShan NEMU's RISC-V hypervisor implementation that allows guest virtual machines to corrupt supervisor-level interrupt state,…

How to fix or mitigate CVE-2026-29646?

Within 24 hours: Verify whether OpenXiangShan NEMU is deployed in your environment-check development, research, and virtualization infrastructure inventories. Within 7 days: For affected deployments, isolate NEMU instances from network access and restrict to trusted development networks only; contact your RISC-V tools vendor for interim mitigation guidance. Within 30 days: Monitor for vendor-released patches and apply commit 55295c4 or later when available; implement additional hypervisor isolation controls pending official patch release.

OpenXiangShan NEMU CVE-2026-29646

EUVD-2026-23958 CRITICAL

Privilege Defined With Unsafe Actions (CWE-267)

2026-04-20 mitre

GHSA-729m-5x6m-wwxv

Denial Of Service N A

9.8

CVSS 3.1 · NVD

Severity by source

NVD PRIMARY

9.8 CRITICAL

AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Primary rating from NVD · only source for this CVE.

CVSS VectorNVD

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

High

Lifecycle Timeline

Analysis Generated

Apr 21, 2026 - 20:23 vuln.today

CVSS changed

Apr 21, 2026 - 20:22 NVD

9.8 (CRITICAL)

EUVD ID Assigned

Apr 20, 2026 - 21:15 euvd

EUVD-2026-23958

Analysis Generated

Apr 20, 2026 - 21:15 vuln.today

CVE Published

Apr 20, 2026 - 00:00 nvd

CRITICAL 9.8

DescriptionCVE.org

In OpenXiangShan NEMU prior to 55295c4, when running with RVH (Hypervisor extension) enabled, a VS-mode guest write to the supervisor interrupt-enable CSR (sie) may be handled incorrectly and can influence machine-level interrupt enable state (mie). This breaks privilege/virtualization isolation and can lead to denial of service or privilege-boundary violation in environments relying on NEMU for correct interrupt virtualization.

AnalysisAI

OpenXiangShan NEMU emulator's RISC-V Hypervisor extension implementation allows VS-mode guest writes to the sie (supervisor interrupt-enable) CSR to corrupt machine-level mie state, breaking privilege isolation between virtualization layers. Fixed in commit 55295c4 per GitHub PR #938. Despite CVSS 9.8 Critical rating with network attack vector (AV:N), the EPSS score of 0.03% (9th percentile) indicates extremely low observed exploitation probability, and the vulnerability specifically affects RISC-V emulator environments rather than typical network-accessible services. No CISA KEV listing or public exploit identified at time of analysis, suggesting this is a theoretical high-severity issue in specialized research/development contexts rather than an imminent widespread threat.

Technical ContextAI

NEMU is an instruction-set emulator from the OpenXiangShan project implementing the RISC-V architecture, including the RVH (Hypervisor) extension for hardware-assisted virtualization. The vulnerability stems from incorrect handling of CSR (Control and Status Register) writes in the privilege separation logic defined by RISC-V specification chapters on Machine, Supervisor, and Hypervisor modes. The sie CSR controls interrupt enables in supervisor mode (S-mode/VS-mode in virtualized contexts), while mie controls machine-mode (M-mode) interrupts-the highest privilege level. CWE-267 (Privilege Defined With Unsafe Actions) applies because the emulator fails to properly constrain lower-privilege CSR writes, allowing guest OS code in VS-mode to manipulate M-mode state. This violates the fundamental RISC-V privilege model where each mode's CSRs must be isolated according to specification-defined access controls. The CPE data shows generic 'n/a:n/a' product identifiers, indicating NVD classification challenges for specialized emulator software rather than mainstream commercial products.

RemediationAI

Update OpenXiangShan NEMU to commit 55295c46580456d8d5a9d5736e1fda924b8825ab or later, which corrects the sie CSR write handling logic per GitHub PR #938 (https://github.com/OpenXiangShan/NEMU/pull/938). Organizations using NEMU for RISC-V development should rebuild from the patched source tree available at the OpenXiangShan GitHub repository. If immediate patching is not feasible, disable RVH (Hypervisor extension) support in NEMU build configurations to eliminate the vulnerable code path-this workaround removes virtualization capabilities but prevents privilege boundary violations. For research environments requiring RVH functionality with unpatched versions, restrict NEMU execution to isolated development systems without network access and treat all guest VMs as potentially able to escape virtualization boundaries. Review RISC-V privilege specification documentation (https://docs.riscv.org/reference/isa/priv/hypervisor.html) to understand correct CSR access control expectations when implementing custom emulator modifications. No backports to stable release branches are mentioned in available references, suggesting mainline source updates are the only remediation path.