CVE-2026-29649

Critical

Published: 20 April 2026

Published

20 April 2026

Modified

24 April 2026

KEV Added

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Patch

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CVSS Score 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS Score 0.0002 6.9th percentile

Risk Priority 20 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-29649 is a critical-severity Protection Mechanism Failure (CWE-693) vulnerability in Xiangshan Nemu. Its CVSS base score is 9.8 (Critical).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 6.9th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified are NIST 800-53 SC-50 (Software-enforced Separation and Policy Enforcement) and SI-2 (Flaw Remediation).

Threat & Defense at a Glance

What attackers do: exploitation maps to Exploit Public-Facing Application (T1190) and 3 other techniques. What defenders deploy: see the NIST 800-53 controls recommended below.

Threat & Defense Details

Mitigating Controls (NIST 800-53 r5)AI

SI-2 Flaw Remediation good match

prevent

Directly mitigates the CVE by identifying, prioritizing, and applying the corrective patch (PR #689) to fix the flawed CSR handling in NEMU.

SC-50 Software-enforced Separation and Policy Enforcement good match

prevent

Enforces software-based separation policies to prevent machine-mode menvcfg writes from improperly modifying hypervisor henvcfg fields, addressing the core virtualization enforcement failure.

SI-6 Security and Privacy Function Verification good match

preventdetect

Verifies correct operation of hypervisor CSR masking and update mechanisms against RISC-V specifications, enabling detection and correction of the configuration propagation flaw.

MITRE ATT&CK Enterprise TechniquesAI

T1190 Exploit Public-Facing Application Initial Access

Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.

attack.mitre.org →

T1068 Exploitation for Privilege Escalation Privilege Escalation

Adversaries may exploit software vulnerabilities in an attempt to elevate privileges.

attack.mitre.org →

T1499.004 Application or System Exploitation Impact

Adversaries may exploit software vulnerabilities that can cause an application or system to crash and deny availability to users.

attack.mitre.org →

T1611 Escape to Host Privilege Escalation

Adversaries may break out of a container or virtualized environment to gain access to the underlying host.

attack.mitre.org →

Why these techniques?

Network-accessible unauthenticated exploitation of hypervisor CSR flaw directly enables T1190; incorrect menvcfg-to-henvcfg propagation allows privilege escalation across virtualization boundaries (T1068) and escape from emulated guest/hypervisor context to host (T1611); resulting traps and availability impact map to application/system exploitation for DoS (T1499.004).

Confidence: MEDIUM · MITRE ATT&CK Enterprise v18.1

NVD Description

NEMU contains an implementation flaw in its RISC-V Hypervisor CSR handling where henvcfg[7:4] (CBIE/CBCFE/CBZE-related fields) is incorrectly masked/updated based on menvcfg[7:4], so a machine-mode write to menvcfg can implicitly modify the hypervisor's environment configuration. This can lead to incorrect enforcement…

of virtualization configuration and may cause unexpected traps or denial of service when executing cache-block management instructions in virtualized contexts (V=1).

Deeper analysisAI

CVE-2026-29649 is a high-severity vulnerability (CVSS 9.8, CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H) in NEMU, an RISC-V emulator, stemming from an implementation flaw in its hypervisor CSR handling. Specifically, the henvcfg[7:4] fields (related to CBIE, CBCFE, and CBZE) are incorrectly masked or updated based on menvcfg[7:4]. This allows a machine-mode write to menvcfg to implicitly modify the hypervisor's environment configuration, leading to improper enforcement of virtualization settings and potential unexpected traps or denial of service during execution of cache-block management instructions in virtualized contexts (V=1). The issue is classified under CWE-693.

Unauthenticated attackers with network access can exploit this vulnerability with low complexity and no user interaction. Exploitation occurs through manipulation of machine-mode writes that propagate to hypervisor configuration, disrupting virtualization enforcement. This enables high-impact outcomes, including confidentiality breaches, integrity violations, and availability disruptions such as traps or denial of service in affected virtualized environments.

Advisories reference RISC-V ISA specifications for hypervisor and machine privilege CSRs, alongside NEMU GitHub issue #681 documenting the flaw and pull request #689 providing a corrective patch. Security practitioners should update NEMU using the fix in PR #689 and verify CSR handling aligns with official RISC-V documentation to mitigate risks.