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Xen Hypervisor EUVDEUVD-2026-37890

| CVE-2026-42490 MEDIUM
Improper Locking (CWE-667)
2026-06-09 GHSA-9xq7-8p44-p7p4
Python Information Disclosure
6.5
CVSS 3.1 · Vendor
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Severity by source

Vendor (CNA) PRIMARY
6.5 MEDIUM
AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
SUSE
4.1 MEDIUM
AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H

Primary rating from Vendor (CNA).

CVSS VectorVendor

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Attack Vector
Network
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

Lifecycle Timeline

1
Analysis Generated
Jun 09, 2026 - 14:16 vuln.today

Description PRE-NVD

Disclosed via oss-security. NVD scoring and full description are pending.

AnalysisAI

Xen Hypervisor's domctl locking mechanism, when XSM/Flask mandatory access control is enabled, acquires the system-wide serialization lock for certain operations before performing any Flask permission checks. This allows a less-privileged guest domain to seize the lock without authorization and stall equally or more privileged entities - including the control domain (dom0) and Xenstore domain - potentially causing a Denial of Service affecting the entire physical host. No public exploit has been identified at time of analysis, and this vulnerability is not listed in CISA KEV.

Technical ContextAI

The Xen Hypervisor uses domctl (domain control) hypercall operations to create and manage guest virtual machines. These operations are issued by the control domain (dom0), a Xenstore domain, or domains that manage specific guests. Because certain domctl operations cannot execute concurrently, Xen employs a system-wide lock to serialize them. XSM/Flask is Xen's optional mandatory access control framework derived from SELinux, designed to enforce fine-grained inter-domain policy. CVE-2026-42490 specifically arises from incorrect ordering: for some domctl operations under XSM/Flask, lock acquisition occurs before the Flask policy engine is consulted to verify the caller has permission to perform the operation at all. This allows an unpermitted less-privileged domain to grab the lock - blocking legitimate privileged callers - without Flask ever denying the attempt. The root cause class is effectively improper ordering of authorization and resource acquisition, analogous to CWE-696 (Incorrect Behavior Order) or CWE-667 (Improper Locking), though no CWE was formally assigned in the available data. This is the companion to CVE-2026-42489, which addresses the lock fairness issue even absent XSM/Flask.

Affected ProductsAI

All Xen Hypervisor versions from 3.3 onwards are confirmed vulnerable per the XSA-492 advisory published by the Xen Project. Versions predating 3.3 use a different locking implementation but may also be vulnerable according to the advisory. Patch sets have been explicitly prepared for Xen 4.17.x, 4.18.x, 4.19.x, 4.20.x, 4.21.x, and the xen-unstable (development) branch. The vulnerability is specific to configurations where XSM/Flask is enabled; deployments without XSM/Flask active are not affected by CVE-2026-42490 (though CVE-2026-42489 covers the related lock fairness issue regardless of XSM/Flask). No CPE strings were included in the available intelligence data.

RemediationAI

Apply the XSA-492 patch set distributed by the Xen Project, available for Xen 4.17.x through 4.21.x and the xen-unstable branch. Patches are identified with SHA256 checksums in the advisory and should be obtained through official Xen Project channels or downstream distribution packages. Importantly, for Xen 4.21 and the staging branch, the patches include a modification to the default Flask policy; environments using custom Flask policies must independently apply an equivalent change to their custom policy or access controls will be misconfigured post-patch. The Xen Project advises updating to the tip of the relevant stable branch before applying patches, as patches target stable branches and may not apply cleanly to release tarballs. The advisory explicitly states there is no known mitigation short of patching. As a partial compensating control with significant trade-offs, disabling XSM/Flask entirely would eliminate CVE-2026-42490's exploitability, but this removes the Flask security boundary and is not recommended for deployments relying on mandatory access control for isolation. Advisory reference: https://seclists.org/oss-sec/2026/q2/858.

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Vendor StatusVendor

SUSE

Severity: Medium
Product Status
SUSE Linux Enterprise High Performance Computing 15 SP5-ESPOS Affected
SUSE Linux Enterprise High Performance Computing 15 SP5-LTSS Affected
SUSE Linux Enterprise Micro 5.5 Affected
SUSE Linux Enterprise Module for Basesystem 15 SP7 Affected
SUSE Linux Enterprise Server 15 SP7 Affected

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EUVD-2026-37890 vulnerability details – vuln.today
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