CVE-2026-31788
Published: 25 March 2026
Summary
CVE-2026-31788 is a high-severity an unspecified weakness vulnerability in Linux Linux Kernel. Its CVSS base score is 8.2 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Privilege Escalation (T1068); ranked at the 4.9th 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 SI-2 (Flaw Remediation) and AC-6 (Least Privilege).
Deeper analysis
CVE-2026-31788 is a vulnerability in the Linux kernel's xen/privcmd driver, which enables user space processes to issue arbitrary hypercalls. Normally restricted to root and denied by the hypervisor for cross-domain effects, the issue arises in unprivileged Xen domU guests booted with secure boot, where a root process can modify kernel memory contents, thereby bypassing secure boot protections. This flaw affects Linux kernels running as Xen guests, particularly unprivileged domUs, and is tracked as Xen Security Advisory XSA-482.
A local attacker with root privileges (PR:H) in an unprivileged domU can exploit this via the privcmd driver to issue hypercalls that alter kernel memory, achieving high impacts on confidentiality, integrity, and availability (C:H/I:H/A:H) with changed scope (S:C). The attack requires low attack complexity and no user interaction (AV:L/AC:L/UI:N), rated at CVSS 8.2. Exploitation is limited to scenarios where the guest acts outside its intended device model role for another guest, as hypercalls would otherwise target only that specific domain.
Kernel patches mitigate the issue by restricting the privcmd driver in non-dom0 environments to hypercalls targeting a specific domain ID obtained from Xenstore at initialization. Updates include deferring Xenstore reads if unavailable, waiting in open() for the target domain, and logging messages if none is found. Relevant stable kernel commits are available at https://git.kernel.org/stable/c/1879319d790f7d57622cdc22807b60ea78b56b6d, https://git.kernel.org/stable/c/389bae9a4409934e8b8d4dbdaaf02a3ae71cf8e4, https://git.kernel.org/stable/c/3ee5b9e3de4b8bdd74183d83205481c91a9effc8, https://git.kernel.org/stable/c/453b8fb68f3641fea970db88b7d9a153ed2a37e8, and https://git.kernel.org/stable/c/4eb245ff0d33b618e097a2c23de5df56d4ad6969.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-15401
Vulnerability details
In the Linux kernel, the following vulnerability has been resolved: xen/privcmd: restrict usage in unprivileged domU The Xen privcmd driver allows to issue arbitrary hypercalls from user space processes. This is normally no problem, as access is usually limited to…
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root and the hypervisor will deny any hypercalls affecting other domains. In case the guest is booted using secure boot, however, the privcmd driver would be enabling a root user process to modify e.g. kernel memory contents, thus breaking the secure boot feature. The only known case where an unprivileged domU is really needing to use the privcmd driver is the case when it is acting as the device model for another guest. In this case all hypercalls issued via the privcmd driver will target that other guest. Fortunately the privcmd driver can already be locked down to allow only hypercalls targeting a specific domain, but this mode can be activated from user land only today. The target domain can be obtained from Xenstore, so when not running in dom0 restrict the privcmd driver to that target domain from the beginning, resolving the potential problem of breaking secure boot. This is XSA-482 --- V2: - defer reading from Xenstore if Xenstore isn't ready yet (Jan Beulich) - wait in open() if target domain isn't known yet - issue message in case no target domain found (Jan Beulich)
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Local root in Xen domU abuses privcmd hypercalls for kernel memory modification (scope change), directly enabling privilege escalation beyond guest, secure boot bypass (subverting trust controls), and host/domain escape.
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Directly mitigates CVE-2026-31788 by requiring timely application of Linux kernel patches that restrict the xen/privcmd driver to target specific domains in unprivileged domUs, preventing arbitrary hypercalls from modifying kernel memory.
Protects kernel memory from unauthorized modification by user-space root processes exploiting the privcmd driver to issue hypercalls that bypass secure boot protections.
Enforces least privilege by restricting root access in unprivileged domUs to only necessary hypercalls targeting specific domains, preventing misuse of the privcmd driver for kernel memory alterations.