MOKSHA-2026-0016: PVinPVH Xen Kernel Command-Line Injection via VM.platform

Summary

A vm-admin can inject arbitrary Xen hypervisor boot parameters into the PVinPVH xen-shim kernel command line by setting VM.platform:pvinpvh-xen-cmdline to an attacker-controlled string. The value is read by xenopsd at xenops_server_xen.ml:2416-2419 with zero validation and passed directly to xenguesthelper as the -cmdline argument. The attacker can disable speculative execution mitigations (spec-ctrl=no), remove Meltdown protection (xpti=false), disable L1TF mitigation (pv-l1tf=false), and disable DMA protection (iommu=no). No map_keys_roles protection exists on VM.platform.

Vulnerability Description

VM.platform is a Map(String, String) field writable by vm-admin - the lowest delegated management role. The pvinpvh-xen-cmdline key controls the kernel command line for the Xen shim used in PV-in-PVH domain builds.

Data Flow

vm-admin calls VM.add_to_platform(vm, "pvinpvh-xen-cmdline", "spec-ctrl=no xpti=false pv-l1tf=false iommu=no")
  -> xenops_server_xen.ml:2416-2419 reads platform["pvinpvh-xen-cmdline"]
  -> Value concatenated with shim_mem parameter
  -> Passed to Domain.BuildPVH { cmdline = pvinpvh_xen_cmdline }
  -> domain.ml:1328-1329 passes as -cmdline argument to xenguesthelper
  -> xenguesthelper boots xen-shim with attacker-controlled command line
  -> Mitigations disabled for the PVinPVH domain

(* xenops_server_xen.ml:2416-2419 *)
let pvinpvh_xen_cmdline =
  let base =
    try List.assoc "pvinpvh-xen-cmdline" vm.Vm.platformdata
    with Not_found -> !Xenopsd.pvinpvh_xen_cmdline
  in
  ...
  String.concat " " [base; shim_mem]

No validation is performed. The entire Xen shim kernel command line is replaced by the attacker-supplied value.

Security-Critical Options

Root Causes

1. Missing RBAC protection. VM.platform has zero map_keys_roles entries. The pvinpvh-xen-cmdline key is writable by vm-admin.

2. Zero consumer-side validation. xenopsd reads the value with List.assoc and passes it directly to the domain builder. No sanitization, no option allowlist, no format check.

3. Write-time vs. start-time validation gap. VM.platform has a sanity_check at VM start time (vm_platform.ml), but pvinpvh-xen-cmdline is not covered by any check.

Affected Systems

Directly Affected

• XenServer / Citrix Hypervisor - all versions (shared XAPI + xenopsd codebase)
• XCP-ng 8.3 - confirmed (live-tested)

Scope Limitation

PV-in-PVH is a backwards-compatibility mechanism for legacy PV guests that cannot run on modern Xen directly. Most production workloads use HVM or PVH guests, which are unaffected by this specific key. However:

• Environments with legacy VMs often contain the oldest, most sensitive data
• The zero-validation pattern represents a systemic architectural issue
• A vm-admin disabling CPU mitigations violates the privilege boundary regardless of guest type

Exploitation Scenarios

Detection

• Monitor VM.platform writes for the pvinpvh-xen-cmdline key
• Alert on values containing security-critical options: spec-ctrl, xpti, pv-l1tf, iommu
• Audit xenguesthelper process arguments for unexpected command-line parameters
• See rule V-01 in disclosure/vendor-detection-guidance.md

Remediation

Short-Term Mitigations

• Audit all VM.platform records for pvinpvh-xen-cmdline keys
• Monitor xenguesthelper process invocations for unexpected parameters
• Restrict vm-admin role grants where PVinPVH guests exist

Long-Term Fix

Add map_keys_roles. Restrict pvinpvh-xen-cmdline in datamodel_vm.ml to _R_POOL_ADMIN.

Consumer-side validation. Implement an allowlist of permitted Xen command-line options in xenopsd. Reject or strip security-critical options that should not be user-controllable.

Write-time validation. Extend vm_platform.ml:sanity_check to cover pvinpvh-xen-cmdline with an option allowlist.

Upstream patches exist. They are held privately pending coordinated disclosure.

Disclosure

Disclosure:

• Public release: 2026-04-24 08:00 CEST
• CERT/CC notified: 2026-04-23
• MITRE CVE reservation filed: 2026-04-09 (no response as of publication)
• EU CVE registers notified: 2026-04-18 (GCVE/CIRCL, ENISA, DIVD - no response)
• Vendor (Cloud Software Group / Citrix): not contacted pre-publication
• Downstream maintainer (Vates/XCP-ng): contacted 2026-04-23 with conditional patch offer

References

• Related MOKSHA IDs: MOKSHA-2026-0001 (BOC-1 privilege escalation)
• XAPI source: xenops_server_xen.ml:2416-2419 (cmdline read), domain.ml:1328-1329 (xenguesthelper invocation), datamodel_vm.ml:2717-2720 (VM.platform field definition, no map_keys_roles)
• Thematic advisory: disclosure/advisories/plat-security-advisory.md (PLAT-2)
• Investigation: research/investigations/vm-platform.md
• PoC scripts: research/plat-2/poc/plat-2-pvinpvh-xen-cmdline.sh (available to CSIRTs on request)
• Evidence: research/plat-2/poc/evidence/plat-2-20260319-200002.log

Credits

Discovered and reported by Jakob Wolffhechel, Moksha.