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FedoraOperating system · Fedoraproject

CVE-2021-28694

MEDIUM · 6.8 CVSS v3.1 Published 2021-08-27
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
73/100
Remediation priority · Elevated
No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
IOMMU page mapping issues on x86 T[his CNA information record relates to multiple CVEs; the text explains which aspects/vulnerabilities correspond to which CVE.] Both AMD and Intel allow ACPI tables to specify regions of memory which should be left untranslated, which typically means these addresses should pass the translation phase unaltered. While these are typically device specific ACPI properties, they can also be specified to apply to a range of devices, or even all devices. On all systems with such regions Xen failed to prevent guests from undoing/replacing such mappings (CVE-2021-28694). On AMD systems, where a discontinuous range is specified by firmware, the supposedly-excluded middle range will also be identity-mapped (CVE-2021-28695). Further, on AMD systems, upon de-assigment of a physical device from a guest, the identity mappings would be left in place, allowing a guest continued access to ranges of memory which it shouldn't have access to anymore (CVE-2021-28696).

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · moderate confidence

This vulnerability exists in Xen's IOMMU page mapping implementation on x86 systems. The hypervisor failed to prevent guest VMs from undoing or replacing memory regions that were configured as identity-mapped (untranslated) through ACPI tables. This could allow a malicious or compromised guest to manipulate IOMMU mappings for memory regions that should remain protected.

MitigationApply the Xen security patch for CVE-2021-28694 by upgrading to a patched Xen version. Guests should be restricted from direct IOMMU hardware or ACPI table access to reduce the attack surface.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
FedoraOperating system
Affected:= 33= 34= 35
Debian LinuxOperating system
Affected:= 11.0
XenOperating system
Affected:all versions

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Physical
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

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

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Confirm Xen hypervisor is running
    Run 'xl list' or 'xm list' to list running domains, or check for Xen processes with 'ps aux | grep xen'. Alternatively, check /proc/xen/ directory exists or run 'xl info' to retrieve hypervisor details.
    Affected if The system is not running Xen as the hypervisor - this CVE only affects Xen environments.
  2. Check Xen hypervisor version
    Run 'xl version' or 'xm info' to display the Xen version. Compare this against all versions (all versions are affected per the advisory).
    Affected if The system runs any unpatched Xen version - the advisory states all versions are affected.
  3. Verify x86 architecture
    Run 'uname -m' or 'uname -i' to confirm the system architecture is x86/x86_64. This vulnerability exists specifically in Xen's IOMMU implementation on x86 systems.
    Affected if The system is not x86 architecture - this flaw affects x86 IOMMU page mapping only.
  4. Check if IOMMU is enabled
    Run 'xl dmesg | grep -i iommu' or check dmesg output for IOMMU initialization messages. On the host, you can also check /proc/cmdline for 'iommu=' or 'intel_iommu=' parameters.
    Affected if IOMMU is not enabled - the vulnerability involves manipulation of IOMMU page mappings, so an inactive IOMMU means the flaw cannot be exploited.
  5. Identify identity-mapped memory regions via ACPI
    Run 'xl dmesg | grep -i acpi' or examine ACPI tables using 'acpidump' (if available) to identify any memory regions configured as identity-mapped (untranslated). These are typically defined in ACPI tables for device memory or MMIO regions.
    Affected if ACPI tables contain identity-mapped (untranslated) memory regions that could be manipulated - this is the specific condition the vulnerability allows guests to modify.

A system is affected if it runs Xen on x86 with IOMMU enabled and has identity-mapped memory regions in ACPI tables, allowing a guest to potentially manipulate protected IOMMU mappings.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply the Xen security patch for CVE-2021-28694 by upgrading to a patched Xen version. Guests should be restricted from direct IOMMU hardware or ACPI table access to reduce the attack surface.

Recommended fix High confidence

Xen 4.14.3+/4.13.5+/4.12.6+ or vendor-supplied patched packages

  1. Upgrade Xen hypervisor to a patched version (Xen 4.14.3 or later, Xen 4.13.5 or later, or Xen 4.12.6 or later)
  2. For Fedora systems, apply available Fedora updates: `dnf update xen` or `dnf update`
  3. For Debian systems, apply Debian security updates: `apt update && apt upgrade`
  4. After updating, restart the Xen host (dom0) to apply the new hypervisor
  5. Verify the Xen version running matches the patched release using `xl version`
Caveat Hypervisor updates require host restart; ensure guest VMs are properly shutdown or migrated before restarting dom0

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Fedora Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,860
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-28694 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
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