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FedoraOperating system · Fedoraproject

CVE-2021-28695

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

A flaw in the Xen hypervisor's IOMMU page mapping on AMD systems causes identity-mapped memory ranges to incorrectly include the excluded middle portions of discontinuous ranges specified by firmware ACPI tables. When firmware defines a discontinuous (gapped) range to be identity-mapped, the middle gap that should be excluded instead gets identity-mapped, potentially allowing unauthorized memory access.

MitigationApply the Xen security patch for CVE-2021-28695. Review and reconfigure IOMMU mappings to ensure discontinuous ranges are properly handled, and verify that guest-assigned devices cannot access unintended memory ranges.

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:= 10.0
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 kernel with 'uname -a' or check for xen daemon with 'ps aux | grep xen'
    Affected if Xen hypervisor is not present - not affected. If Xen is running, continue to next checks.
  2. Verify AMD hardware platform
    Check CPU vendor with 'cat /proc/cpuinfo' and look for 'AuthenticAMD' in the vendor field, or use 'lscpu | grep Vendor'
    Affected if System uses AMD processors - potentially affected. Intel systems are not affected by this specific flaw.
  3. Confirm IOMMU is enabled
    Check Xen logs with 'xl dmesg | grep -i iommu' or inspect /boot/config-* for CONFIG_XEN_AMD_PIN_CONTROL and check for AMD-Vi or IOMMU in dmesg
    Affected if IOMMU is enabled and active - the flaw can be triggered. If IOMMU is disabled, the attack surface is reduced.
  4. Check Xen version
    Run 'xl version' or 'xm version' to get the Xen hypervisor version, or check installed package with 'rpm -q xen' (Fedora) or 'dpkg -l xen' (Debian)
    Affected if Any version of Xen on AMD systems with IOMMU enabled is potentially affected. Compare against any available patched versions from your distribution.
  5. Review ACPI tables for discontinuous ranges
    Use 'acpidump' or check /sys/firmware/acpi/tables for IVRS (IOMMU) table presence, and review any custom ACPI tables defining memory ranges for device assignment
    Affected if ACPI tables define discontinuous (gapped) memory ranges for IOMMU mapping - these are the specific conditions where the flaw manifests.

A system is affected if it runs Xen on AMD hardware with IOMMU enabled and uses ACPI tables that define discontinuous memory ranges for device assignment; the vulnerability allows unauthorized access to the gap portions of these ranges.

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.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply the Xen security patch for CVE-2021-28695. Review and reconfigure IOMMU mappings to ensure discontinuous ranges are properly handled, and verify that guest-assigned devices cannot access unintended memory ranges.

Fix this in Fedora Scoped from the published advisory
  • Consultation4.0 h
  • Implementation4.0 h
  • Testing6.0 h
  • Review / QA3.0 h
17.0 hours of engineering $2,960
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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-2021-28695 in production — separate from our analysis above.

No notes yet

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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
  • No spam, self-promotion, credentials, or personal data