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FedoraOperating system · Fedoraproject

CVE-2021-28696

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 · high confidence

On AMD systems, when a physical device is de-assigned from a guest virtual machine, the Xen hypervisor fails to remove the identity mappings that were previously established for that device. This leaves the guest with continued access to memory ranges it should no longer be able to access after device de-assignment, creating an information disclosure or privilege escalation risk.

MitigationApply the relevant Xen security patches to remove identity mappings upon device de-assignment, then restart affected guests to ensure clean IOMMU state.

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. Verify Xen hypervisor is in use
    Run 'xl list' or 'xm list' to confirm a Xen hypervisor environment is running
    Affected if The system is not running Xen as the hypervisor, then this CVE does not apply
  2. Confirm AMD processor architecture
    Check /proc/cpuinfo for the vendor 'AuthenticAMD' or run 'lscpu | grep -i vendor'
    Affected if This flaw only affects AMD systems; Intel systems are not vulnerable to this specific issue
  3. Check if PCI device passthrough is configured
    Run 'xl pci-assignable-list' or check /sys/bus/pci/devices/ for assigned devices; also verify IOMMU is enabled with 'xl dmesg | grep -i iommu'
    Affected if IOMMU and PCI device assignment must be actively in use for the flaw to be exploitable; without assigned devices, there is no attack surface
  4. Identify recently de-assigned devices
    Review guest configuration files in /etc/xen/ for removed 'pci' lines or use 'xl pci-list <guestname>' to compare with known assignments
    Affected if Guests that had physical devices removed via 'xl pci-detach' without subsequent host reboot may retain stale identity mappings

A user is affected if they run Xen on AMD hardware with active PCI device passthrough and have performed device de-assignment operations on guests.

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 relevant Xen security patches to remove identity mappings upon device de-assignment, then restart affected guests to ensure clean IOMMU state.

Fix this in Fedora Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA3.0 h
21.0 hours of engineering $3,680
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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-2021-28696 in production — separate from our analysis above.

No notes yet

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What this is

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What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
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