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FedoraOperating system · Fedoraproject

CVE-2021-28698

MEDIUM · 5.5 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 →
57/100
Remediation priority · Elevated
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
long running loops in grant table handling In order to properly monitor resource use, Xen maintains information on the grant mappings a domain may create to map grants offered by other domains. In the process of carrying out certain actions, Xen would iterate over all such entries, including ones which aren't in use anymore and some which may have been created but never used. If the number of entries for a given domain is large enough, this iterating of the entire table may tie up a CPU for too long, starving other domains or causing issues in the hypervisor itself. Note that a domain may map its own grants, i.e. there is no need for multiple domains to be involved here. A pair of "cooperating" guests may, however, cause the effects to be more severe.

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

Xen's grant table handling iterates over all entries (including unused ones) when performing certain actions. When a domain creates many grant mappings, this full table iteration can monopolize a CPU, causing denial of service by starving other domains or destabilizing the hypervisor itself.

MitigationApply the appropriate Xen hypervisor patch to address the grant table iteration issue. For production environments, schedule maintenance windows for patching and ensure proper testing in non-production environments first.

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:>= 3.2.0

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
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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 list running domains, or check /sys/hypervisor/ for Xen-specific files. If these commands fail or the directory does not exist, Xen is not the active hypervisor.
    Affected if The system is running Xen as the hypervisor and the command outputs domain information or the /sys/hypervisor/ path exists.
  2. Determine the installed Xen version
    Run 'xl info' or 'xm info' and look for the 'xen_version' field, or use 'dmesg | grep -i xen' to find the version string at boot.
    Affected if The Xen version is 3.2.0 or higher (any version from 3.2.0 onward is within the affected range).
  3. Identify if grant tables are actively used
    Check domain configurations with 'xl list -l' for each domain and look for 'grant_tables' in the output, or examine the xl config files in /etc/xen/ for entries containing 'grant_tables' or 'hypervisor grant table' settings.
    Affected if Grant table support is enabled in any active domain configuration.
  4. Confirm the host OS matches affected distributions
    Check the host operating system with 'cat /etc/os-release' (Fedora 33, 34, 35) or 'cat /etc/debian_version' (Debian 11.0). If the host OS is one of these versions, the Xen installation shipped with it is affected regardless of Xen version.
    Affected if The host runs Fedora 33, 34, or 35, or Debian 11.0, with Xen installed as the hypervisor.

The environment is affected if Xen is running as the hypervisor at version 3.2.0 or higher (or if the host OS is Fedora 33/34/35 or Debian 11.0 with Xen installed) AND grant table functionality is enabled for any domain.

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 appropriate Xen hypervisor patch to address the grant table iteration issue. For production environments, schedule maintenance windows for patching and ensure proper testing in non-production environments first.

Recommended fix Moderate confidence

Xen 4.14.3, Xen 4.15.1, or later; or equivalent distribution-provided Xen package updates

  1. Update the Xen hypervisor package to the latest available version from your distribution's security repository
  2. For Fedora systems: run 'dnf update xen' or 'dnf update' to get all security updates
  3. For Debian systems: run 'apt update && apt upgrade' or 'apt update && apt full-upgrade'
  4. For Xen from xen.org: upgrade to Xen 4.14.3, Xen 4.15.1, or later which contain the fix for this vulnerability
  5. After updating, reboot the host to load the new hypervisor
  6. Verify the running Xen version matches the updated package using 'xl info' or 'xl version'
Caveat Major Xen version upgrades may require configuration changes; ensure guest XML is backed up before rebooting

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 / QA2.0 h
20.0 hours of engineering $3,500
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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-2021-28698 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