FedoraOperating system · Fedoraproject
CVE-2021-28697
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 · uneditedgrant table v2 status pages may remain accessible after de-allocation Guest get permitted access to certain Xen-owned pages of memory. The majority of such pages remain allocated / associated with a guest for its entire lifetime. Grant table v2 status pages, however, get de-allocated when a guest switched (back) from v2 to v1. The freeing of such pages requires that the hypervisor know where in the guest these pages were mapped. The hypervisor tracks only one use within guest space, but racing requests from the guest to insert mappings of these pages may result in any of them to become mapped in multiple locations. Upon switching back from v2 to v1, the guest would then retain access to a page that was freed and perhaps re-used for other purposes.
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 confidenceA race condition in Xen hypervisor's grant table v2 implementation allows guest VMs to retain access to freed memory pages. When a guest switches from grant table v2 to v1, status pages are de-allocated, but due to a flaw where the hypervisor only tracks one mapping location, racing mapping requests can cause pages to remain mapped in multiple locations. Upon switching back to v1, the guest retains access to freed and potentially re-used pages, enabling information disclosure or privilege escalation.
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
= 33= 34= 35
Debian LinuxOperating system
= 11.0>= 4.0.0, <= 4.15.0CVSS 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
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:L/AC:L/PR:L/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 checksWork through these to decide whether this CVE applies to you.
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Identify Xen hypervisor versionRun 'xl info' or 'xe host-list' and look for the hypervisor version string, or check /sys/hypervisor/version if availableAffected if version is between 4.0.0 and 4.15.0 inclusive
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Confirm grant table v2 is in useReview guest VM configuration files in /etc/xen/ or via 'xl list -v' to check for 'grant_table_version=2' or 'type=grant_table_v2' in the VM definitionAffected if any guest VM is configured to use grant table version 2 (gnttab v2)
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Check for grant table status page mappingsExamine hypervisor logs (xl dmesg or /var/log/xen/console) for activity involving grant table status pages during v1/v2 transitionsAffected if guests frequently switch between grant table versions, causing potential race condition window
You are affected if your Xen hypervisor version falls between 4.0.0 and 4.15.0 AND any guest VMs use grant table v2, enabling the race condition during version switches.
Generated from the published advisory. Verify against your own configuration.
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 dataUpgrade Xen hypervisor to a patched version. This is a hypervisor-level vulnerability requiring host-side remediation; guest-side workarounds are limited to isolation and monitoring until patching is feasible.
Fix this in Fedora
Scoped from the published advisory
- Consultation4.0 h
- Implementation8.0 h
- Testing4.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $5,120.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-28697 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-28697 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- 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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