dbcve.org
Debian LinuxOperating system · Debian

CVE-2021-28701

HIGH · 7.8 CVSS v3.1 Published 2021-09-08
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
80/100
Remediation priority · High
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
Another race in XENMAPSPACE_grant_table handling Guests are 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, are de-allocated when a guest switches (back) from v2 to v1. Freeing such pages requires that the hypervisor enforce that no parallel request can result in the addition of a mapping of such a page to a guest. That enforcement was missing, allowing guests to retain access to pages that were freed and perhaps re-used for other purposes. Unfortunately, when XSA-379 was being prepared, this similar issue was not noticed.

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

A race condition in XENMAPSPACE_grant_table handling allows guests to retain access to freed grant table v2 status pages when switching from v2 to v1. The hypervisor failed to enforce that no parallel request could add a mapping during the window when pages are being freed, potentially allowing guests to access re-used memory pages.

MitigationApply Xen security patches addressing this race condition. Ensure grant table operations are properly synchronized and guests cannot map freed v2 status pages.

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
Debian LinuxOperating system
Affected:= 11.0
FedoraOperating system
Affected:= 33= 34= 35
XenOperating system
Affected:>= 4.0.0

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
High
Privileges
Low
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/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 running
    Check for running Xen-related processes: `ps aux | grep -E 'xen|xl|xm' | grep -v grep` or check xen daemon status with `systemctl status xen`
    Affected if No Xen processes are running - not affected as a hypervisor guest (this is a hypervisor vulnerability)
  2. Determine installed Xen version
    Run `xl version` or `xm version` or check package version with `dpkg -l | grep xen` (Debian) or `rpm -qa | grep xen` (Fedora)
    Affected if Xen version is >= 4.0.0 according to the affected range, verify exact version against this threshold
  3. Identify guests using grant tables
    List running guests with `xl list` and check their configurations for grant table usage: look for 'grant_table' in domain config files in /etc/xen or /var/lib/xen
    Affected if Guests are configured with paravirtualization and grant table access - the vulnerability applies when v2 grant tables are in use
  4. Check for concurrent grant table operations
    Review guest workloads and migration activities: check for scripts or automation performing grant table version switches (v2 to v1 transitions)
    Affected if Guests perform grant table version switching operations - the race condition occurs during the v2 to v1 transition window
  5. Confirm if host uses grant table v2
    Examine Xen boot parameters and runtime config: check /boot/grub/grub.cfg for xen.gz parameters or run `xl list -l` on domains to inspect grant_table version settings
    Affected if Grant table version 2 (v2) is enabled for any guest - required for the vulnerability to be exploitable

A system is affected if it runs Xen >= 4.0.0 with guests utilizing grant table v2 that perform concurrent grant table operations or v2-to-v1 transitions.

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 Xen security patches addressing this race condition. Ensure grant table operations are properly synchronized and guests cannot map freed v2 status pages.

Recommended fix High confidence

Xen 4.15.x or later; Debian 11.5+ with security updates; Fedora 35 with latest updates

  1. 1. Identify the Xen hypervisor version currently running: `xl info | grep xen_version`
  2. 2. For Debian 11 (bullseye): Run `apt-get update && apt-get upgrade` to install the latest security patches, or specifically `apt-get install xen-hypervisor-$(uname -r)`
  3. 3. For Fedora 33/34/35: Run `dnf update xen` or `dnf upgrade` to apply security updates
  4. 4. For upstream Xen: Upgrade to Xen 4.15 or later which contains the XSA-389 fix
  5. 5. After updating, reboot the host to load the new hypervisor
  6. 6. Verify the new version is running: `xl info | grep xen_version`
Caveat Major Xen version upgrades may require guest configuration adjustments; ensure backups before upgrading production systems

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

Fix this in Debian Linux Scoped from the published advisory
  • Consultation2.0 h
  • Implementation3.0 h
  • Testing2.0 h
  • Review / QA1.0 h
8.0 hours of engineering $1,420
Get help mitigating

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,272.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2021-28701 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.

Get it on GitHub Scan in the browser

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-2021-28701 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

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