CVE-2021-28704

Host denial of service via misaligned PoD requests

Published Nov 24, 2021 · Updated Aug 3, 2024

Denial of service in Xen Project Xen Hypervisor allows guest OS users controlling a kernel to disrupt the host via crafted PoD hypercalls. The XENMEM_decrease_reservation path accepts a base guest frame number that is not aligned to the requested page order, although downstream PoD handling assumes alignment. Exploitation requires an x86 HVM or PVH guest started with populate-on-demand memory, and it can deny service to the entire host; privilege escalation and information disclosure remain unconfirmed.

CVSS severity8.8
High
EPSS probability0.33%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Denial of service in Xen Project Xen Hypervisor allows guest OS users controlling a kernel to disrupt the host via crafted PoD hypercalls. The XENMEM_decrease_reservation path accepts a base guest frame number that is not aligned to the requested page order, although downstream PoD handling assumes alignment. Exploitation requires an x86 HVM or PVH guest started with populate-on-demand memory, and it can deny service to the entire host; privilege escalation and information disclosure remain unconfirmed.

The record

CVE
CVE-2021-28704
Published
Nov 24, 2021
Updated
Aug 3, 2024
Vendor
Xen Project
Product
Xen Hypervisor
Classifications
T1499
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Nov 24, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=4.12.x <unspecified
  2. Affected versionversion=4.15.x
  3. Affected versionversion=xen-unstable

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

Xen Project · Xen Hypervisorversion=4.12.x <unspecified; version=4.15.x; version=xen-unstable

Published CVSS scores

6.9NIST NVDAV:L/AC:M/Au:N/C:C/I:C/A:C
8.8NIST NVDCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1499Endpoint Denial of Service

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo