CVE-2021-28708

Host crash via unsupported reservation page orders

Published Nov 24, 2021 · Updated Nov 18, 2024

Denial of service in Xen Hypervisor 4.7 and later allows guest OS users to crash the host through a crafted memory hypercall. XENMEM_decrease_reservation accepts a populate-on-demand page order other than 4 KiB, 2 MiB, or 1 GiB and reaches handling that assumes a supported order. Exploitation requires an x86 HVM or PVH guest running in populate-on-demand mode, which is enabled when maxmem exceeds memory, and interrupts the entire host.

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

What happened

Denial of service in Xen Hypervisor 4.7 and later allows guest OS users to crash the host through a crafted memory hypercall. XENMEM_decrease_reservation accepts a populate-on-demand page order other than 4 KiB, 2 MiB, or 1 GiB and reaches handling that assumes a supported order. Exploitation requires an x86 HVM or PVH guest running in populate-on-demand mode, which is enabled when maxmem exceeds memory, and interrupts the entire host.

The record

CVE
CVE-2021-28708
Published
Nov 24, 2021
Updated
Nov 18, 2024
Vendor
Xen Project
Product
Xen Hypervisor
Classifications
T1499.004
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Nov 24, 2021CVE publishedPublication date reported by the CVE source.
  2. Nov 18, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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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

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

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

T1499.004Application or System Exploitation

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

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Your stack

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