CVE-2021-28951

System hang via disabled SQPOLL startup deadlock

Published Mar 20, 2021 · Updated Aug 3, 2024

Improper locking in Linux kernel io_uring through 5.11.8 allows local users to deadlock the system through an SQPOLL ring. During exit, io_disable_sqo_submit marks submission dead while a disabled SQPOLL thread still awaits startup, leaving exit waiting to park a thread that never starts. Low-privileged local execution and an io_uring instance using submission-queue polling with initially disabled rings are required; the consequence is a persistent system hang.

CVSS severity5.5
Medium
EPSS probability0.27%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper locking in Linux kernel io_uring through 5.11.8 allows local users to deadlock the system through an SQPOLL ring. During exit, io_disable_sqo_submit marks submission dead while a disabled SQPOLL thread still awaits startup, leaving exit waiting to park a thread that never starts. Low-privileged local execution and an io_uring instance using submission-queue polling with initially disabled rings are required; the consequence is a persistent system hang.

The record

CVE
CVE-2021-28951
Published
Mar 20, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-667, T1499.004
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Mar 20, 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

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

4.9NIST NVDAV:L/AC:L/Au:N/C:N/I:N/A:C
5.5NIST NVDCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
5.5UbuntuCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
5.5Amazon LinuxCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
5.5NetAppCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-667Improper Locking
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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