CVE-2021-20317

Privileged system freeze via timer-tree corruption

Published Sep 27, 2021 · Updated Aug 3, 2024

Race condition in the Linux Kernel before 5.4-rc1 allows local users with special privileges to freeze the system through timer corruption. timerqueue_add maintains a separate next-timer pointer beside the red-black tree, allowing concurrent timer changes to corrupt ordering and miss a task wakeup. Reaching the flaw requires CAP_SYS_ADMIN; successful exploitation progressively starves resources until the host stops responding.

CVSS severity4.4
Medium
EPSS probability0.40%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Race condition in the Linux Kernel before 5.4-rc1 allows local users with special privileges to freeze the system through timer corruption. timerqueue_add maintains a separate next-timer pointer beside the red-black tree, allowing concurrent timer changes to corrupt ordering and miss a task wakeup. Reaching the flaw requires CAP_SYS_ADMIN; successful exploitation progressively starves resources until the host stops responding.

The record

CVE
CVE-2021-20317
Published
Sep 27, 2021
Updated
Aug 3, 2024
Vendor
The Linux Kernel Organization
Product
Linux Kernel
Classifications
CWE-665, T1499.004
Attack vector
local
Privileges
admin

Timeline

How it unfolded

  1. Sep 27, 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?

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  1. Affected versionversion=Kernel 5.3 rc1

What conditions does exploitation require?

Attack vectorlocal
Required privilegesadmin

What is affected?

The Linux Kernel Organization · Linux Kernelversion=Kernel 5.3 rc1

Published CVSS scores

4.4Oracle LinuxCVSS:3.0/AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
4.9National Vulnerability DatabaseAV:L/AC:L/Au:N/C:N/I:N/A:C
4.4National Vulnerability DatabaseCVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
4.4Amazon LinuxCVSS:3.1/AV:L/AC:L/PR:H/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.

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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
Privilegesadmin
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-665Improper Initialization
T1499.004Endpoint Denial of Service: Application 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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