CVE-2021-47209

Kernel crash via cfs_rq teardown race

Published Apr 10, 2024 · Updated Aug 5, 2026

Use-after-free in Linux 5.13 through 5.15.4 allows local users to crash the kernel by racing CPU task-group teardown. unregister_fair_sched_group() can be interrupted before a dying task group is unlinked, letting sched_cfs_period_timer() re-add its cfs_rq objects before free_fair_sched_group() releases them. Reaching the race requires low-privilege local access to manipulate CPU cgroups; subsequent scheduler list traversal can trigger an access violation or kernel crash.

CVSS severity5.5
Medium
EPSS probability0.21%
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

Use-after-free in Linux 5.13 through 5.15.4 allows local users to crash the kernel by racing CPU task-group teardown. unregister_fair_sched_group() can be interrupted before a dying task group is unlinked, letting sched_cfs_period_timer() re-add its cfs_rq objects before free_fair_sched_group() releases them. Reaching the race requires low-privilege local access to manipulate CPU cgroups; subsequent scheduler list traversal can trigger an access violation or kernel crash.

The record

CVE
CVE-2021-47209
Published
Apr 10, 2024
Updated
Aug 5, 2026
Vendor
The Linux Kernel Organization
Product
Linux
Classifications
T1499
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Apr 10, 2024CVE publishedPublication date reported by the CVE source.
  2. Aug 5, 2026Record 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=5.13
  2. Affected versionversion=a7b359fc6a37faaf472125867c8dc5a068c90982 <512e21c150c1c3ee298852660f3a796e267e62ec
  3. Affected versionversion=a7b359fc6a37faaf472125867c8dc5a068c90982 <b027789e5e50494c2325cc70c8642e7fd6059479

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

The Linux Kernel Organization · Linuxversion=5.13; version=a7b359fc6a37faaf472125867c8dc5a068c90982 <512e21c150c1c3ee298852660f3a796e267e62ec; version=a7b359fc6a37faaf472125867c8dc5a068c90982 <b027789e5e50494c2325cc70c8642e7fd6059479

Published CVSS scores

5.5NVDCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
5.5SUSECVSS: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

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