CVE-2021-47557

Kernel crash via stale sch_ets active-list entries

Published May 24, 2024 · Updated May 11, 2026

NULL-pointer dereference in the Linux kernel sch_ets scheduler allows local users to crash the kernel by changing configured DRR classes. ets_qdisc_change() leaves removed DRR classes on the round-robin active list, and ets_qdisc_dequeue() later calls qdisc->ops->peek through a class whose qdisc pointer is NULL. Exploitation requires local access and permission to reconfigure an ETS qdisc; the demonstrated consequence is a kernel crash.

CVSS severity5.5
Medium
EPSS probability0.24%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

NULL-pointer dereference in the Linux kernel sch_ets scheduler allows local users to crash the kernel by changing configured DRR classes. ets_qdisc_change() leaves removed DRR classes on the round-robin active list, and ets_qdisc_dequeue() later calls qdisc->ops->peek through a class whose qdisc pointer is NULL. Exploitation requires local access and permission to reconfigure an ETS qdisc; the demonstrated consequence is a kernel crash.

The record

CVE
CVE-2021-47557
Published
May 24, 2024
Updated
May 11, 2026
Vendor
The Linux Kernel Organization
Product
Linux
Classifications
CWE-476, T1499.004
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. May 24, 2024CVE publishedPublication date reported by the CVE source.
  2. May 11, 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=5.6
  2. Affected versionversion=dcc68b4d8084e1ac9af0d4022d6b1aff6a139a33 <ae2659d2c670252759ee9c823c4e039c0e05a6f2
  3. Affected versionversion=dcc68b4d8084e1ac9af0d4022d6b1aff6a139a33 <de6d25924c2a8c2988c6a385990cafbe742061bf
  4. Affected versionversion=dcc68b4d8084e1ac9af0d4022d6b1aff6a139a33 <e25bdbc7e951ae5728fee1f4c09485df113d013c

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

The Linux Kernel Organization · Linuxversion=5.6; version=dcc68b4d8084e1ac9af0d4022d6b1aff6a139a33 <ae2659d2c670252759ee9c823c4e039c0e05a6f2; version=dcc68b4d8084e1ac9af0d4022d6b1aff6a139a33 <de6d25924c2a8c2988c6a385990cafbe742061bf; version=dcc68b4d8084e1ac9af0d4022d6b1aff6a139a33 <e25bdbc7e951ae5728fee1f4c09485df113d013c

Published CVSS scores

5.5CanonicalCVSS: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-476NULL Pointer Dereference
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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