CVE-2025-21657

Local service interruption via incorrect scheduler locking

Published Jan 21, 2025 · Updated May 11, 2026

Improper locking in the Linux kernel 6.12 scheduler allows local users to interrupt service through sched_ext bypass handling. In kernel/sched/ext.c, scx_ops_bypass() calls rq_lock() while iterating every CPU even though that lock is intended only for online CPUs, triggering a scheduler warning. Reachability requires local low-privileged access, an active sched_ext BPF scheduler, and an offline CPU or a task running in a higher scheduler class.

CVSS severity5.5
Medium
EPSS probability0.18%
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 the Linux kernel 6.12 scheduler allows local users to interrupt service through sched_ext bypass handling. In kernel/sched/ext.c, scx_ops_bypass() calls rq_lock() while iterating every CPU even though that lock is intended only for online CPUs, triggering a scheduler warning. Reachability requires local low-privileged access, an active sched_ext BPF scheduler, and an offline CPU or a task running in a higher scheduler class.

The record

CVE
CVE-2025-21657
Published
Jan 21, 2025
Updated
May 11, 2026
Vendor
The Linux Kernel Organization
Product
Linux
Classifications
T1499
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Jan 21, 2025CVE 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

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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=0e7ffff1b8117b05635c87d3c9099f6aa9c9b689 <6268d5bc10354fc2ab8d44a0cd3b042d49a0417e
  2. Affected versionversion=0e7ffff1b8117b05635c87d3c9099f6aa9c9b689 <d9e446dd63cee7161717a6a8414ba9c6435af764
  3. Affected versionversion=6.12

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

The Linux Kernel Organization · Linuxversion=0e7ffff1b8117b05635c87d3c9099f6aa9c9b689 <6268d5bc10354fc2ab8d44a0cd3b042d49a0417e; version=0e7ffff1b8117b05635c87d3c9099f6aa9c9b689 <d9e446dd63cee7161717a6a8414ba9c6435af764; version=6.12

Published CVSS scores

5.5NVDCVSS: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 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

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

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Your stack

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