CVE-2025-21908

System hang via recursive NFS writeback

Published Apr 1, 2025 · Updated May 11, 2026

Improper locking in Linux kernel NFS from 6.3 allows local users to cause a system hang through kcompactd-triggered writeback. nfs_release_folio() calls nfs_wb_folio() while kcompactd is reclaiming a folio, causing writeback to recurse into NFS and wait indefinitely. Reachability requires kernel compaction plus an NFS loopback mount on the same host that drives NFSD into XFS folio acquisition, and the bounded impact is an availability-only hang.

CVSS severity5.5
Medium
EPSS probability0.15%
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 NFS from 6.3 allows local users to cause a system hang through kcompactd-triggered writeback. nfs_release_folio() calls nfs_wb_folio() while kcompactd is reclaiming a folio, causing writeback to recurse into NFS and wait indefinitely. Reachability requires kernel compaction plus an NFS loopback mount on the same host that drives NFSD into XFS folio acquisition, and the bounded impact is an availability-only hang.

The record

CVE
CVE-2025-21908
Published
Apr 1, 2025
Updated
May 11, 2026
Vendor
The Linux Kernel Organization
Product
Linux
Classifications
CWE-667, T1499
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Apr 1, 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=6.3
  2. Affected versionversion=96780ca55e3cbf4f150fd5a833a61492c9947b5b <5ae31c54cff745832b9bd5b32e71f3d1b607cd1e
  3. Affected versionversion=96780ca55e3cbf4f150fd5a833a61492c9947b5b <8253ff29edcb429a9a6c75710941c6a16a9a34b1
  4. Affected versionversion=96780ca55e3cbf4f150fd5a833a61492c9947b5b <ab0727d6e2196682351c25c1dd112136f6991f11
  5. Affected versionversion=96780ca55e3cbf4f150fd5a833a61492c9947b5b <ce6d9c1c2b5cc785016faa11b48b6cd317eb367e

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

The Linux Kernel Organization · Linuxversion=6.3; version=96780ca55e3cbf4f150fd5a833a61492c9947b5b <5ae31c54cff745832b9bd5b32e71f3d1b607cd1e; version=96780ca55e3cbf4f150fd5a833a61492c9947b5b <8253ff29edcb429a9a6c75710941c6a16a9a34b1; version=96780ca55e3cbf4f150fd5a833a61492c9947b5b <ab0727d6e2196682351c25c1dd112136f6991f11; version=96780ca55e3cbf4f150fd5a833a61492c9947b5b <ce6d9c1c2b5cc785016faa11b48b6cd317eb367e

Published CVSS scores

5.5UbuntuCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
5.5CISA ADPCVSS: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

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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
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-667Improper Locking
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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