CVE-2025-21900Network attack vector

Service hang via synchronous NFSv4 delegation return

Published Apr 1, 2025 · Updated Aug 5, 2026

Deadlock in the Linux kernel NFSv4 client allows remote attackers to hang NFS client operations through server-triggered state recovery. put_nfs_open_context() performs a synchronous delegation return during open reclaim without marking the call privileged, causing lock recursion when close races with recovery. A mounted NFSv4 server reboot or session reset must coincide with closing a sillyrenamed delete-on-close file; the bounded consequence is stalled client work without data disclosure or modification.

CVSS severity7.5
High
EPSS probability0.34%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Deadlock in the Linux kernel NFSv4 client allows remote attackers to hang NFS client operations through server-triggered state recovery. put_nfs_open_context() performs a synchronous delegation return during open reclaim without marking the call privileged, causing lock recursion when close races with recovery. A mounted NFSv4 server reboot or session reset must coincide with closing a sillyrenamed delete-on-close file; the bounded consequence is stalled client work without data disclosure or modification.

The record

CVE
CVE-2025-21900
Published
Apr 1, 2025
Updated
Aug 5, 2026
Vendor
The Linux Kernel Organization
Product
Linux
Classifications
T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 1, 2025CVE 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=6.11
  2. Affected versionversion=adb4b42d19aea91826621a8d0bac94cf2c08f8bc <4fe4ae6c2e01d028856b73b6328b12b8945df871
  3. Affected versionversion=adb4b42d19aea91826621a8d0bac94cf2c08f8bc <8f8df955f078e1a023ee55161935000a67651f38
  4. Affected versionversion=adb4b42d19aea91826621a8d0bac94cf2c08f8bc <f41a60bc43e7abbc636fee78bed0d74c31e738b0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Linux Kernel Organization · Linuxversion=6.11; version=adb4b42d19aea91826621a8d0bac94cf2c08f8bc <4fe4ae6c2e01d028856b73b6328b12b8945df871; version=adb4b42d19aea91826621a8d0bac94cf2c08f8bc <8f8df955f078e1a023ee55161935000a67651f38; version=adb4b42d19aea91826621a8d0bac94cf2c08f8bc <f41a60bc43e7abbc636fee78bed0d74c31e738b0

Published CVSS scores

7.5LinuxCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
6.8SUSECVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
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)
Vectornetwork
Privilegesunauthenticated
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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