CVE-2025-21795Network attack vector

NFS server denial of service via callback hang

Published Feb 27, 2025 · Updated Sep 8, 2026

Improper input validation in Linux kernel NFSD allows remote attackers to interrupt NFS service through crafted NFSv4 client state. The callback worker sends an RPC for a courtesy-state client, leaving cl_cb_inflight nonzero and nfsd4_shutdown_callback blocked until TCP detects the dropped connection. An exposed NFSv4 server is required, and repeated conflicting operations can stall NFSD state and lock cleanup for about 15 minutes.

CVSS severity7.5
High
EPSS probability0.63%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper input validation in Linux kernel NFSD allows remote attackers to interrupt NFS service through crafted NFSv4 client state. The callback worker sends an RPC for a courtesy-state client, leaving cl_cb_inflight nonzero and nfsd4_shutdown_callback blocked until TCP detects the dropped connection. An exposed NFSv4 server is required, and repeated conflicting operations can stall NFSD state and lock cleanup for about 15 minutes.

The record

CVE
CVE-2025-21795
Published
Feb 27, 2025
Updated
Sep 8, 2026
Vendor
Siemens
Product
SIPLUS S7-1500 CPU 1518-4 PN/DP MFP
Classifications
CWE-20, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 27, 2025CVE publishedPublication date reported by the CVE source.
  2. Sep 8, 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=V3.1.5 <*
  2. Affected versionversion=V3.1.6 <*

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Siemens · SIPLUS S7-1500 CPU 1518-4 PN/DP MFPversion=V3.1.5 <*; version=V3.1.6 <*
Siemens · SIMATIC S7-1500 TM MFP - GNU/Linux subsystemversion=0 <*
The Linux Kernel Organization · Linuxversion=26540b8940a2e21582afa61a6fb8af87310bac72 <efa8a261c575f816c7e79a87aeb3ef8a0bd6b221; version=5.10.220 <5.10.235; version=5.15.154 <5.15.179; version=5.19; version=66af25799940b26efd41ea6e648f75c41a48a2c2 <036ac2778f7b28885814c6fbc07e156ad1624d03; version=66af25799940b26efd41ea6e648f75c41a48a2c2 <23ad7797c74cd8f7f90617f1e59a8703e2b43908; version=66af25799940b26efd41ea6e648f75c41a48a2c2 <38d345f612503b850c2973e5a879f88e441b34d7; version=66af25799940b26efd41ea6e648f75c41a48a2c2 <cedfbb92cf97a6bff3d25633001d9c44442ee854; version=66af25799940b26efd41ea6e648f75c41a48a2c2 <e88d2451cd42e025465d6b51fd716a47b0b3800d; version=67ef9e5fd737eab2495f2586df7e9ea30caa1b77 <abed68027ea3ab893ac85cc46a00e2e64a324239
Siemens · SIMATIC S7-1500 CPU 1518F-4 PN/DP MFPversion=V3.1.5 <*; version=V3.1.6 <*
Siemens · SIMATIC S7-1500 CPU 1518-4 PN/DP MFPversion=V3.1.5 <*; version=V3.1.6 <*

Published CVSS scores

5.5Siemens ProductCERTCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
7.5Linux kernelCVSS:3.1/AV:N/AC:L/PR:N/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.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-20Improper Input Validation
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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