CVE-2021-47506Network attack vector

Kernel crash via nfsd delegation race

Published May 24, 2024 · Updated Aug 5, 2026

Use-after-free in Linux nfsd allows remote attackers to crash the kernel by racing conflicting NFSv4 delegation requests. nfsd4_cb_recall_prepare adds a breaking delegation to del_recall_lru, but nfs4_set_delegation can free it after failing to hash it without first removing the list entry. The nfsd service must be handling concurrent delegation conflicts; the stale entry later causes use-after-free or list corruption in the laundromat thread.

CVSS severity7.8
High
EPSS probability0.78%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Use-after-free in Linux nfsd allows remote attackers to crash the kernel by racing conflicting NFSv4 delegation requests. nfsd4_cb_recall_prepare adds a breaking delegation to del_recall_lru, but nfs4_set_delegation can free it after failing to hash it without first removing the list entry. The nfsd service must be handling concurrent delegation conflicts; the stale entry later causes use-after-free or list corruption in the laundromat thread.

The record

CVE
CVE-2021-47506
Published
May 24, 2024
Updated
Aug 5, 2026
Vendor
The Linux Kernel Organization
Product
Linux
Classifications
CWE-416, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 24, 2024CVE 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

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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=3.17
  2. Affected versionversion=dff1399f8addf7129c49bb2227469da79cc30b47 <04a8d07f3d58308b92630045560799a3faa3ebce
  3. Affected versionversion=dff1399f8addf7129c49bb2227469da79cc30b47 <148c816f10fd11df27ca6a9b3238cdd42fa72cd3
  4. Affected versionversion=dff1399f8addf7129c49bb2227469da79cc30b47 <2becaa990b93cbd2928292c0b669d3abb6cf06d4
  5. Affected versionversion=dff1399f8addf7129c49bb2227469da79cc30b47 <33645d3e22720cac1e4548f8fef57bf0649536ee
  6. Affected versionversion=dff1399f8addf7129c49bb2227469da79cc30b47 <348714018139c39533c55661a0c7c990671396b4
  7. Affected versionversion=dff1399f8addf7129c49bb2227469da79cc30b47 <548ec0805c399c65ed66c6641be467f717833ab5
  8. Affected versionversion=dff1399f8addf7129c49bb2227469da79cc30b47 <e0759696de6851d7536efddfdd2dfed4c4df1f09
  9. Affected versionversion=dff1399f8addf7129c49bb2227469da79cc30b47 <eeb0711801f5e19ef654371b627682aed3b11373

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Linux Kernel Organization · Linuxversion=3.17; version=dff1399f8addf7129c49bb2227469da79cc30b47 <04a8d07f3d58308b92630045560799a3faa3ebce; version=dff1399f8addf7129c49bb2227469da79cc30b47 <148c816f10fd11df27ca6a9b3238cdd42fa72cd3; version=dff1399f8addf7129c49bb2227469da79cc30b47 <2becaa990b93cbd2928292c0b669d3abb6cf06d4; version=dff1399f8addf7129c49bb2227469da79cc30b47 <33645d3e22720cac1e4548f8fef57bf0649536ee; version=dff1399f8addf7129c49bb2227469da79cc30b47 <348714018139c39533c55661a0c7c990671396b4; version=dff1399f8addf7129c49bb2227469da79cc30b47 <548ec0805c399c65ed66c6641be467f717833ab5; version=dff1399f8addf7129c49bb2227469da79cc30b47 <e0759696de6851d7536efddfdd2dfed4c4df1f09; version=dff1399f8addf7129c49bb2227469da79cc30b47 <eeb0711801f5e19ef654371b627682aed3b11373

Published CVSS scores

7.8NIST NVDCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/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 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

CWE-416Use After Free
T1499.004Application 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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