NVD / CVE Program· CNA Linux

CVE-2021-47506: nfsd: fix use-after-free due to delegation race

HighCVSS 7.8 · v3.1—No exploit Fix available

Noroxi assessment

Priority: Monitor (action score 31/100)

The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
Probability of exploitation in the next 30 days: 0.78% (FIRST EPSS); higher than 56.4% of all records.
Someone with local access to the system can trigger it. A low-privileged account is enough. No user action is required. No special conditions are required; it is repeatable.
Fixed version: Linux Linux 04a8d07f3d58308b92630045560799a3faa3ebce, 148c816f10fd11df27ca6a9b3238cdd42fa72cd3, 2becaa990b93cbd2928292c0b669d3abb6cf06d4.
Severity is disputed: the assigning authority (CNA) scores it 9.8, NVD 7.8.
Mentioned 1 time in Turkish national cyber security advisories (siberguvenlik.gov.tr).

Low priority for now. Upgrade to Linux Linux 04a8d07f3d58308b92630045560799a3faa3ebce, 148c816f10fd11df27ca6a9b3238cdd42fa72cd3, 2becaa990b93cbd2928292c0b669d3abb6cf06d4 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

In the Linux kernel, the following vulnerability has been resolved: nfsd: fix use-after-free due to delegation race A delegation break could arrive as soon as we've called vfs_setlease. A delegation break runs a callback which immediately (in nfsd4_cb_recall_prepare) adds the delegation to del_recall_lru. If we then exit nfs4_set_delegation without hashing the delegation, it will be freed as soon as the callback is done with it, without ever being removed from del_recall_lru. Symptoms show up later as use-after-free or list corruption warnings, usually in the laundromat thread. I suspect aba2072f4523 "nfsd: grant read delegations to clients holding writes" made this bug easier to hit, but I looked as far back as v3.0 and it looks to me it already had the same problem. So I'm not sure where the bug was introduced; it may have been there from the beginning.

Memory safety

Published: May 24, 2024
Updated: Aug 5, 2026
EPSS: 0.7% · 56th percentile
CWE: CWE-416

Report tools

JSON

Action score

Monitor

Low priority for now.

CVSS: 31 / 40 · 7.8 / 10
CISA KEV: 0 / 30 · Not listed
EPSS: 0 / 30 · 0.7%

CISA SSVC decision

Exploitation: none
Automatable: no
Technical impact: total

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD: 7.8
CNA · Linux: 9.8; 2.0 point gap

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

Not in KEV: no record of exploitation in the wild
No public exploit code on record
A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Is your version affected? Check it

Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

Vendor	Product	CPE
linux	linux kernel	cpe:2.3:o:linux:linux_kernel

Versions reported by the vendor

Affected version ranges reported by the assigning authority (Linux). Independent of NVD's CPE analysis and usually ahead of it.

Linux Linux
- 3.17affected
- dff1399f8addf7129c49bb2227469da79cc30b47 and later · before 04a8d07f3d58308b92630045560799a3faa3ebceaffected · git
- dff1399f8addf7129c49bb2227469da79cc30b47 and later · before 348714018139c39533c55661a0c7c990671396b4affected · git
- dff1399f8addf7129c49bb2227469da79cc30b47 and later · before 33645d3e22720cac1e4548f8fef57bf0649536eeaffected · git
- dff1399f8addf7129c49bb2227469da79cc30b47 and later · before 2becaa990b93cbd2928292c0b669d3abb6cf06d4affected · git
- dff1399f8addf7129c49bb2227469da79cc30b47 and later · before e0759696de6851d7536efddfdd2dfed4c4df1f09affected · git
- dff1399f8addf7129c49bb2227469da79cc30b47 and later · before eeb0711801f5e19ef654371b627682aed3b11373affected · git
- dff1399f8addf7129c49bb2227469da79cc30b47 and later · before 148c816f10fd11df27ca6a9b3238cdd42fa72cd3affected · git
- dff1399f8addf7129c49bb2227469da79cc30b47 and later · before 548ec0805c399c65ed66c6641be467f717833ab5affected · git
- before 3.17not affected · semver

Affected versions · NVD (8)

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

linux linux kernelbefore 4.4.296
linux linux kernel4.5 and later · before 4.9.294
linux linux kernel4.10 and later · before 4.14.259
linux linux kernel4.15 and later · before 4.19.222
linux linux kernel4.20 and later · before 5.4.168
linux linux kernel5.5 and later · before 5.10.85
linux linux kernel5.11 and later · before 5.15.8
linux linux kernel5.15

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

Ecosystem	Package	Affected range	Fix
Debian:12	linux	before 5.15.15-1	5.15.15-1
SUSE:Linux Enterprise High Performance Computing 15 SP2-LTSS	kernel-default-base	before 5.3.18-150200.24.200.1.150200.9.103.1	5.3.18-150200.24.200.1.150200.9.103.1
SUSE:Linux Enterprise High Performance Computing 15 SP2-LTSS	kernel-docs	before 5.3.18-150200.24.200.1	5.3.18-150200.24.200.1
SUSE:Linux Enterprise High Performance Computing 15 SP2-LTSS	kernel-obs-build	before 5.3.18-150200.24.200.1	5.3.18-150200.24.200.1
SUSE:Linux Enterprise High Performance Computing 15 SP2-LTSS	kernel-preempt	before 5.3.18-150200.24.200.1	5.3.18-150200.24.200.1
SUSE:Linux Enterprise High Performance Computing 15 SP2-LTSS	kernel-source	before 5.3.18-150200.24.200.1	5.3.18-150200.24.200.1
SUSE:Linux Enterprise High Performance Computing 15 SP2-LTSS	kernel-syms	before 5.3.18-150200.24.200.1	5.3.18-150200.24.200.1
SUSE:Linux Enterprise High Performance Computing 15 SP3-LTSS	kernel-64kb	before 5.3.18-150300.59.170.1	5.3.18-150300.59.170.1
SUSE:Linux Enterprise High Performance Computing 15 SP3-LTSS	kernel-default-base	before 5.3.18-150300.59.170.1.150300.18.100.1	5.3.18-150300.59.170.1.150300.18.100.1
SUSE:Linux Enterprise High Performance Computing 15 SP3-LTSS	kernel-docs	before 5.3.18-150300.59.170.2	5.3.18-150300.59.170.2
SUSE:Linux Enterprise High Performance Computing 15 SP3-LTSS	kernel-obs-build	before 5.3.18-150300.59.170.1	5.3.18-150300.59.170.1
SUSE:Linux Enterprise High Performance Computing 15 SP3-LTSS	kernel-preempt	before 5.3.18-150300.59.170.1	5.3.18-150300.59.170.1
SUSE:Linux Enterprise High Performance Computing 15 SP3-LTSS	kernel-source	before 5.3.18-150300.59.170.1	5.3.18-150300.59.170.1
SUSE:Linux Enterprise High Performance Computing 15 SP3-LTSS	kernel-syms	before 5.3.18-150300.59.170.1	5.3.18-150300.59.170.1
SUSE:Linux Enterprise Live Patching 15 SP2	kernel-default	before 5.3.18-150200.24.200.1	5.3.18-150200.24.200.1
SUSE:Linux Enterprise Live Patching 15 SP2	kernel-livepatch-SLE15-SP2_Update_51	before 1-150200.5.3.1	1-150200.5.3.1
SUSE:Linux Enterprise Live Patching 15 SP3	kernel-default	before 5.3.18-150300.59.170.1	5.3.18-150300.59.170.1
SUSE:Linux Enterprise Live Patching 15 SP3	kernel-livepatch-SLE15-SP3_Update_47	before 1-150300.7.3.1	1-150300.7.3.1
SUSE:Linux Enterprise Server 15 SP3-LTSS	kernel-zfcpdump	before 5.3.18-150300.59.170.1	5.3.18-150300.59.170.1

Same product

linux: all records

Other highest-scoring records for the same primary product.

CVE-2026-31431crypto: algif_aead - Revert to operating out-of-placeKEV
90Now
CVE-2022-0847A flaw was found in the way the "flags" member of the new pipe buffer structure was lacking proper initialization in copy_page_to_iter_pipe…KEV
89Now
CVE-2021-22555Heap Out-Of-Bounds Write in Netfilter IP6T_SO_SET_REPLACEKEV
85Now

Published before and after this one in the same product

← EarlierCVE-2021-47503May 24, 2024 · scsi: pm80xx: Do not call scsi_remove_host() in pm8001_alloc()
24Monitor
← EarlierCVE-2021-47505May 24, 2024 · aio: fix use-after-free due to missing POLLFREE handling
31Monitor
Later →CVE-2021-47507May 24, 2024 · nfsd: Fix nsfd startup race (again)
18Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / package	Fixed version	Source
Linux Linux	04a8d07f3d58308b92630045560799a3faa3ebce	Vendor (CNA)
Linux Linux	148c816f10fd11df27ca6a9b3238cdd42fa72cd3	Vendor (CNA)
Linux Linux	2becaa990b93cbd2928292c0b669d3abb6cf06d4	Vendor (CNA)
Linux Linux	33645d3e22720cac1e4548f8fef57bf0649536ee	Vendor (CNA)
Linux Linux	348714018139c39533c55661a0c7c990671396b4	Vendor (CNA)
Linux Linux	548ec0805c399c65ed66c6641be467f717833ab5	Vendor (CNA)
Linux Linux	e0759696de6851d7536efddfdd2dfed4c4df1f09	Vendor (CNA)
Linux Linux	eeb0711801f5e19ef654371b627682aed3b11373	Vendor (CNA)
debian:linux	5.15.15-1 · Debian:12	Package registry (OSV)
suse:kernel-64kb	5.3.18-150300.59.170.1 · SUSE:Linux Enterprise High Performance Computing 15 SP3-LTSS	Package registry (OSV)
suse:kernel-default	5.3.18-150200.24.200.1 · SUSE:Linux Enterprise Live Patching 15 SP2	Package registry (OSV)
suse:kernel-default-base	5.3.18-150200.24.200.1.150200.9.103.1 · SUSE:Linux Enterprise High Performance Computing 15 SP2-LTSS	Package registry (OSV)
suse:kernel-docs	5.3.18-150200.24.200.1 · SUSE:Linux Enterprise High Performance Computing 15 SP2-LTSS	Package registry (OSV)
suse:kernel-livepatch-SLE15-SP2_Update_51	1-150200.5.3.1 · SUSE:Linux Enterprise Live Patching 15 SP2	Package registry (OSV)
suse:kernel-livepatch-SLE15-SP3_Update_47	1-150300.7.3.1 · SUSE:Linux Enterprise Live Patching 15 SP3	Package registry (OSV)
suse:kernel-obs-build	5.3.18-150200.24.200.1 · SUSE:Linux Enterprise High Performance Computing 15 SP2-LTSS	Package registry (OSV)
suse:kernel-preempt	5.3.18-150200.24.200.1 · SUSE:Linux Enterprise High Performance Computing 15 SP2-LTSS	Package registry (OSV)
suse:kernel-source	5.3.18-150200.24.200.1 · SUSE:Linux Enterprise High Performance Computing 15 SP2-LTSS	Package registry (OSV)
suse:kernel-syms	5.3.18-150200.24.200.1 · SUSE:Linux Enterprise High Performance Computing 15 SP2-LTSS	Package registry (OSV)
suse:kernel-zfcpdump	5.3.18-150300.59.170.1 · SUSE:Linux Enterprise Server 15 SP3-LTSS	Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

National notice (Türkiye Cybersecurity Directorate / USOM)

Official security notices citing this record; remediation advice is on the agency's page.

TR-24-0551 · May 27, 2024(Linux Kernel Güncellemesi)

All national notices →

Not on record for this CVE: EPSS history · patch commit or PR link · researcher credit · variant or chain candidate · public bounty program.

Technical details

Attack conditions

Someone with local access to the system can trigger it.
A low-privileged account is enough.
No user action is required.
No special conditions are required; it is repeatable.

If successful

Confidentiality: high · data can be read
Integrity: high · data or configuration can be modified
Availability: high · the service can be disrupted

Attack vector: Local
Attack complexity: Low
Privileges required: Low
User interaction: None
Scope: Unchanged
Confidentiality impact: High
Integrity impact: High
Availability impact: High

Weakness class (CWE)

CWE-416 · Use After Free

CVSS vector

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

nvd-primary

Root cause

After a memory region is freed, another reference pointing to it is still used. By then, the same region may have been allocated for different data.

The bug in code

The whole CWE-416 class

A representative example of this vulnerability class. Not the vendor's source code; it shows the faulty pattern and its fix.

Vulnerable

1free(dev->buf);
2/* ... another thread is still running ... */
3process(dev->buf);

Fixed

1lock(&dev->lock);
2free(dev->buf);
3dev->buf = NULL;
4unlock(&dev->lock);

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log (1)

Fix✗ → ✓Sep 28, 2026, 10:10 PM UTC

For records you follow, these changes also arrive as notifications. →

References

git.kernel.org/stable/c/04a8d07f3d58308b92630045560799a3faa3ebce

Show all references (7 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records