CVE-2021-47506 (GCVE-0-2021-47506)
Vulnerability from cvelistv5 – Published: 2024-05-24 15:01 – Updated: 2026-08-05 08:48
VLAI
EPSS
VEX
Title
nfsd: fix use-after-free due to delegation race
Summary
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.
Severity
9.8 (Critical)
SSVC
Exploitation: none
Automatable: no
Technical Impact: total
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2024-05-29 17:04 UTC
Assigner
References
8 references
Impacted products
2 products
| Vendor | Product | Version | CPE status | |
|---|---|---|---|---|
| Linux | Linux |
Affected:
dff1399f8addf7129c49bb2227469da79cc30b47 , < 04a8d07f3d58308b92630045560799a3faa3ebce
(git)
Affected: dff1399f8addf7129c49bb2227469da79cc30b47 , < 348714018139c39533c55661a0c7c990671396b4 (git) Affected: dff1399f8addf7129c49bb2227469da79cc30b47 , < 33645d3e22720cac1e4548f8fef57bf0649536ee (git) Affected: dff1399f8addf7129c49bb2227469da79cc30b47 , < 2becaa990b93cbd2928292c0b669d3abb6cf06d4 (git) Affected: dff1399f8addf7129c49bb2227469da79cc30b47 , < e0759696de6851d7536efddfdd2dfed4c4df1f09 (git) Affected: dff1399f8addf7129c49bb2227469da79cc30b47 , < eeb0711801f5e19ef654371b627682aed3b11373 (git) Affected: dff1399f8addf7129c49bb2227469da79cc30b47 , < 148c816f10fd11df27ca6a9b3238cdd42fa72cd3 (git) Affected: dff1399f8addf7129c49bb2227469da79cc30b47 , < 548ec0805c399c65ed66c6641be467f717833ab5 (git) |
guessed | |
| Linux | Linux |
Affected:
3.17
Unaffected: 0 , < 3.17 (semver) Unaffected: 4.4.296 , ≤ 4.4.* (semver) Unaffected: 4.9.294 , ≤ 4.9.* (semver) Unaffected: 4.14.259 , ≤ 4.14.* (semver) Unaffected: 4.19.222 , ≤ 4.19.* (semver) Unaffected: 5.4.168 , ≤ 5.4.* (semver) Unaffected: 5.10.85 , ≤ 5.10.* (semver) Unaffected: 5.15.8 , ≤ 5.15.* (semver) Unaffected: 5.16 , ≤ * (original_commit_for_fix) |
guessed |
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"orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
"shortName": "CISA-ADP"
},
"title": "CISA ADP Vulnrichment"
}
],
"cna": {
"affected": [
{
"defaultStatus": "unaffected",
"product": "Linux",
"programFiles": [
"fs/nfsd/nfs4state.c"
],
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"vendor": "Linux",
"versions": [
{
"lessThan": "04a8d07f3d58308b92630045560799a3faa3ebce",
"status": "affected",
"version": "dff1399f8addf7129c49bb2227469da79cc30b47",
"versionType": "git"
},
{
"lessThan": "348714018139c39533c55661a0c7c990671396b4",
"status": "affected",
"version": "dff1399f8addf7129c49bb2227469da79cc30b47",
"versionType": "git"
},
{
"lessThan": "33645d3e22720cac1e4548f8fef57bf0649536ee",
"status": "affected",
"version": "dff1399f8addf7129c49bb2227469da79cc30b47",
"versionType": "git"
},
{
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"status": "affected",
"version": "dff1399f8addf7129c49bb2227469da79cc30b47",
"versionType": "git"
},
{
"lessThan": "e0759696de6851d7536efddfdd2dfed4c4df1f09",
"status": "affected",
"version": "dff1399f8addf7129c49bb2227469da79cc30b47",
"versionType": "git"
},
{
"lessThan": "eeb0711801f5e19ef654371b627682aed3b11373",
"status": "affected",
"version": "dff1399f8addf7129c49bb2227469da79cc30b47",
"versionType": "git"
},
{
"lessThan": "148c816f10fd11df27ca6a9b3238cdd42fa72cd3",
"status": "affected",
"version": "dff1399f8addf7129c49bb2227469da79cc30b47",
"versionType": "git"
},
{
"lessThan": "548ec0805c399c65ed66c6641be467f717833ab5",
"status": "affected",
"version": "dff1399f8addf7129c49bb2227469da79cc30b47",
"versionType": "git"
}
]
},
{
"defaultStatus": "affected",
"product": "Linux",
"programFiles": [
"fs/nfsd/nfs4state.c"
],
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"vendor": "Linux",
"versions": [
{
"status": "affected",
"version": "3.17"
},
{
"lessThan": "3.17",
"status": "unaffected",
"version": "0",
"versionType": "semver"
},
{
"lessThanOrEqual": "4.4.*",
"status": "unaffected",
"version": "4.4.296",
"versionType": "semver"
},
{
"lessThanOrEqual": "4.9.*",
"status": "unaffected",
"version": "4.9.294",
"versionType": "semver"
},
{
"lessThanOrEqual": "4.14.*",
"status": "unaffected",
"version": "4.14.259",
"versionType": "semver"
},
{
"lessThanOrEqual": "4.19.*",
"status": "unaffected",
"version": "4.19.222",
"versionType": "semver"
},
{
"lessThanOrEqual": "5.4.*",
"status": "unaffected",
"version": "5.4.168",
"versionType": "semver"
},
{
"lessThanOrEqual": "5.10.*",
"status": "unaffected",
"version": "5.10.85",
"versionType": "semver"
},
{
"lessThanOrEqual": "5.15.*",
"status": "unaffected",
"version": "5.15.8",
"versionType": "semver"
},
{
"lessThanOrEqual": "*",
"status": "unaffected",
"version": "5.16",
"versionType": "original_commit_for_fix"
}
]
}
],
"cpeApplicability": [
{
"nodes": [
{
"cpeMatch": [
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "4.4.296",
"versionStartIncluding": "3.17",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "4.9.294",
"versionStartIncluding": "3.17",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "4.14.259",
"versionStartIncluding": "3.17",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "4.19.222",
"versionStartIncluding": "3.17",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "5.4.168",
"versionStartIncluding": "3.17",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "5.10.85",
"versionStartIncluding": "3.17",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "5.15.8",
"versionStartIncluding": "3.17",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "5.16",
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"vulnerable": true
}
],
"negate": false,
"operator": "OR"
}
]
}
],
"descriptions": [
{
"lang": "en",
"value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnfsd: fix use-after-free due to delegation race\n\nA delegation break could arrive as soon as we\u0027ve called vfs_setlease. A\ndelegation break runs a callback which immediately (in\nnfsd4_cb_recall_prepare) adds the delegation to del_recall_lru. If we\nthen exit nfs4_set_delegation without hashing the delegation, it will be\nfreed as soon as the callback is done with it, without ever being\nremoved from del_recall_lru.\n\nSymptoms show up later as use-after-free or list corruption warnings,\nusually in the laundromat thread.\n\nI suspect aba2072f4523 \"nfsd: grant read delegations to clients holding\nwrites\" made this bug easier to hit, but I looked as far back as v3.0\nand it looks to me it already had the same problem. So I\u0027m not sure\nwhere the bug was introduced; it may have been there from the beginning."
}
],
"metrics": [
{
"cvssV3_1": {
"baseScore": 9.8,
"baseSeverity": "CRITICAL",
"vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
"version": "3.1"
},
"scenarios": [
{
"lang": "en",
"value": "AV:N - The vulnerable path is in the in-kernel NFS server (nfs4_set_delegation / nfsd4_cb_recall_prepare in fs/nfsd/nfs4state.c), reached when a remote NFSv4 client issues OPEN compounds over TCP/UDP 2049 that attempt to grant a read delegation.\nAC:L - A remote attacker controls both sides of the race by opening/requesting a delegation on one NFS client/session while concurrently issuing a conflicting open (e.g. write) from another client or session slot on separate nfsd threads, so success does not depend on conditions outside attacker influence.\nPR:N - Typical nfsd deployments use AUTH_SYS/AUTH_NULL with IP-based exports and no cryptographic authentication, so any host allowed by the export can EXCHANGE_ID/CREATE_SESSION/OPEN and exercise the delegation-grant path by self-asserting credentials.\nUI:N - The attacker alone drives the OPEN/delegation grant and the conflicting open that breaks the lease; no victim user or administrator action on the server is required.\nS:U - The use-after-free corrupts kernel slab memory (nfs4_delegation on del_recall_lru) within the host nfsd security authority and does not cross a VM, IOMMU, or other separate boundary.\nC:H - The unhashed delegation is freed while still linked on del_recall_lru; later laundromat traversal is a classic slab UAF that can be groomed/reallocated to disclose arbitrary kernel memory.\nI:H - The same freed nfs4_delegation object remains on del_recall_lru so laundromat list operations write into attacker-reclaimable heap, yielding an arbitrary-write / control-flow hijack primitive.\nA:H - Use-after-free and list corruption of del_recall_lru reliably cause kernel oops/panic in the laundromat thread, fully disrupting availability of the NFS server host."
}
]
}
],
"providerMetadata": {
"dateUpdated": "2026-08-05T08:48:10.240Z",
"orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"shortName": "Linux"
},
"references": [
{
"url": "https://git.kernel.org/stable/c/04a8d07f3d58308b92630045560799a3faa3ebce"
},
{
"url": "https://git.kernel.org/stable/c/348714018139c39533c55661a0c7c990671396b4"
},
{
"url": "https://git.kernel.org/stable/c/33645d3e22720cac1e4548f8fef57bf0649536ee"
},
{
"url": "https://git.kernel.org/stable/c/2becaa990b93cbd2928292c0b669d3abb6cf06d4"
},
{
"url": "https://git.kernel.org/stable/c/e0759696de6851d7536efddfdd2dfed4c4df1f09"
},
{
"url": "https://git.kernel.org/stable/c/eeb0711801f5e19ef654371b627682aed3b11373"
},
{
"url": "https://git.kernel.org/stable/c/148c816f10fd11df27ca6a9b3238cdd42fa72cd3"
},
{
"url": "https://git.kernel.org/stable/c/548ec0805c399c65ed66c6641be467f717833ab5"
}
],
"title": "nfsd: fix use-after-free due to delegation race",
"x_generator": {
"engine": "bippy-1.2.0"
}
}
},
"cveMetadata": {
"assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"assignerShortName": "Linux",
"cveId": "CVE-2021-47506",
"datePublished": "2024-05-24T15:01:52.746Z",
"dateReserved": "2024-05-22T06:20:56.205Z",
"dateUpdated": "2026-08-05T08:48:10.240Z",
"state": "PUBLISHED"
},
"dataType": "CVE_RECORD",
"dataVersion": "5.2"
}
}
}
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Experimental. This forecast is provided for visualization only and may change without notice. Do not use it for operational decisions.
Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the
vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base
model, served locally by ML-Gateway.
They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
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Related by attack behaviour
Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.
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