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vuln-cve/data/2021/47xxx/CVE-2021-47506.json
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{
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"cveId": "CVE-2021-47506",
"assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"state": "PUBLISHED",
"assignerShortName": "Linux",
"dateReserved": "2024-05-22T06:20:56.205Z",
"datePublished": "2024-05-24T15:01:52.746Z",
"dateUpdated": "2026-08-05T08:48:10.240Z"
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"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've 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'm not sure\nwhere the bug was introduced; it may have been there from the beginning."
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"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."
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