high CVSS 7.8

CVE-2021-47506·Kernel vulnerability

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
high
Software
Kernel
Fixed in
5.15.8
Published
2024-05-24

Affected versions

From: 5.11

Until: 5.15.8

Fixed in: 5.15.8

How to fix this CVE

Update your Linux kernel to version 5.15.8 or later to resolve a use-after-free vulnerability in the NFS delegation handling code. This bug occurs when a delegation break races with the delegation setup process, causing memory corruption in the kernel's NFS server subsystem. Kernel versions between 5.11 and 5.15.7 are vulnerable and should be patched immediately.

sudo dnf update kernel kernel-devel

Defensia detects this vulnerability

How to check if you are affected

  1. Check your current kernel version: uname -r — if it returns 5.11.x through 5.15.7, your system is vulnerable
  2. Verify NFS server is running: systemctl status nfs-server — this CVE only affects systems with nfsd enabled
  3. Search kernel logs for delegation-related crashes: grep -i 'use-after-free\|list_corruption\|nfs4_set_delegation' /var/log/kern.log or journalctl -b | grep -i 'use-after-free'
  4. After patching, reboot and confirm: uname -r should show 5.15.8 or later, and dmesg should not contain delegation-related warnings

FAQ

What is CVE-2021-47506?

This vulnerability is a use-after-free condition in the Linux kernel's NFS server delegation code where a delegation break callback can free memory while it remains referenced on an internal list, leading to heap corruption and system instability.

Is CVE-2021-47506 being actively exploited?

No, this vulnerability is not listed on the CISA Known Exploited Vulnerabilities catalog and no public exploits are available. However, it causes memory corruption that manifests as kernel crashes and data corruption on affected systems.

What versions of Kernel are affected by CVE-2021-47506?

Linux kernel versions 5.11 through 5.15.7 are affected. The vulnerability was fixed in kernel 5.15.8 and later stable releases.

How do I check if my server is vulnerable to CVE-2021-47506?

Run `uname -r` and check if the output is between 5.11 and 5.15.7; additionally verify NFS server is active with `systemctl is-active nfs-server`

Does Defensia detect CVE-2021-47506?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Kernel is installed on a monitored server, CVE-2021-47506 will appear in your dashboard with remediation steps.

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CVE-2026-43501CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: ipv6: rpl: reserve mac_len headroom when recompressed SRH grows ipv6_rpl_srh_rcv() decompresses an RFC 6554 Source Routing Header, swaps the next segment into ipv6_hdr->daddr, recompresses, then pulls the old header and pushes the new one plus the IPv6 header back. The recompressed header can be larger than the received one when the swap reduces the common-prefix length the segments share with daddr (CmprI=0, CmprE>0, seg[0][0] != daddr[0] gives the maximum +8 bytes). pskb_expand_head() was gated on segments_left == 0, so on earlier segments the push consumed unchecked headroom. Once skb_push() leaves fewer than skb->mac_len bytes in front of data, skb_mac_header_rebuild()'s call to: skb_set_mac_header(skb, -skb->mac_len); will store (data - head) - mac_len into the u16 mac_header field, which wraps to ~65530, and the following memmove() writes mac_len bytes ~64KiB past skb->head. A single AF_INET6/SOCK_RAW/IPV6_HDRINCL packet over lo with a two segment type-3 SRH (CmprI=0, CmprE=15) reaches headroom 8 after one pass; KASAN reports a 14-byte OOB write in ipv6_rthdr_rcv. Fix this by expanding the head whenever the remaining room is less than the push size plus mac_len, and request that much extra so the rebuilt MAC header fits afterwards.
CVE-2026-64216CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: netfs: Fix potential UAF in netfs_unlock_abandoned_read_pages() netfs_unlock_abandoned_read_pages(rreq) accesses the index of the folios it is wanting to unlock and compares that to rreq->no_unlock_folio so that it doesn't unlock a folio being read for netfs_perform_write() or netfs_write_begin(). However, given that netfs_unlock_abandoned_read_pages() is called _after_ NETFS_RREQ_IN_PROGRESS is cleared, the one folio that it's not allowed to dereference is the one specified by ->no_unlock_folio as ownership immediately reverts to the caller. Fix this by storing the folio pointer instead and using that rather than the index. Also fix netfs_unlock_read_folio() where the same applies.

References

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