high CVSS 7.8

CVE-2021-47107·Kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved: NFSD: Fix READDIR buffer overflow If a client sends a READDIR count argument that is too small (say, zero), then the buffer size calculation in the new init_dirlist helper functions results in an underflow, allowing the XDR stream functions to write beyond the actual buffer. This calculation has always been suspect. NFSD has never sanity- checked the READDIR count argument, but the old entry encoders managed the problem correctly. With the commits below, entry encoding changed, exposing the underflow to the pointer arithmetic in xdr_reserve_space(). Modern NFS clients attempt to retrieve as much data as possible for each READDIR request. Also, we have no unit tests that exercise the behavior of READDIR at the lower bound of @count values. Thus this case was missed during testing.

Severity
high
Software
Kernel
Fixed in
5.15.12
Published
2024-03-04

Affected versions

From: 5.13

Until: 5.15.12

Fixed in: 5.15.12

How to fix this CVE

Update your Linux kernel to version 5.15.12 or later to patch a buffer overflow vulnerability in the NFSD READDIR implementation. This vulnerability occurs when clients send unusually small READDIR count values, causing memory corruption in the XDR stream encoding. Apply the patch immediately if you are running kernel versions 5.13 through 5.15.11 and expose NFS services.

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
  2. Verify if NFSD is active: systemctl status nfs-server or ps aux | grep nfsd
  3. Check kernel logs for memory corruption or XDR-related warnings: dmesg | grep -i 'xdr\|nfsd\|buffer\|overflow'
  4. After patching, reboot and confirm the new kernel: uname -r (should show 5.15.12 or higher)
  5. Verify NFSD functionality post-patch: showmount -e localhost

FAQ

What is CVE-2021-47107?

CVE-2021-47107 is a buffer overflow vulnerability in the Linux kernel's NFSD (Network File System Daemon) READDIR handler that occurs when clients send abnormally small count values, causing the XDR encoder to write beyond allocated buffer boundaries.

Is CVE-2021-47107 being actively exploited?

No, this vulnerability is not listed on the CISA Known Exploited Vulnerabilities (KEV) catalog and there are no public exploits available. However, it remains a high-severity issue due to its potential for local privilege escalation or denial of service.

What versions of Kernel are affected by CVE-2021-47107?

Linux kernel versions 5.13 through 5.15.11 are vulnerable. The vulnerability was resolved in version 5.15.12 and later.

How do I check if my server is vulnerable to CVE-2021-47107?

Run 'uname -r' to check your kernel version. If it shows any version between 5.13 and 5.15.11 and NFSD is enabled (check with 'systemctl status nfs-server'), your system is vulnerable.

Does Defensia detect CVE-2021-47107?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If the Linux kernel is installed on a monitored server, CVE-2021-47107 will appear in your dashboard with remediation steps.

Related Kernel CVEs

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CVE-2026-100075CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: RDMA/srpt: Fix srpt_alloc_rw_ctxs() unwind counters When srpt_alloc_rw_ctxs() fails partway through a multi-buffer indirect descriptor, the unwind path destroys RDMA contexts but leaves stale n_rw_ctx and n_rdma values (and a dangling rw_ctxs pointer). Later sq_wr_avail accounting in srpt_queue_response() or srpt_write_pending() can then subtract the wrong number of send queue credits. Reset the counters and clear rw_ctxs after freeing the heap allocation before returning an error.
CVE-2026-64033CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: RDMA/rtrs: Fix use-after-free in path file creation cleanup In the error path of rtrs_srv_create_path_files(), the sysfs root folders may already have been created and srv_path->kobj may already have been initialized. If a later step fails, the cleanup currently calls kobject_put(&srv_path->kobj) before rtrs_srv_destroy_once_sysfs_root_folders(srv_path). kobject_put() may drop the last reference to srv_path->kobj and invoke the release callback, rtrs_srv_release(), which frees srv_path. The following call to rtrs_srv_destroy_once_sysfs_root_folders(srv_path) then dereferences srv_path internally to access srv_path->srv, resulting in a use-after-free. This failure path is reached before rtrs_srv_create_path_files() returns success, so the successful-path lifetime handling is not involved. Fix this by destroying the sysfs root folders before calling kobject_put(&srv_path->kobj), so srv_path is still valid while the helper accesses it. This issue was found by a static analysis tool I am developing.
CVE-2026-64007CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: netfilter: synproxy: refresh tcphdr after skb_ensure_writable synproxy_tstamp_adjust() rewrites the TCP timestamp option in place and then patches the TCP checksum via inet_proto_csum_replace4() on the caller-supplied tcphdr pointer. Both ipv4_synproxy_hook() and ipv6_synproxy_hook() obtain that pointer with skb_header_pointer() before calling in, so it may either alias skb->head directly or point at the caller's on-stack _tcph buffer. Between obtaining the pointer and using it, the function calls skb_ensure_writable(skb, optend), which on a cloned or non-linear skb invokes pskb_expand_head() and frees the old skb->head. After that point the cached th is stale: caller (ipv[46]_synproxy_hook) th = skb_header_pointer(skb, ..., &_tcph) synproxy_tstamp_adjust(skb, protoff, th, ...) skb_ensure_writable(skb, optend) pskb_expand_head() /* kfree(old skb->head) */ ... inet_proto_csum_replace4(&th->check, ...) /* writes into freed head, or into the caller's stack copy leaving the on-wire checksum stale */ The option bytes are written through skb->data and are fine; only the checksum update goes through th and so lands in the wrong place. The result is either a write into freed slab memory or a packet leaving with a checksum that does not match its payload. Fix by re-deriving th from skb->data + protoff immediately after skb_ensure_writable() succeeds, so the subsequent checksum update targets the linear, writable header.
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References

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