high CVSS 7.8 EPSS <0.1%

CVE-2021-47394·Kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved: netfilter: nf_tables: unlink table before deleting it syzbot reports following UAF: BUG: KASAN: use-after-free in memcmp+0x18f/0x1c0 lib/string.c:955 nla_strcmp+0xf2/0x130 lib/nlattr.c:836 nft_table_lookup.part.0+0x1a2/0x460 net/netfilter/nf_tables_api.c:570 nft_table_lookup net/netfilter/nf_tables_api.c:4064 [inline] nf_tables_getset+0x1b3/0x860 net/netfilter/nf_tables_api.c:4064 nfnetlink_rcv_msg+0x659/0x13f0 net/netfilter/nfnetlink.c:285 netlink_rcv_skb+0x153/0x420 net/netlink/af_netlink.c:2504 Problem is that all get operations are lockless, so the commit_mutex held by nft_rcv_nl_event() isn't enough to stop a parallel GET request from doing read-accesses to the table object even after synchronize_rcu(). To avoid this, unlink the table first and store the table objects in on-stack scratch space.

Severity
high
Software
Kernel
Fixed in
5.14.10
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at <0.1% — higher than 3% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Read the two together. CVE-2021-47394 scores 7.8 on CVSS, so a successful attack would be serious — but at <0.1%, almost nobody is currently attempting it. That is a different situation from a flaw under mass exploitation, and it is usually the difference between patching on your next maintenance window and patching tonight.

Affected versions

From: 5.12

Until: 5.14.10

Fixed in: 5.14.10

How to fix this CVE

Update your Linux kernel to version 5.14.10 or later to resolve this use-after-free vulnerability in the netfilter nf_tables subsystem. The vulnerability allows local authenticated attackers to trigger a memory access violation through concurrent table lookup operations. Ensure you reboot the system after applying the kernel update to activate the patched version.

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 netfilter nf_tables is loaded: lsmod | grep nf_tables
  3. Review kernel logs for UAF (use-after-free) KASAN errors: sudo dmesg | grep -i 'kasan\|use-after-free'
  4. Confirm the patched version is installed after update: uname -r | grep -E '5\.14\.(1[0-9]|[2-9])|5\.(1[5-9]|[2-9][0-9])'
  5. Verify nftables rules are still functional: sudo nft list tables

FAQ

What is CVE-2021-47394?

This is a use-after-free vulnerability in the Linux kernel's netfilter nf_tables subsystem that occurs when table deletion and concurrent GET operations race without proper synchronization, allowing local users to read freed memory.

Is CVE-2021-47394 being actively exploited?

No, according to CISA KEV data, CVE-2021-47394 is not listed as actively exploited in the wild. However, it remains a high-severity local privilege escalation vector that should be patched promptly.

What versions of Kernel are affected by CVE-2021-47394?

Linux kernel versions 5.12 through 5.14.9 are vulnerable. Versions 5.14.10 and later include the fix.

How do I check if my server is vulnerable to CVE-2021-47394?

Run 'uname -r' and check if the version is between 5.12 and 5.14.9. If yes, your system is vulnerable. Additionally, confirm nf_tables is in use with 'sudo nft list tables'.

Does Defensia detect CVE-2021-47394?

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-2021-47394 will appear in your dashboard with remediation steps.

Related Kernel CVEs

CVE-2026-63795CVSS 10In the Linux kernel, the following vulnerability has been resolved: 9p: avoid putting oldfid in p9_client_walk() error path When p9_client_walk() is called with clone set to false, fid aliases oldfid. If the walk subsequently fails after the request has been sent, the error path jumps to clunk_fid, which currently calls p9_fid_put(fid) unconditionally. This drops a reference to oldfid even though ownership of oldfid remains with the caller. If this is the last reference, oldfid can be clunked and destroyed while the caller still expects it to be valid. A later use or put of oldfid can then trigger a use-after-free or refcount underflow. Fix this by only putting fid in the clunk_fid error path when it does not alias oldfid, matching the existing guard in the error path below. This can be triggered when a multi-component walk is split into multiple p9_client_walk() calls and a later non-cloning walk fails. A reproducer and refcount warning logs are available on request.
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CVE-2026-53151CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: rxrpc: Fix the ACK parser to extract the SACK table for parsing Fix modification of the received skbuff in rxrpc_input_soft_acks() and a potential incorrect access of the buffer in a fragmented UDP packet (the packet would probably have to be deliberately pre-generated as fragmented) when AF_RXRPC tries to extract the contents of the SACK table by copying out the contents of the SACK table into a buffer before attempting to parse AF_RXRPC assumes that it can just call skb_condense() and then validly access the SACK table from skb->data and that it will be a flat buffer - but skb_condense() can silently fail to do anything under some circumstances. Note that whilst rxrpc_input_soft_acks() should be able to parse extended ACKs, the rest of AF_RXRPC doesn't currently support that. Further, there's then no need to call skb_condense() in rxrpc_input_ack(), so don't.
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References

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