# CVE-2021-47129: netfilter: nft_ct: skip expectations for confirmed conntrack

> A logic error in the Linux kernel netfilter module causes a kernel warning and potential crash when processing confirmed conntrack entries.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Mar 2024
- **Updated:** 5 Aug 2026
- **Also known as:** DEBIAN-CVE-2021-47129, GHSA-w7x4-qp38-gw56, UBUNTU-CVE-2021-47129

## What is CVE-2021-47129?

CVE-2021-47129 is a high-severity vulnerability in Linux kernel, published on 15 Mar 2024. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

netfilter: nft_ct: skip expectations for confirmed conntrack

nft_ct_expect_obj_eval() calls nf_ct_ext_add() for a confirmed
conntrack entry. However, nf_ct_ext_add() can only be called for
!nf_ct_is_confirmed().

[ 1825.349056] WARNING: CPU: 0 PID: 1279 at net/netfilter/nf_conntrack_extend.c:48 nf_ct_xt_add+0x18e/0x1a0 [nf_conntrack]
[ 1825.351391] RIP: 0010:nf_ct_ext_add+0x18e/0x1a0 [nf_conntrack]
[ 1825.351493] Code: 41 5c 41 5d 41 5e 41 5f c3 41 bc 0a 00 00 00 e9 15 ff ff ff ba 09 00 00 00 31 f6 4c 89 ff e8 69 6c 3d e9 eb 96 45 31 ed eb cd <0f> 0b e9 b1 fe ff ff e8 86 79 14 e9 eb bf 0f 1f 40 00 0f 1f 44 00
[ 1825.351721] RSP: 0018:ffffc90002e1f1e8 EFLAGS: 00010202
[ 1825.351790] RAX: 000000000000000e RBX: ffff88814f5783c0 RCX: ffffffffc0e4f887
[ 1825.351881] RDX: dffffc0000000000 RSI: 0000000000000008 RDI: ffff88814f578440
[ 1825.351971] RBP: 0000000000000000 R08: 0000000000000000 R09: ffff88814f578447
[ 1825.352060] R10: ffffed1029eaf088 R11: 0000000000000001 R12: ffff88814f578440
[ 1825.352150] R13: ffff8882053f3a00 R14: 0000000000000000 R15: 0000000000000a20
[ 1825.352240] FS:  00007f992261c900(0000) GS:ffff889faec00000(0000) knlGS:0000000000000000
[ 1825.352343] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1825.352417] CR2: 000056070a4d1158 CR3: 000000015efe0000 CR4: 0000000000350ee0
[ 1825.352508] Call Trace:
[ 1825.352544]  nf_ct_helper_ext_add+0x10/0x60 [nf_conntrack]
[ 1825.352641]  nft_ct_expect_obj_eval+0x1b8/0x1e0 [nft_ct]
[ 1825.352716]  nft_do_chain+0x232/0x850 [nf_tables]

Add the ct helper extension only for unconfirmed conntrack. Skip rule
evaluation if the ct helper extension does not exist. Thus, you can
only create expectations from the first packet.

It should be possible to remove this limitation by adding a new action
to attach a generic ct helper to the first packet. Then, use this ct
helper extension from follow up packets to create the ct expectation.

While at it, add a missing check to skip the template conntrack too
and remove check for IPCT_UNTRACK which is implicit to !ct.

Weakness: CWE-273 (Improper Check for Dropped Privileges)

## Which versions are affected by CVE-2021-47129?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2021-47129?

Apply the vendor's patch.

## Is CVE-2021-47129 being exploited?

There is no known exploitation of CVE-2021-47129 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-47129 and fix it if it is affected.

What it is: CVE-2021-47129 is a high-severity vulnerability in Linux kernel, published on 15 Mar 2024. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-47129 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-47129/ (Markdown: https://insights.sbomify.com/CVE-2021-47129/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-47129
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-47129
- OSV: https://osv.dev/vulnerability/CVE-2021-47129

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-47129/
Summaries are generated and may contain mistakes; the official description above is the record.