---
id: CVE-2025-68206
title: 'netfilter: nft_ct: add seqadj extension for natted connections'
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  netfilter: nft_ct: add seqadj extension for natted connections

  Sequence adjustment may be required for FTP traffic with PASV/EPSV modes.
  due to need to re-write packet…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'
cvssSource: cna
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 1a64edf54f55d7956cf5a0d95898bc1f84f9b818 <
    83273af0b60c093ba0085c205864d8542e1b1653
  - >-
    Linux >= 1a64edf54f55d7956cf5a0d95898bc1f84f9b818 <
    b19492c25eff04852e0cb58f9bb8238b6695ed2d
  - >-
    Linux >= 1a64edf54f55d7956cf5a0d95898bc1f84f9b818 <
    4de80f0dc3868408dd7fe9817e507123c9dd8bb0
  - >-
    Linux >= 1a64edf54f55d7956cf5a0d95898bc1f84f9b818 <
    b477ef7fa612fa45b6b3134d90d1eeb09396500a
  - >-
    Linux >= 1a64edf54f55d7956cf5a0d95898bc1f84f9b818 <
    4ab2cd906e4e1a19ddbda6eb532851b0e9cda110
  - >-
    Linux >= 1a64edf54f55d7956cf5a0d95898bc1f84f9b818 <
    2b52d89cbbb0dbe3e948d8d9a91e704316dccfe6
  - >-
    Linux >= 1a64edf54f55d7956cf5a0d95898bc1f84f9b818 <
    90918e3b6404c2a37837b8f11692471b4c512de2
  - Linux 4.12
published: '2025-12-16'
updated: '2026-09-08'
sourceUpdated: '2026-09-08T08:43:04.122Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2025-68206'
references:
  - url: 'https://git.kernel.org/stable/c/83273af0b60c093ba0085c205864d8542e1b1653'
  - url: 'https://git.kernel.org/stable/c/b19492c25eff04852e0cb58f9bb8238b6695ed2d'
  - url: 'https://git.kernel.org/stable/c/4de80f0dc3868408dd7fe9817e507123c9dd8bb0'
  - url: 'https://git.kernel.org/stable/c/b477ef7fa612fa45b6b3134d90d1eeb09396500a'
  - url: 'https://git.kernel.org/stable/c/4ab2cd906e4e1a19ddbda6eb532851b0e9cda110'
  - url: 'https://git.kernel.org/stable/c/2b52d89cbbb0dbe3e948d8d9a91e704316dccfe6'
  - url: 'https://git.kernel.org/stable/c/90918e3b6404c2a37837b8f11692471b4c512de2'
tags:
  - cve.org
epss: 0.00693
epssPercentile: 0.5079
ingestedAt: '2026-09-08T15:33:26.996Z'
---

## Overview

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

netfilter: nft_ct: add seqadj extension for natted connections

Sequence adjustment may be required for FTP traffic with PASV/EPSV modes.
due to need to re-write packet payload (IP, port) on the ftp control
connection. This can require changes to the TCP length and expected
seq / ack_seq.

The easiest way to reproduce this issue is with PASV mode.
Example ruleset:
table inet ftp_nat {
        ct helper ftp_helper {
                type "ftp" protocol tcp
                l3proto inet
        }

        chain prerouting {
                type filter hook prerouting priority 0; policy accept;
                tcp dport 21 ct state new ct helper set "ftp_helper"
        }
}
table ip nat {
        chain prerouting {
                type nat hook prerouting priority -100; policy accept;
                tcp dport 21 dnat ip prefix to ip daddr map {
			192.168.100.1 : 192.168.13.2/32 }
        }

        chain postrouting {
                type nat hook postrouting priority 100 ; policy accept;
                tcp sport 21 snat ip prefix to ip saddr map {
			192.168.13.2 : 192.168.100.1/32 }
        }
}

Note that the ftp helper gets assigned *after* the dnat setup.

The inverse (nat after helper assign) is handled by an existing
check in nf_nat_setup_info() and will not show the problem.

Topoloy:

 +-------------------+     +----------------------------------+
 | FTP: 192.168.13.2 | <-> | NAT: 192.168.13.3, 192.168.100.1 |
 +-------------------+     +----------------------------------+
                                      |
                         +-----------------------+
                         | Client: 192.168.100.2 |
                         +-----------------------+

ftp nat changes do not work as expected in this case:
Connected to 192.168.100.1.
[..]
ftp> epsv
EPSV/EPRT on IPv4 off.
ftp> ls
227 Entering passive mode (192,168,100,1,209,129).
421 Service not available, remote server has closed connection.

Kernel logs:
Missing nfct_seqadj_ext_add() setup call
WARNING: CPU: 1 PID: 0 at net/netfilter/nf_conntrack_seqadj.c:41
[..]
 __nf_nat_mangle_tcp_packet+0x100/0x160 [nf_nat]
 nf_nat_ftp+0x142/0x280 [nf_nat_ftp]
 help+0x4d1/0x880 [nf_conntrack_ftp]
 nf_confirm+0x122/0x2e0 [nf_conntrack]
 nf_hook_slow+0x3c/0xb0
 ..

Fix this by adding the required extension when a conntrack helper is assigned
to a connection that has a nat binding.

## Affected

- `Linux >= 1a64edf54f55d7956cf5a0d95898bc1f84f9b818 < 83273af0b60c093ba0085c205864d8542e1b1653`
- `Linux >= 1a64edf54f55d7956cf5a0d95898bc1f84f9b818 < b19492c25eff04852e0cb58f9bb8238b6695ed2d`
- `Linux >= 1a64edf54f55d7956cf5a0d95898bc1f84f9b818 < 4de80f0dc3868408dd7fe9817e507123c9dd8bb0`
- `Linux >= 1a64edf54f55d7956cf5a0d95898bc1f84f9b818 < b477ef7fa612fa45b6b3134d90d1eeb09396500a`
- `Linux >= 1a64edf54f55d7956cf5a0d95898bc1f84f9b818 < 4ab2cd906e4e1a19ddbda6eb532851b0e9cda110`
- `Linux >= 1a64edf54f55d7956cf5a0d95898bc1f84f9b818 < 2b52d89cbbb0dbe3e948d8d9a91e704316dccfe6`
- `Linux >= 1a64edf54f55d7956cf5a0d95898bc1f84f9b818 < 90918e3b6404c2a37837b8f11692471b4c512de2`
- `Linux 4.12`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.