# CVE-2025-68206

## Summary

- **CVE ID:** CVE-2025-68206
- **Severity:** HIGH
- **CVSS Score:** 7.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H)
- **CWE:** N/A
- **Published:** Dec 16, 2025
- **Last Modified:** Sep 8, 2026

## Description

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 Products

- Linux — Linux (1a64edf54f55d7956cf5a0d95898bc1f84f9b818)
- Linux — Linux (4.12)
- Linux — Linux (0)
- Linux — Linux (6.12.64)
- Linux — Linux (6.17.9)
- Linux — Linux (6.18)
- Linux — Linux (6.1.167)
- Linux — Linux (6.6.130)
- Linux — Linux (5.10.253)
- Linux — Linux (5.15.203)

## References

- [CNA](https://git.kernel.org/stable/c/4ab2cd906e4e1a19ddbda6eb532851b0e9cda110)
- [CNA](https://git.kernel.org/stable/c/2b52d89cbbb0dbe3e948d8d9a91e704316dccfe6)
- [CNA](https://git.kernel.org/stable/c/90918e3b6404c2a37837b8f11692471b4c512de2)
- [CNA](https://git.kernel.org/stable/c/4de80f0dc3868408dd7fe9817e507123c9dd8bb0)
- [CNA](https://git.kernel.org/stable/c/b477ef7fa612fa45b6b3134d90d1eeb09396500a)
- [CNA](https://git.kernel.org/stable/c/83273af0b60c093ba0085c205864d8542e1b1653)
- [CNA](https://git.kernel.org/stable/c/b19492c25eff04852e0cb58f9bb8238b6695ed2d)
- [siemens-SADP](https://cert-portal.siemens.com/productcert/html/ssa-082556.html)
- [siemens-SADP](https://cert-portal.siemens.com/productcert/html/ssa-019113.html)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.69%
- **EPSS Percentile:** 51.2

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-18._