linux-cve-announce - CVE-2025-68206: netfilter: nft_ct: add seqadj extension for natted connections

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Message-ID: <2025121632-CVE-2025-68206-47ba@gregkh>
Date: Tue, 16 Dec 2025 14:54:35 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-68206: netfilter: nft_ct: add seqadj extension for natted connections

From: Greg Kroah-Hartman <gregkh@...nel.org>

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.

The Linux kernel CVE team has assigned CVE-2025-68206 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 4.12 with commit 1a64edf54f55d7956cf5a0d95898bc1f84f9b818 and fixed in 6.17.9 with commit 2b52d89cbbb0dbe3e948d8d9a91e704316dccfe6
	Issue introduced in 4.12 with commit 1a64edf54f55d7956cf5a0d95898bc1f84f9b818 and fixed in 6.18 with commit 90918e3b6404c2a37837b8f11692471b4c512de2

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2025-68206
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	net/netfilter/nft_ct.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/2b52d89cbbb0dbe3e948d8d9a91e704316dccfe6
	https://git.kernel.org/stable/c/90918e3b6404c2a37837b8f11692471b4c512de2