Linux Kernel CVE-2025-68206

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 affected product is the Linux kernel netfilter/nftables subsystem across multiple stable branches; exact affected version ranges are defined by the fixing commits on git.kernel.org (2b52d89cbbb0, 4ab2cd906e4e, 4de80f0dc386, 90918e3b6404, b477ef7fa612, 83273af0b60c, b19492c25eff). Distribution-level exposure is confirmed for Ubuntu via USN-8179-1 (https://ubuntu.com/security/notices/USN-8179-1), and Siemens has published product-specific advisories at https://cert-portal.siemens.com/productcert/html/ssa-019113.html and https://cert-portal.siemens.com/productcert/html/ssa-082556.html for embedded/industrial products shipping affected kernels. No standalone CPE strings were provided in the input, so precise version boundaries should be taken from the referenced stable commits and each distributor's advisory rather than inferred.