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.

A low severity flaw was found in the Linux kernel's netfilter nft_ct handling of NATed connections. When a conntrack helper (such as the FTP connection tracking helper) is assigned to a connection that already has a NAT binding, the sequence adjustment (seqadj) extension is not set up. For FTP traffic in PASV/EPSV mode, the helper must rewrite the IP address and port inside the FTP control connection payload, which changes the TCP length and the expected sequence and acknowledgment numbers; without the seqadj extension this triggers a kernel WARNING in nf_conntrack_seqadj and the FTP control connection fails and is closed by the remote server, resulting in a denial of service for the affected connections. Red Hat rates this issue as Low severity (CVSS 3.1 score 5.5, AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H), lower than the CVE.org score of 7.5, because triggering the problem requires a specific nftables configuration that combines the conntrack FTP helper with NAT such that the helper is assigned after the NAT binding, and the consequence is limited to failure of those FTP sessions rather than a broader system compromise. The kernel and kernel-rt packages in Red Hat Enterprise Linux 8, 9, and 10 and the kernel package in Red Hat In-Vehicle OS 1.1 are affected, with the fix deferred on those streams due to the low severity and the narrow, configuration-specific availability impact; the residual risk is limited to FTP connections that traverse NAT with the conntrack FTP helper. Red Hat Enterprise Linux 6 and 7 Extended Lifecycle Support are not affected because the vulnerable code is not present in those versions.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.