Linux Kernel up to 6.18.5/6.19-rc4 frags nf_conntrack_cleanup_net_list state issue

Summaryinfo

A vulnerability was found in Linux Kernel up to 6.18.5/6.19-rc4. It has been declared as critical. This impacts the function nf_conntrack_cleanup_net_list of the component frags. Executing a manipulation can lead to state issue. The identification of this vulnerability is CVE-2026-23016. There is no exploit available. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability classified as critical was found in Linux Kernel up to 6.18.5/6.19-rc4. This vulnerability affects the function nf_conntrack_cleanup_net_list of the component frags. The manipulation with an unknown input leads to a state issue vulnerability. The CWE definition for the vulnerability is CWE-371. The impact remains unknown. CVE summarizes:

In the Linux kernel, the following vulnerability has been resolved: inet: frags: drop fraglist conntrack references Jakub added a warning in nf_conntrack_cleanup_net_list() to make debugging leaked skbs/conntrack references more obvious. syzbot reports this as triggering, and I can also reproduce this via ip_defrag.sh selftest: conntrack cleanup blocked for 60s WARNING: net/netfilter/nf_conntrack_core.c:2512 [..] conntrack clenups gets stuck because there are skbs with still hold nf_conn references via their frag_list. net.core.skb_defer_max=0 makes the hang disappear. Eric Dumazet points out that skb_release_head_state() doesn't follow the fraglist. ip_defrag.sh can only reproduce this problem since commit 6471658dc66c ("udp: use skb_attempt_defer_free()"), but AFAICS this problem could happen with TCP as well if pmtu discovery is off. The relevant problem path for udp is: 1. netns emits fragmented packets 2. nf_defrag_v6_hook reassembles them (in output hook) 3. reassembled skb is tracked (skb owns nf_conn reference) 4. ip6_output refragments 5. refragmented packets also own nf_conn reference (ip6_fragment calls ip6_copy_metadata()) 6. on input path, nf_defrag_v6_hook skips defragmentation: the fragments already have skb->nf_conn attached 7. skbs are reassembled via ipv6_frag_rcv() 8. skb_consume_udp -> skb_attempt_defer_free() -> skb ends up in pcpu freelist, but still has nf_conn reference. Possible solutions: 1 let defrag engine drop nf_conn entry, OR 2 export kick_defer_list_purge() and call it from the conntrack netns exit callback, OR 3 add skb_has_frag_list() check to skb_attempt_defer_free() 2 & 3 also solve ip_defrag.sh hang but share same drawback: Such reassembled skbs, queued to socket, can prevent conntrack module removal until userspace has consumed the packet. While both tcp and udp stack do call nf_reset_ct() before placing skb on socket queue, that function doesn't iterate frag_list skbs. Therefore drop nf_conn entries when they are placed in defrag queue. Keep the nf_conn entry of the first (offset 0) skb so that reassembled skb retains nf_conn entry for sake of TX path. Note that fixes tag is incorrect; it points to the commit introducing the 'ip_defrag.sh reproducible problem': no need to backport this patch to every stable kernel.

The advisory is available at git.kernel.org. This vulnerability was named CVE-2026-23016 since 01/13/2026. Technical details are known, but there is no available exploit. The structure of the vulnerability defines a possible price range of USD $0-$5k at the moment (estimation calculated on 03/26/2026).

The vulnerability scanner Nessus provides a plugin with the ID 297488 (Linux Distros Unpatched Vulnerability : CVE-2026-23016), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 6.18.6 or 6.19-rc5 eliminates this vulnerability. Applying the patch 088ca99dbb039c444c3ff987c5412a73f4f0cbf8/2ef02ac38d3c17f34a00c4b267d961a8d4b45d1a is able to eliminate this problem. The bugfix is ready for download at git.kernel.org. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the databases at Tenable (297488), EUVD (EUVD-2026-5078) and CERT Bund (WID-SEC-2026-0280). If you want to get best quality of vulnerability data, you may have to visit VulDB.

Affected

• Open Source Linux Kernel

Productinfo

Type

• Operating System

Vendor

• Linux

Name

• Kernel

Version

• 6.18.0
• 6.18.1
• 6.18.2
• 6.18.3
• 6.18.4
• 6.18.5
• 6.19-rc1
• 6.19-rc2
• 6.19-rc3
• 6.19-rc4

License

• open-source

Website

• Vendor: https://www.kernel.org/

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Entryinfo

If you want to get best quality of vulnerability data, you may have to visit VulDB.

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