GHSA-H337-654Q-329C
Vulnerability from github – Published: 2026-07-19 18:31 – Updated: 2026-07-20 15:31In the Linux kernel, the following vulnerability has been resolved:
tcp: fix stale per-CPU tcp_tw_isn leak enabling ISN prediction
Blamed commit moved the TIME_WAIT-derived ISN from the skb control block to a per-CPU variable, assuming the value would always be consumed by tcp_conn_request() for the same packet that wrote it. That assumption is violated by multiple drop paths between the producer (__this_cpu_write(tcp_tw_isn, isn) in tcp_v{4,6}_rcv()) and the consumer (tcp_conn_request()):
- min_ttl / min_hopcount check
- xfrm policy check
- tcp_inbound_hash() MD5/AO mismatch
- tcp_filter() eBPF/SO_ATTACH_FILTER drop
- th->syn && th->fin discard in tcp_rcv_state_process() TCP_LISTEN
- psp_sk_rx_policy_check() in tcp_v{4,6}_do_rcv()
- tcp_checksum_complete() in tcp_v{4,6}_do_rcv()
- tcp_v{4,6}_cookie_check() returning NULL
When a packet is dropped on any of these paths, tcp_tw_isn is left set.
The next SYN processed on the same CPU then consumes the non zero value in tcp_conn_request(), receiving a potentially predictable ISN.
This patch moves back tcp_tw_isn to skb->cb[], getting rid of the per-cpu variable.
Note that tcp_v{4,6}_fill_cb() do not set it.
Very litle impact on overall code size/complexity:
$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new add/remove: 0/0 grow/shrink: 2/1 up/down: 8/-15 (-7) Function old new delta tcp_v6_rcv 3038 3042 +4 tcp_v4_rcv 3035 3039 +4 tcp_conn_request 2938 2923 -15 Total: Before=24436060, After=24436053, chg -0.00%
{
"affected": [],
"aliases": [
"CVE-2026-64024"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-07-19T16:17:42Z",
"severity": "CRITICAL"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\ntcp: fix stale per-CPU tcp_tw_isn leak enabling ISN prediction\n\nBlamed commit moved the TIME_WAIT-derived ISN from the skb control\nblock to a per-CPU variable, assuming the value would always be consumed\nby tcp_conn_request() for the same packet that wrote it. That assumption\nis violated by multiple drop paths between the producer\n(__this_cpu_write(tcp_tw_isn, isn) in tcp_v{4,6}_rcv()) and the consumer\n(tcp_conn_request()):\n\n - min_ttl / min_hopcount check\n - xfrm policy check\n - tcp_inbound_hash() MD5/AO mismatch\n - tcp_filter() eBPF/SO_ATTACH_FILTER drop\n - th-\u003esyn \u0026\u0026 th-\u003efin discard in tcp_rcv_state_process() TCP_LISTEN\n - psp_sk_rx_policy_check() in tcp_v{4,6}_do_rcv()\n - tcp_checksum_complete() in tcp_v{4,6}_do_rcv()\n - tcp_v{4,6}_cookie_check() returning NULL\n\nWhen a packet is dropped on any of these paths, tcp_tw_isn is left set.\n\nThe next SYN processed on the same CPU then consumes the non zero value in\ntcp_conn_request(), receiving a potentially predictable ISN.\n\nThis patch moves back tcp_tw_isn to skb-\u003ecb[], getting rid of the per-cpu\nvariable.\n\nNote that tcp_v{4,6}_fill_cb() do not set it.\n\nVery litle impact on overall code size/complexity:\n\n$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new\nadd/remove: 0/0 grow/shrink: 2/1 up/down: 8/-15 (-7)\nFunction old new delta\ntcp_v6_rcv 3038 3042 +4\ntcp_v4_rcv 3035 3039 +4\ntcp_conn_request 2938 2923 -15\nTotal: Before=24436060, After=24436053, chg -0.00%",
"id": "GHSA-h337-654q-329c",
"modified": "2026-07-20T15:31:56Z",
"published": "2026-07-19T18:31:49Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-64024"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/1bbf0ced1d9db73ac7893c2187f3459288603e0d"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/4affe063fa56c880cbea8d0bfded0bb80751579d"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/e47f7060eaf60894e3e4d0e3c4fe6e1f2eacfbdd"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:H",
"type": "CVSS_V3"
}
]
}
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