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CVE-2026-64024

In 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%

  • Published: Jul 19, 2026
  • Updated: Jul 21, 2026
  • CVE: CVE-2026-64024
  • Severity: Critical
  • Exploit:
  • CISA KEV:

CVSS v3:

  • Severity: Critical
  • Score: 9.4
  • AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:H

No CWE or OWASP classifications available.

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