CVE-2026-64024 in Linux
Summary
by MITRE • 07/19/2026
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%
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Analysis
by VulDB Data Team • 07/19/2026
This vulnerability affects the linux kernel's tcp implementation and represents a significant security flaw that enables predictable sequence number generation. The issue stems from a flawed design decision where the TIME_WAIT-derived ISN value was moved from the socket buffer control block to a per-cpu variable to improve performance. This change introduced a race condition between packet producers and consumers that could lead to sequence number prediction attacks. The vulnerability is classified under CWE-377 as insecure temporary file handling and CWE-327 as use of a broken or weak cryptographic algorithm, though the primary concern here involves predictable network protocol behavior rather than cryptographic weakness.
The technical flaw occurs because multiple packet drop paths exist between where tcp_tw_isn is set using __this_cpu_write(tcp_tw_isn, isn) in tcp_v4_rcv() and tcp_v6_rcv() functions and where it's consumed by tcp_conn_request(). These drop paths include min_ttl/min_hopcount checks, xfrm policy checks, tcp_inbound_hash() MD5/AO mismatch handling, eBPF/SO_ATTACH_FILTER drops, th->syn && th->fin discard conditions in tcp_rcv_state_process(), psp_sk_rx_policy_check() operations, and tcp_checksum_complete() processing. When packets are dropped on any of these paths, the tcp_tw_isn value remains set in the per-cpu variable. Subsequent SYN packets processed on the same CPU then consume this non-zero value, resulting in potentially predictable sequence numbers that can be exploited by attackers.
The operational impact of this vulnerability is substantial as it enables ISN prediction attacks that can compromise TCP security mechanisms. Attackers can potentially perform connection hijacking, session takeover, or man-in-the-middle attacks by predicting sequence numbers generated from stale TIME_WAIT values. This vulnerability directly relates to the MITRE ATT&CK technique T1210 for exploiting known vulnerabilities and could be leveraged in conjunction with other network-based attacks. The flaw essentially creates a window of opportunity where predictable sequence numbers can be generated, undermining fundamental TCP security assumptions.
The fix implemented addresses this by moving tcp_tw_isn back to the socket buffer control block (skb->cb[]) which ensures proper lifecycle management of the ISN value. This approach eliminates the per-cpu variable that was causing the race condition and ensures that ISN values are properly managed through the normal packet processing flow. The patch maintains minimal impact on code size and complexity, with only minor changes to function sizes and overall kernel binary footprint. The solution follows secure coding practices by removing the problematic per-cpu state management and restoring proper socket buffer-based handling of sequence numbers, which aligns with best practices for avoiding race conditions in concurrent systems.