---
cve: "CVE-2026-64024"
severity: "CRITICAL"
cvss: 9.4
epss: "39%"
vendor: "Linux"
kev: false
exploited: false
published: "2026-07-19 15:39:16"
tags: [cve, security, critical]
source: tsecurity.de CVE-Dossier
exported: "2026-09-07T09:52:53+02:00"
---

# CVE-2026-64024

> 9.4 CRITICAL

## Beschreibung

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%

## CVSS-Vektor

```
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:H
```

| Metrik | Wert | Bewertung |
|---|---|---|
| AV Angriffsvektor | Netzwerk | bad |
| AC Komplexität | Gering | bad |
| PR Privilegien | Keine | bad |
| UI Interaktion | Keine | bad |
| S Scope | Unverändert | good |
| C Vertraulichkeit | Gering | warn |
| I Integrität | Hoch | bad |
| A Verfügbarkeit | Hoch | bad |

## Patch verfügbar (OSV)

- Kernel ≥ 6.18.34
- Kernel ≥ 7.0.11

## Referenzen

- <https://git.kernel.org/stable/c/e47f7060eaf60894e3e4d0e3c4fe6e1f2eacfbdd>
- <https://git.kernel.org/stable/c/4affe063fa56c880cbea8d0bfded0bb80751579d>
- <https://git.kernel.org/stable/c/1bbf0ced1d9db73ac7893c2187f3459288603e0d>

---
_Exportiert aus dem [tsecurity.de CVE-Dossier](https://tsecurity.de/cve?cve=CVE-2026-64024) · Datenquellen: EUVD (ENISA), NVD, OSV, CISA KEV, FIRST EPSS, Exploit-DB, BSI BITS_