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CVE-2026-64024
tcp: fix stale per-CPU tcp_tw_isn leak enabling ISN prediction
Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem include/net. Debian urgency: unimportant.
What to do, per Debian release
Debian's own status for CVE-2026-64024 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | not affected | 6.1.190-1 | Nothing to do. bookworm never shipped the vulnerable code. |
| Debian 13trixie | vulnerable | 6.12.113-1 | No fix published anywhere yet, neither in Debian nor upstream for the series trixie tracks. Watch the Debian tracker page for this CVE. Advisories for trixie have historically landed a median of 11 days after publication. |
| Debian 14forky | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
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%
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
include/net/tcp.h
net/ipv4/tcp.c
net/ipv4/tcp_input.c
net/ipv4/tcp_ipv4.c
net/ipv6/tcp_ipv6.c
- What this is
- NETWORKING [TCP]MAINTAINERS section matched on include/net/tcp.h · netdev@vger.kernel.org · Maintained
Check it on the machine
grep -w CONFIG_INET /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
Show whether Debian builds this, per release
Debian builds this code. Every release below compiles the affected file into the kernels it ships.
- bookworm, trixie, forky, sidy
- y
- built into the kernel image on every flavour - only a patch removes it
- Build switch
- CONFIG_INET
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-07-19 (3 months ago)source
- CVSS
- 9.4 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.39%, higher than 31.0% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- unimportantsource
- Subsystem
- include/net
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 6.10 | 6.18.34 | e47f7060eaf6 |
| 6.10 | 7.0.11 | 4affe063fa56 |
| 6.10 | 7.1 | 1bbf0ced1d9d |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:NAttack vector, network
- The vulnerable code is reached from the standard IPv4/IPv6 TCP receive path (tcp_v4_rcv/tcp_v6_rcv) when remote attackers send crafted TCP packets to any listening service on the host.
- AC:LAttack complexity, low
- An attacker fully controls the trigger sequence—establishing TIME_WAIT state, deliberately causing drops on documented paths (e.g., SYN+FIN or checksum failure), and spraying SYNs to land on the same CPU via RPS flow hashing—without depending on uncontrollable timing or memory layout.
- PR:NPrivileges required, none
- Exploitation requires only the ability to send TCP packets to a reachable port; no local shell access, capabilities, or authentication is needed on the target system.
- UI:NUser interaction, none
- No victim user action is required; the attack is conducted entirely by sending network packets to an internet-facing or LAN-reachable server.
- S:UScope, unchanged
- Impact is confined to the kernel TCP stack on the target host (predictable ISNs, syncookie bypass, session manipulation) and does not cross a VM, container, or IOMMU security boundary.
- C:LConfidentiality impact, low
- Stale TIME_WAIT ISNs leak predictable sequence-number information that an off-path attacker can use to infer or intercept data on affected TCP connections, bypassing the cryptographically generated ISNs from secure_tcp_seq_and_ts_off().
- I:HIntegrity impact, high
- Predictable server ISNs enable TCP session hijacking and injection of forged segments into established or newly accepted connections, plus syncookie bypass allows unauthorized connection state to be created under flood conditions.
- A:HAvailability impact, high
- Skipping syncookie and SYN-backlog protections when a stale non-zero ISN is consumed allows unauthenticated resource exhaustion and connection-disruption attacks against listening services.