---
id: CVE-2025-71088
title: 'mptcp: fallback earlier on simult connection'
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  mptcp: fallback earlier on simult connection

  Syzkaller reports a simult-connect race leading to inconsistent fallback
  status:

    WARNING: CPU: 3 PID: 33 at net/mptcp/s…
severity: none
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 01b7822700f2256900089e00390e119e1ad545df <
    b5f46a08269265e2f5e87d855287d6d22de0a32b
  - >-
    Linux >= 1e777f39b4d75e599a3aac8e0f67d739474f198c <
    c9bf315228287653522894df9d851e9b43db9516
  - >-
    Linux >= 1e777f39b4d75e599a3aac8e0f67d739474f198c <
    79f80a7a47849ef1b3c25a0bedcc448b9cb551c1
  - >-
    Linux >= 1e777f39b4d75e599a3aac8e0f67d739474f198c <
    25f1ae942c097b7ae4ce5c2b9c6fefb8e3672b86
  - >-
    Linux >= 1e777f39b4d75e599a3aac8e0f67d739474f198c <
    71154bbe49423128c1c8577b6576de1ed6836830
  - Linux >= 6.1.110 < 6.1.160
  - Linux 6.2
published: '2026-01-13'
updated: '2026-09-08'
sourceUpdated: '2026-09-08T08:43:47.761Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2025-71088'
references:
  - url: 'https://git.kernel.org/stable/c/b5f46a08269265e2f5e87d855287d6d22de0a32b'
  - url: 'https://git.kernel.org/stable/c/c9bf315228287653522894df9d851e9b43db9516'
  - url: 'https://git.kernel.org/stable/c/79f80a7a47849ef1b3c25a0bedcc448b9cb551c1'
  - url: 'https://git.kernel.org/stable/c/25f1ae942c097b7ae4ce5c2b9c6fefb8e3672b86'
  - url: 'https://git.kernel.org/stable/c/71154bbe49423128c1c8577b6576de1ed6836830'
tags:
  - cve.org
epss: 0.00142
epssPercentile: 0.03888
ingestedAt: '2026-09-08T15:33:26.995Z'
---

## Overview

In the Linux kernel, the following vulnerability has been resolved:

mptcp: fallback earlier on simult connection

Syzkaller reports a simult-connect race leading to inconsistent fallback
status:

  WARNING: CPU: 3 PID: 33 at net/mptcp/subflow.c:1515 subflow_data_ready+0x40b/0x7c0 net/mptcp/subflow.c:1515
  Modules linked in:
  CPU: 3 UID: 0 PID: 33 Comm: ksoftirqd/3 Not tainted syzkaller #0 PREEMPT(full)
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
  RIP: 0010:subflow_data_ready+0x40b/0x7c0 net/mptcp/subflow.c:1515
  Code: 89 ee e8 78 61 3c f6 40 84 ed 75 21 e8 8e 66 3c f6 44 89 fe bf 07 00 00 00 e8 c1 61 3c f6 41 83 ff 07 74 09 e8 76 66 3c f6 90 <0f> 0b 90 e8 6d 66 3c f6 48 89 df e8 e5 ad ff ff 31 ff 89 c5 89 c6
  RSP: 0018:ffffc900006cf338 EFLAGS: 00010246
  RAX: 0000000000000000 RBX: ffff888031acd100 RCX: ffffffff8b7f2abf
  RDX: ffff88801e6ea440 RSI: ffffffff8b7f2aca RDI: 0000000000000005
  RBP: 0000000000000000 R08: 0000000000000005 R09: 0000000000000007
  R10: 0000000000000004 R11: 0000000000002c10 R12: ffff88802ba69900
  R13: 1ffff920000d9e67 R14: ffff888046f81800 R15: 0000000000000004
  FS:  0000000000000000(0000) GS:ffff8880d69bc000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000560fc0ca1670 CR3: 0000000032c3a000 CR4: 0000000000352ef0
  Call Trace:
   <TASK>
   tcp_data_queue+0x13b0/0x4f90 net/ipv4/tcp_input.c:5197
   tcp_rcv_state_process+0xfdf/0x4ec0 net/ipv4/tcp_input.c:6922
   tcp_v6_do_rcv+0x492/0x1740 net/ipv6/tcp_ipv6.c:1672
   tcp_v6_rcv+0x2976/0x41e0 net/ipv6/tcp_ipv6.c:1918
   ip6_protocol_deliver_rcu+0x188/0x1520 net/ipv6/ip6_input.c:438
   ip6_input_finish+0x1e4/0x4b0 net/ipv6/ip6_input.c:489
   NF_HOOK include/linux/netfilter.h:318 [inline]
   NF_HOOK include/linux/netfilter.h:312 [inline]
   ip6_input+0x105/0x2f0 net/ipv6/ip6_input.c:500
   dst_input include/net/dst.h:471 [inline]
   ip6_rcv_finish net/ipv6/ip6_input.c:79 [inline]
   NF_HOOK include/linux/netfilter.h:318 [inline]
   NF_HOOK include/linux/netfilter.h:312 [inline]
   ipv6_rcv+0x264/0x650 net/ipv6/ip6_input.c:311
   __netif_receive_skb_one_core+0x12d/0x1e0 net/core/dev.c:5979
   __netif_receive_skb+0x1d/0x160 net/core/dev.c:6092
   process_backlog+0x442/0x15e0 net/core/dev.c:6444
   __napi_poll.constprop.0+0xba/0x550 net/core/dev.c:7494
   napi_poll net/core/dev.c:7557 [inline]
   net_rx_action+0xa9f/0xfe0 net/core/dev.c:7684
   handle_softirqs+0x216/0x8e0 kernel/softirq.c:579
   run_ksoftirqd kernel/softirq.c:968 [inline]
   run_ksoftirqd+0x3a/0x60 kernel/softirq.c:960
   smpboot_thread_fn+0x3f7/0xae0 kernel/smpboot.c:160
   kthread+0x3c2/0x780 kernel/kthread.c:463
   ret_from_fork+0x5d7/0x6f0 arch/x86/kernel/process.c:148
   ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
   </TASK>

The TCP subflow can process the simult-connect syn-ack packet after
transitioning to TCP_FIN1 state, bypassing the MPTCP fallback check,
as the sk_state_change() callback is not invoked for * -> FIN_WAIT1
transitions.

That will move the msk socket to an inconsistent status and the next
incoming data will hit the reported splat.

Close the race moving the simult-fallback check at the earliest possible
stage - that is at syn-ack generation time.

About the fixes tags: [2] was supposed to also fix this issue introduced
by [3]. [1] is required as a dependence: it was not explicitly marked as
a fix, but it is one and it has already been backported before [3]. In
other words, this commit should be backported up to [3], including [2]
and [1] if that's not already there.

## Affected

- `Linux >= 01b7822700f2256900089e00390e119e1ad545df < b5f46a08269265e2f5e87d855287d6d22de0a32b`
- `Linux >= 1e777f39b4d75e599a3aac8e0f67d739474f198c < c9bf315228287653522894df9d851e9b43db9516`
- `Linux >= 1e777f39b4d75e599a3aac8e0f67d739474f198c < 79f80a7a47849ef1b3c25a0bedcc448b9cb551c1`
- `Linux >= 1e777f39b4d75e599a3aac8e0f67d739474f198c < 25f1ae942c097b7ae4ce5c2b9c6fefb8e3672b86`
- `Linux >= 1e777f39b4d75e599a3aac8e0f67d739474f198c < 71154bbe49423128c1c8577b6576de1ed6836830`
- `Linux >= 6.1.110 < 6.1.160`
- `Linux 6.2`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.