# CVE-2025-40258: mptcp: fix race condition in mptcp_schedule_work()

> A race condition in the Linux kernel MPTCP subsystem can lead to a use-after-free, potentially causing a system crash.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 4 Dec 2025
- **Updated:** 5 Aug 2026
- **Also known as:** AZL-71380, BELL-CVE-2025-40258, DEBIAN-CVE-2025-40258, GHSA-86fp-6jqc-qg25, ROOT-OS-DEBIAN-11-CVE-2025-40258, ROOT-OS-DEBIAN-12-CVE-2025-40258, ROOT-OS-DEBIAN-13-CVE-2025-40258, ROOT-OS-UBUNTU-2204-CVE-2025-40258, ROOT-OS-UBUNTU-2404-CVE-2025-40258, UBUNTU-CVE-2025-40258

## What is CVE-2025-40258?

CVE-2025-40258 is a critical vulnerability in Linux, published on 4 Dec 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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

mptcp: fix race condition in mptcp_schedule_work()

syzbot reported use-after-free in mptcp_schedule_work() [1]

Issue here is that mptcp_schedule_work() schedules a work,
then gets a refcount on sk->sk_refcnt if the work was scheduled.
This refcount will be released by mptcp_worker().

[A] if (schedule_work(...)) {
[B]     sock_hold(sk);
        return true;
    }

Problem is that mptcp_worker() can run immediately and complete before [B]

We need instead :

    sock_hold(sk);
    if (schedule_work(...))
        return true;
    sock_put(sk);

[1]
refcount_t: addition on 0; use-after-free.
 WARNING: CPU: 1 PID: 29 at lib/refcount.c:25 refcount_warn_saturate+0xfa/0x1d0 lib/refcount.c:25
Call Trace:
 <TASK>
 __refcount_add include/linux/refcount.h:-1 [inline]
  __refcount_inc include/linux/refcount.h:366 [inline]
  refcount_inc include/linux/refcount.h:383 [inline]
  sock_hold include/net/sock.h:816 [inline]
  mptcp_schedule_work+0x164/0x1a0 net/mptcp/protocol.c:943
  mptcp_tout_timer+0x21/0xa0 net/mptcp/protocol.c:2316
  call_timer_fn+0x17e/0x5f0 kernel/time/timer.c:1747
  expire_timers kernel/time/timer.c:1798 [inline]
  __run_timers kernel/time/timer.c:2372 [inline]
  __run_timer_base+0x648/0x970 kernel/time/timer.c:2384
  run_timer_base kernel/time/timer.c:2393 [inline]
  run_timer_softirq+0xb7/0x180 kernel/time/timer.c:2403
  handle_softirqs+0x22f/0x710 kernel/softirq.c:622
  __do_softirq kernel/softirq.c:656 [inline]
  run_ktimerd+0xcf/0x190 kernel/softirq.c:1138
  smpboot_thread_fn+0x542/0xa60 kernel/smpboot.c:160
  kthread+0x711/0x8a0 kernel/kthread.c:463
  ret_from_fork+0x4bc/0x870 arch/x86/kernel/process.c:158
  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

## Which versions are affected by CVE-2025-40258?


## How do I fix CVE-2025-40258?

Apply the vendor's patch.

## Is CVE-2025-40258 being exploited?

There is no known exploitation of CVE-2025-40258 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40258 and fix it if it is affected.

What it is: CVE-2025-40258 is a critical vulnerability in Linux, published on 4 Dec 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2025-40258 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-40258/ (Markdown: https://insights.sbomify.com/CVE-2025-40258/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40258
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40258
- OSV: https://osv.dev/vulnerability/CVE-2025-40258

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40258/
Summaries are generated and may contain mistakes; the official description above is the record.