# CVE-2025-68227: mptcp: Fix proto fallback detection with BPF

> A local attacker can trigger a kernel warning and potential denial of service in the Linux kernel's MPTCP subsystem when BPF sockmap is used with TCP fallback.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 16 Dec 2025
- **Updated:** 11 May 2026
- **Also known as:** AZL-72503, BELL-CVE-2025-68227, DEBIAN-CVE-2025-68227, GHSA-fpqf-4vc3-w3r3, ROOT-OS-DEBIAN-12-CVE-2025-68227, ROOT-OS-DEBIAN-13-CVE-2025-68227, ROOT-OS-UBUNTU-2204-CVE-2025-68227, ROOT-OS-UBUNTU-2404-CVE-2025-68227, UBUNTU-CVE-2025-68227

## What is CVE-2025-68227?

CVE-2025-68227 is a denial of service vulnerability in Linux, published on 16 Dec 2025. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

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

mptcp: Fix proto fallback detection with BPF

The sockmap feature allows bpf syscall from userspace, or based
on bpf sockops, replacing the sk_prot of sockets during protocol stack
processing with sockmap's custom read/write interfaces.
'''
tcp_rcv_state_process()
  syn_recv_sock()/subflow_syn_recv_sock()
    tcp_init_transfer(BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB)
      bpf_skops_established       <== sockops
        bpf_sock_map_update(sk)   <== call bpf helper
          tcp_bpf_update_proto()  <== update sk_prot
'''

When the server has MPTCP enabled but the client sends a TCP SYN
without MPTCP, subflow_syn_recv_sock() performs a fallback on the
subflow, replacing the subflow sk's sk_prot with the native sk_prot.
'''
subflow_syn_recv_sock()
  subflow_ulp_fallback()
    subflow_drop_ctx()
      mptcp_subflow_ops_undo_override()
'''

Then, this subflow can be normally used by sockmap, which replaces the
native sk_prot with sockmap's custom sk_prot. The issue occurs when the
user executes accept::mptcp_stream_accept::mptcp_fallback_tcp_ops().
Here, it uses sk->sk_prot to compare with the native sk_prot, but this
is incorrect when sockmap is used, as we may incorrectly set
sk->sk_socket->ops.

This fix uses the more generic sk_family for the comparison instead.

Additionally, this also prevents a WARNING from occurring:

result from ./scripts/decode_stacktrace.sh:
------------[ cut here ]------------
WARNING: CPU: 0 PID: 337 at net/mptcp/protocol.c:68 mptcp_stream_accept \
(net/mptcp/protocol.c:4005)
Modules linked in:
...

PKRU: 55555554
Call Trace:
<TASK>
do_accept (net/socket.c:1989)
__sys_accept4 (net/socket.c:2028 net/socket.c:2057)
__x64_sys_accept (net/socket.c:2067)
x64_sys_call (arch/x86/entry/syscall_64.c:41)
do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
RIP: 0033:0x7f87ac92b83d

---[ end trace 0000000000000000 ]---

## Which versions are affected by CVE-2025-68227?


## How do I fix CVE-2025-68227?

Apply the vendor's patch.

## Is CVE-2025-68227 being exploited?

There is no known exploitation of CVE-2025-68227 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-68227 and fix it if it is affected.

What it is: CVE-2025-68227 is a denial of service vulnerability in Linux, published on 16 Dec 2025. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2025-68227 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-2025-68227/ (Markdown: https://insights.sbomify.com/CVE-2025-68227/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-68227
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-68227
- OSV: https://osv.dev/vulnerability/CVE-2025-68227

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-68227/
Summaries are generated and may contain mistakes; the official description above is the record.