# CVE-2025-40186: tcp: Don't call reqsk_fastopen_remove() in tcp_conn_request().

> A use-after-free vulnerability in the Linux kernel TCP stack allows attackers to cause a denial of service or potentially execute code via race conditions during connection handling.

- **Severity:** High 8.1 (`CVSS:3.1/AV:N/AC:H/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:** 12 Nov 2025
- **Updated:** 15 Oct 2026
- **Also known as:** BELL-CVE-2025-40186, DEBIAN-CVE-2025-40186, GHSA-qx52-pj36-489j, ROOT-OS-DEBIAN-11-CVE-2025-40186, ROOT-OS-DEBIAN-13-CVE-2025-40186, SUSE-SU-2026:0525-1, SUSE-SU-2026:20541-1, SUSE-SU-2026:20558-1, SUSE-SU-2026:20606-1, UBUNTU-CVE-2025-40186

## What is CVE-2025-40186?

CVE-2025-40186 is a high-severity use after free vulnerability in Linux, published on 12 Nov 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:

tcp: Don't call reqsk_fastopen_remove() in tcp_conn_request().

syzbot reported the splat below in tcp_conn_request(). [0]

If a listener is close()d while a TFO socket is being processed in
tcp_conn_request(), inet_csk_reqsk_queue_add() does not set reqsk->sk
and calls inet_child_forget(), which calls tcp_disconnect() for the
TFO socket.

After the cited commit, tcp_disconnect() calls reqsk_fastopen_remove(),
where reqsk_put() is called due to !reqsk->sk.

Then, reqsk_fastopen_remove() in tcp_conn_request() decrements the
last req->rsk_refcnt and frees reqsk, and __reqsk_free() at the
drop_and_free label causes the refcount underflow for the listener
and double-free of the reqsk.

Let's remove reqsk_fastopen_remove() in tcp_conn_request().

Note that other callers make sure tp->fastopen_rsk is not NULL.

[0]:
refcount_t: underflow; use-after-free.
WARNING: CPU: 12 PID: 5563 at lib/refcount.c:28 refcount_warn_saturate (lib/refcount.c:28)
Modules linked in:
CPU: 12 UID: 0 PID: 5563 Comm: syz-executor Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
RIP: 0010:refcount_warn_saturate (lib/refcount.c:28)
Code: ab e8 8e b4 98 ff 0f 0b c3 cc cc cc cc cc 80 3d a4 e4 d6 01 00 75 9c c6 05 9b e4 d6 01 01 48 c7 c7 e8 df fb ab e8 6a b4 98 ff <0f> 0b e9 03 5b 76 00 cc 80 3d 7d e4 d6 01 00 0f 85 74 ff ff ff c6
RSP: 0018:ffffa79fc0304a98 EFLAGS: 00010246
RAX: d83af4db1c6b3900 RBX: ffff9f65c7a69020 RCX: d83af4db1c6b3900
RDX: 0000000000000000 RSI: 00000000ffff7fff RDI: ffffffffac78a280
RBP: 000000009d781b60 R08: 0000000000007fff R09: ffffffffac6ca280
R10: 0000000000017ffd R11: 0000000000000004 R12: ffff9f65c7b4f100
R13: ffff9f65c7d23c00 R14: ffff9f65c7d26000 R15: ffff9f65c7a64ef8
FS:  00007f9f962176c0(0000) GS:ffff9f65fcf00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000000180 CR3: 000000000dbbe006 CR4: 0000000000372ef0
Call Trace:
 <IRQ>
 tcp_conn_request (./include/linux/refcount.h:400 ./include/linux/refcount.h:432 ./include/linux/refcount.h:450 ./include/net/sock.h:1965 ./include/net/request_sock.h:131 net/ipv4/tcp_input.c:7301)
 tcp_rcv_state_process (net/ipv4/tcp_input.c:6708)
 tcp_v6_do_rcv (net/ipv6/tcp_ipv6.c:1670)
 tcp_v6_rcv (net/ipv6/tcp_ipv6.c:1906)
 ip6_protocol_deliver_rcu (net/ipv6/ip6_input.c:438)
 ip6_input (net/ipv6/ip6_input.c:500)
 ipv6_rcv (net/ipv6/ip6_input.c:311)
 __netif_receive_skb (net/core/dev.c:6104)
 process_backlog (net/core/dev.c:6456)
 __napi_poll (net/core/dev.c:7506)
 net_rx_action (net/core/dev.c:7569 net/core/dev.c:7696)
 handle_softirqs (kernel/softirq.c:579)
 do_softirq (kernel/softirq.c:480)
 </IRQ>

## Which versions are affected by CVE-2025-40186?


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `kgraft-patch-SLE12-SP5_Update_73` | SUSE:Linux Enterprise Live Patching 12 SP5 | All versions | 3-2.1 |
| `kernel-livepatch-SLE16_Update_1` | SUSE:Linux Enterprise Server 16.0 | All versions | 4-160000.1.1 |
| `kernel-livepatch-SLE16_Update_1` | SUSE:Linux Enterprise Server for SAP applications 16.0 | All versions | 4-160000.1.1 |
| `kernel-livepatch-SLE16-RT_Update_1` | SUSE:Linux Micro 6.2 | All versions | 4-160000.1.1 |
| `kernel-livepatch-SLE16_Update_1` | SUSE:Linux Micro 6.2 | All versions | 4-160000.1.1 |

## How do I fix CVE-2025-40186?

Apply the vendor's patch.

## Is CVE-2025-40186 being exploited?

There is no known exploitation of CVE-2025-40186 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40186 and fix it if it is affected.

What it is: CVE-2025-40186 is a high-severity use after free vulnerability in Linux, published on 12 Nov 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-2025-40186 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-2025-40186/ (Markdown: https://insights.sbomify.com/CVE-2025-40186/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40186
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40186
- OSV: https://osv.dev/vulnerability/CVE-2025-40186

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40186/
Summaries are generated and may contain mistakes; the official description above is the record.