# CVE-2026-31711: smb: server: fix active_num_conn leak on transport allocation failure

> An unauthenticated remote attacker can cause a denial of service in the Linux kernel SMB server by exhausting connection slots through memory allocation failures.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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:** 1 May 2026
- **Updated:** 15 Oct 2026
- **Also known as:** BELL-CVE-2026-31711, DEBIAN-CVE-2026-31711, GHSA-f3m4-27m2-3p6q, ROOT-OS-DEBIAN-12-CVE-2026-31711, ROOT-OS-UBUNTU-2204-CVE-2026-31711, ROOT-OS-UBUNTU-2404-CVE-2026-31711, UBUNTU-CVE-2026-31711

## What is CVE-2026-31711?

CVE-2026-31711 is a high-severity denial of service vulnerability in Linux kernel, published on 1 May 2026. 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:

smb: server: fix active_num_conn leak on transport allocation failure

Commit 77ffbcac4e56 ("smb: server: fix leak of active_num_conn in
ksmbd_tcp_new_connection()") addressed the kthread_run() failure
path.  The earlier alloc_transport() == NULL path in the same
function has the same leak, is reachable pre-authentication via any
TCP connect to port 445, and was empirically reproduced on UML
(ARCH=um, v7.0-rc7): a small number of forced allocation failures
were sufficient to put ksmbd into a state where every subsequent
connection attempt was rejected for the remainder of the boot.

ksmbd_kthread_fn() increments active_num_conn before calling
ksmbd_tcp_new_connection() and discards the return value, so when
alloc_transport() returns NULL the socket is released and -ENOMEM
returned without decrementing the counter.  Each such failure
permanently consumes one slot from the max_connections pool; once
cumulative failures reach the cap, atomic_inc_return() hits the
threshold on every subsequent accept and every new connection is
rejected.  The counter is only reset by module reload.

An unauthenticated remote attacker can drive the server toward the
memory pressure that makes alloc_transport() fail by holding open
connections with large RFC1002 lengths up to MAX_STREAM_PROT_LEN
(0x00FFFFFF); natural transient allocation failures on a loaded
host produce the same drift more slowly.

Mirror the existing rollback pattern in ksmbd_kthread_fn(): on the
alloc_transport() failure path, decrement active_num_conn gated on
server_conf.max_connections.

Repro details: with the patch reverted, forced alloc_transport()
NULL returns leaked counter slots and subsequent connection
attempts -- including legitimate connects issued after the
forced-fail window had closed -- were all rejected with "Limit the
maximum number of connections".  With this patch applied, the same
connect sequence produces no rejections and the counter cycles
cleanly between zero and one on every accept.

Weakness: CWE-401 (Missing Release of Memory after Effective Lifetime)

## Which versions are affected by CVE-2026-31711?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31711?

Apply the vendor's patch.

## Is CVE-2026-31711 being exploited?

There is no known exploitation of CVE-2026-31711 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-31711 and fix it if it is affected.

What it is: CVE-2026-31711 is a high-severity denial of service vulnerability in Linux kernel, published on 1 May 2026. 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-2026-31711 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2026-31711/ (Markdown: https://insights.sbomify.com/CVE-2026-31711/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31711
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31711
- OSV: https://osv.dev/vulnerability/CVE-2026-31711

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31711/
Summaries are generated and may contain mistakes; the official description above is the record.