# CVE-2026-64401: smb: client: resolve SWN tcon from live registrations

> A use-after-free in the Linux kernel SMB client allows attackers to cause denial of service or execute code via crafted network shares.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 Jul 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-64401, DEBIAN-CVE-2026-64401, GHSA-gww6-qc28-623p, ROOT-OS-DEBIAN-12-CVE-2026-64401, ROOT-OS-DEBIAN-13-CVE-2026-64401, ROOT-OS-UBUNTU-2204-CVE-2026-64401, UBUNTU-CVE-2026-64401

## What is CVE-2026-64401?

CVE-2026-64401 is a high-severity use after free vulnerability in Linux kernel, published on 25 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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

smb: client: resolve SWN tcon from live registrations

cifs_swn_notify() looks up a witness registration by id under
cifs_swnreg_idr_mutex, drops the mutex, and then uses the registration's
cached tcon pointer.  That pointer is not a lifetime reference, and it is
not a stable representative once cifs_get_swn_reg() lets multiple tcons
for the same net/share name share one registration id.

A same-share second mount can keep the cifs_swn_reg alive after the first
tcon unregisters and is freed.  The registration then still points at the
freed first tcon, so taking tc_lock or incrementing tc_count through
swnreg->tcon only moves the use-after-free earlier.  Taking tc_lock while
holding cifs_swnreg_idr_mutex also violates the documented CIFS lock
order.

Fix this by making the registration store only the stable witness
identity: id, net name, share name, and notify flags.  When a notify
arrives, copy that identity under cifs_swnreg_idr_mutex, drop the mutex,
then find and pin a live witness tcon that currently matches the net/share
pair under the normal cifs_tcp_ses_lock -> tc_lock order.  The notification
path uses that pinned tcon directly and drops the reference when done.

Registration and unregister messages now use the live tcon passed by the
caller instead of a cached tcon in the registration.  The final unregister
send is folded into cifs_swn_unregister() while the registration is still
protected by cifs_swnreg_idr_mutex.  This removes the previous
find/drop/reacquire raw-pointer window.  The release path only removes the
idr entry and frees the stable identity strings.

This preserves the intended one-registration/many-tcon behavior: a
registration id represents a net/share pair, and notify handling acts on a
live representative selected at use time.  It also preserves CLIENT_MOVE
ordering for the representative tcon because the old-IP unregister is sent
before cifs_swn_register() sends the new-IP register.

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2026-64401?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-64401?

Apply the vendor's patch.

## Is CVE-2026-64401 being exploited?

There is no known exploitation of CVE-2026-64401 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-2026-64401 and fix it if it is affected.

What it is: CVE-2026-64401 is a high-severity use after free vulnerability in Linux kernel, published on 25 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-64401 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-2026-64401/ (Markdown: https://insights.sbomify.com/CVE-2026-64401/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-64401
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64401
- OSV: https://osv.dev/vulnerability/CVE-2026-64401

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-64401/
Summaries are generated and may contain mistakes; the official description above is the record.