# CVE-2026-31392: smb: client: fix krb5 mount with username option

> The Linux kernel SMB client incorrectly reuses Kerberos sessions, allowing mounts with different usernames to bypass authentication checks.

- **Severity:** High 8.1 (`CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 3 Apr 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-31392, DEBIAN-CVE-2026-31392, GHSA-h833-487p-56g8, ROOT-OS-DEBIAN-11-CVE-2026-31392, ROOT-OS-DEBIAN-12-CVE-2026-31392, ROOT-OS-DEBIAN-13-CVE-2026-31392, ROOT-OS-UBUNTU-2204-CVE-2026-31392, ROOT-OS-UBUNTU-2404-CVE-2026-31392, UBUNTU-CVE-2026-31392

## What is CVE-2026-31392?

CVE-2026-31392 is a high-severity authentication bypass vulnerability in Linux kernel, published on 3 Apr 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: fix krb5 mount with username option

Customer reported that some of their krb5 mounts were failing against
a single server as the client was trying to mount the shares with
wrong credentials.  It turned out the client was reusing SMB session
from first mount to try mounting the other shares, even though a
different username= option had been specified to the other mounts.

By using username mount option along with sec=krb5 to search for
principals from keytab is supported by cifs.upcall(8) since
cifs-utils-4.8.  So fix this by matching username mount option in
match_session() even with Kerberos.

For example, the second mount below should fail with -ENOKEY as there
is no 'foobar' principal in keytab (/etc/krb5.keytab).  The client
ends up reusing SMB session from first mount to perform the second
one, which is wrong.

```
$ ktutil
ktutil:  add_entry -password -p testuser -k 1 -e aes256-cts
Password for testuser@ZELDA.TEST:
ktutil:  write_kt /etc/krb5.keytab
ktutil:  quit
$ klist -ke
Keytab name: FILE:/etc/krb5.keytab
KVNO Principal
 ---- ----------------------------------------------------------------
   1 testuser@ZELDA.TEST (aes256-cts-hmac-sha1-96)
$ mount.cifs //w22-root2/scratch /mnt/1 -o sec=krb5,username=testuser
$ mount.cifs //w22-root2/scratch /mnt/2 -o sec=krb5,username=foobar
$ mount -t cifs | grep -Po 'username=\K\w+'
testuser
testuser
```

Weakness: CWE-287 (Improper Authentication)

## Which versions are affected by CVE-2026-31392?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31392?

Apply the vendor's patch.

## Is CVE-2026-31392 being exploited?

There is no known exploitation of CVE-2026-31392 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-2026-31392 and fix it if it is affected.

What it is: CVE-2026-31392 is a high-severity authentication bypass vulnerability in Linux kernel, published on 3 Apr 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-2026-31392 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-2026-31392/ (Markdown: https://insights.sbomify.com/CVE-2026-31392/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31392
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31392
- OSV: https://osv.dev/vulnerability/CVE-2026-31392

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31392/
Summaries are generated and may contain mistakes; the official description above is the record.