# CVE-2026-64389: ksmbd: validate NTLMv2 response before updating session key

> Attackers can bypass NTLMv2 authentication in ksmbd to modify session keys, potentially gaining unauthorized access or privileges.

- **Severity:** High 8.2 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/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:** 25 Jul 2026
- **Updated:** 21 Oct 2026
- **Also known as:** BELL-CVE-2026-64389, DEBIAN-CVE-2026-64389, GHSA-fxvf-vm38-6pxw, ROOT-OS-DEBIAN-12-CVE-2026-64389, ROOT-OS-DEBIAN-13-CVE-2026-64389, ROOT-OS-UBUNTU-2204-CVE-2026-64389, UBUNTU-CVE-2026-64389

## What is CVE-2026-64389?

CVE-2026-64389 is a high-severity authentication bypass vulnerability in Linux kernel, published on 25 Jul 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:

ksmbd: validate NTLMv2 response before updating session key

ksmbd_auth_ntlmv2() derives the NTLMv2 session key into
sess->sess_key before it verifies the NTLMv2 response.
ksmbd_decode_ntlmssp_auth_blob() then continues into KEY_XCH even
when ksmbd_auth_ntlmv2() failed.

With SMB3 multichannel binding, the failed authentication operates on
an existing session and the session setup error path does not expire
binding sessions. A client can send a binding session setup with a
bad NT proof and KEY_XCH and still modify sess->sess_key before
STATUS_LOGON_FAILURE is returned.

Relevant path:

  smb2_sess_setup()
    -> conn->binding = true
    -> ntlm_authenticate()
       -> session_user()
       -> ksmbd_decode_ntlmssp_auth_blob()
          -> ksmbd_auth_ntlmv2()
             -> calc_ntlmv2_hash()
             -> hmac_md5_usingrawkey(..., sess->sess_key)
             -> crypto_memneq() returns mismatch
          -> KEY_XCH arc4_crypt(..., sess->sess_key, ...)
    -> out_err without expiring the binding session

Derive the base session key into a local buffer and copy it to
sess->sess_key only after the proof matches. Return immediately on
authentication failure so KEY_XCH is only processed after successful
authentication.

## Which versions are affected by CVE-2026-64389?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-64389?

Apply the vendor's patch.

## Is CVE-2026-64389 being exploited?

There is no known exploitation of CVE-2026-64389 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-2026-64389 and fix it if it is affected.

What it is: CVE-2026-64389 is a high-severity authentication bypass vulnerability in Linux kernel, published on 25 Jul 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-2026-64389 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-2026-64389/ (Markdown: https://insights.sbomify.com/CVE-2026-64389/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-64389
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64389
- OSV: https://osv.dev/vulnerability/CVE-2026-64389

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-64389/
Summaries are generated and may contain mistakes; the official description above is the record.