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

CVE-2026-64389 is fixed in linux on 1 platform, first by DEBIAN-CVE-2026-64389, released August 7, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-64389
- Priority: High
- CVSS: 8.2 (Important)
- EPSS: 0.0049
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jul 25, 2026
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-64389

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 13.5 (trixie) | 6.12.115-2 | [DEBIAN-CVE-2026-64389](https://patchblox.com/security/data/fix/linux/debian-cve-2026-64389.md) | Aug 7, 2026 |
| Linux | linux Debian 14 | 7.1.4-1 | [DEBIAN-CVE-2026-64389](https://patchblox.com/security/data/fix/linux/debian-cve-2026-64389.md) | Aug 7, 2026 |

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Data as of 2026-11-09T01:18:40Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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