CVE-2026-64389
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.
CSIRTS triage
- What
- NTLMv2 responses are not validated before updating the session key.
- Who is affected
- Deployments using KSMBD.
- Urgency
- Remediation is important to prevent potential security issues, but no exploitation is reported.
- Action
- Update KSMBD to the latest version.
AI-assisted analysis generated from the source advisory — verify against the original.
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Exploitation outlook
- Low exploitation risk0.45% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 38% of all EPSS-scored CVEs.
Advisory coverage (2)
External references
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