mschap: ERROR: MS-CHAP2-Response is incorrect

FreeRADIUS 'MS-CHAP2-Response is incorrect' — the real causes

The log lines:

(8) mschap: ERROR: MS-CHAP2-Response is incorrect
(8) eap_peap: ERROR: Tunneled authentication was rejected
(8) Login incorrect (mschap: ERROR: MS-CHAP2-Response is incorrect): [alice]

What it actually means: everything up to the password worked. The client trusted your certificate, the TLS tunnel came up, the inner EAP conversation ran — and the MSCHAPv2 proof the client sent doesn’t match what the server computed from its copy of the password. Certificates, shared secrets, and clients.conf are all exonerated.

Three things produce a failed proof:

1. The password (or the username variant) is simply wrong

Including the sneaky versions: a stale cached Wi-Fi credential after a password change, or the same user existing as alice, CORP\alice, and [email protected] where only one resolves to the right account.

Check: take the exact User-Name from the debug output and test those credentials on the server itself — radtest for a quick loop, or ntlm_auth --username=alice --password=... if you authenticate against AD. If it fails there too, it’s genuinely the credential.

2. Your backend can’t produce an NT hash — so MSCHAPv2 can never succeed

This is the one that eats weekends. MSCHAPv2 is mathematically bound to the NT hash of the password. The server must have either the cleartext password or the NT hash. If your LDAP directory stores passwords as SSHA, MD5-crypt, or bcrypt — as security best practice says it should — FreeRADIUS cannot verify MSCHAPv2 against them. Every user fails, always, with this exact error.

Check: look earlier in the debug output for warnings like No Cleartext-Password / No NT-Password. If present, your options are: store NT hashes (unpalatable), authenticate via ntlm_auth against AD/Samba, switch the inner method to TTLS-PAP (the directory verifies the cleartext), or move to EAP-TLS and stop fighting password formats.

3. The AD account is locked, disabled, or expired

When FreeRADIUS delegates to AD via ntlm_auth/winbind, account-state problems surface as a generic MSCHAP failure rather than a helpful message.

Check: run ntlm_auth manually on the RADIUS host and read the NT_STATUS_* code it returns — NT_STATUS_ACCOUNT_LOCKED_OUT and NT_STATUS_PASSWORD_EXPIRED name the problem precisely.

Triage shortcut

One user failing → cause 1 or 3. Every user failing, since forever or since a directory migration → cause 2, and no password reset will ever fix it.