mORMot Open Source

testgary

Member

Registered: 2025-02-06

Posts: 41

TRestServerAuthenticationDefault bug ?!

TRestServerAuthenticationDefault.Auth

Join([aClientNonce,  User.LogonName, User.PasswordHashHexa], salt); 

innerHash = TAuthUser.PasswordHashHexa
PassWord = Sha256(ModelRoot+Nonce+ClientNonce+UserName+innerHash)

If the PasswordHashHexa password in the database is compromised (without needing to know the original password), it appears that frontend login is still possible!

Last edited by testgary (2025-11-05 07:48:05)

ab

Administrator

From: France

Registered: 2010-06-21

Posts: 15,261

Website

Re: TRestServerAuthenticationDefault bug ?!

Your remark makes perfect sense.

It currently lacks a "client proof" as in SCRAM.
What we could easily do is, in pseudo code:

SaltedPassword = KDF(Password, randomsalt, iterations) // using MCF prefix parameters

// initial server storage
ClientKey = HMAC(SaltedPassword,"Client Key") 
StoredKey = H(ClientKey) 
ServerKey = HMAC(SaltedPassword,"Server Key")
PersistedKey = "#MCF prefix" + base64uri(StoredKey + ServerKey) 
save PersistedKey in the DB for this user

// client side
ClientKey = HMAC(SaltedPassword,"Client Key") 
StoredKey = H(ClientKey) 
ClientSignature = HMAC(StoredKey, AuthMessage) // here AuthMessage = model.root + username + client nonce + server nonce
return ClientProof = ClientKey XOR ClientSignature

// server side
ClientSignature = HMAC(StoredKey, AuthMessage) // we would try both current and previous server nonce
candidate_ClientKey = ClientProof XOR ClientSignature
Checks: H(candidate_ClientKey) = StoredKey
return ServerProof = ServerKey XOR ClientSignature

// client side
ServerKey = HMAC(SaltedPassword,"Server Key")
Checks: ServerProof XOR ClientSignature = ServerKey

I guess this could be implemented in TRestServerAuthenticationDefault.CheckPassword and TRestClientAuthenticationDefault.ClientComputeSessionKey methods, with proper backward compatibility.
But we would need to store not the crypto hash in PasswordHashHexa, but PersistedKey on server side - this is a breaking change, so we propose to use the # prefix instead of $ for such values.
And it would also need a new ModularCryptStoredKey() to compute the StoredKey, still with the modular crypt parameters prefix.

Could you please create an issue on GitHub about this?

testgary

Member

Registered: 2025-02-06

Posts: 41

Re: TRestServerAuthenticationDefault bug ?!

https://github.com/synopse/mORMot2/issues/405