feat: add hashed refresh tokens

[hf]

Today refresh tokens are stored raw in the database. Should the database be stolen, leaked or misused by an external or internal party, it would be nearly trivial to issue a valid access token for any user.

This PR stores the refresh tokens (the token column in refresh_tokens) as a SHA256/224 hash. It is thus impossible to impersonate a user using this value, but it is possible to verify that the user has a valid token.

For backward compatibility purposes, a token lookup occurs using both the hashed and raw value of the token; but all new tokens will be using the hashed and secure form. To prevent hashed tokens being used instead of real tokens, they are stored with a H: prefix in the database, and backward-compatible lookups are done only after stripping the same prefix before hashing.

Furthermore, the old behavior with the reuse window of refresh tokens is now cleaned up. A new refresh token is generated if a revoked refresh token is issued within the revocation period. (This only happens if the browser / client has not synced up the refresh tokens they use for requests -- in a multi-tab or concurrent request environment.)

[hf]

hmm so we're just hashing the token in the refresh_tokens table? what happens when the user tries to use the refresh token in /token?grant_type=refresh_token ?

It's hashed and then looked up in the database.

func FindUserWithRefreshToken(tx *storage.Connection, token string) (*User, *RefreshToken, error) {
	hashedToken := crypto.HashSHA224Base64(token)

	refreshToken := &RefreshToken{}
	if err := tx.Where("token = ? OR token = ?", hashedToken, token).First(refreshToken); err != nil {

For backward compatibility the non-hashed version is also looked up. Note that since these strings are random and 128 bits or more, it is impossible to have the same value appearing in hashed or unhashed.

will we hash the refresh_token sent by the user to check if it matches the hashed_token in the db?

(Yes.)

how do we also determine whether the refresh_token is an existing one (no corresponding hash in the db) or a newly issued one (has a corresponding hash in the db)?

Well there is an interesting property. If you know the original value of the token (i.e. what the user has) and you successfully find it in the database, then you have access to both the hashed and original values. Thus, it does not matter if a token is one of the old ones or a new one. If you really needed to distinguish, you can always compare Token and HashedToken. If they're the same, it was an old refresh token.

[pixtron]

For backward compatibility the non-hashed version is also looked up

@hf Couldn't an attacker just send the hashed token (from DB) as the non-hashed token (in Request) and mint new tokens with it?

[hf]

For backward compatibility the non-hashed version is also looked up

@hf Couldn't an attacker just send the hashed token (from DB) as the non-hashed token (in Request) and mint new tokens with it?

Yes, that is a good catch (tho only valid in the backward compatibility period)... it's an easy fix though if the hashed tokens are stored with some prefix in the database. For example, H:<hash> then the lookup would fail because let's say your scenario happens:

1. Attacker steals hashed value h = hash(r)
2. GoTrue receives h and looks up token = h OR token = 'H:' + hash(h).

Since it's practically impossible to have an already existing token with the value of h and hash(h) can't ever be r, the attack wouldn't work.

I'll amend the code to include the prefix.

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Obviously this only works if any H: prefix is stripped before doing the lookup.

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Fixed and added test cases for it.

[hf]

Old PR.