FriendsOfFlarum OAuth allows users to log in to Flarum with GitHub, Twitter, Facebook, and other providers. Prior to 1.7.4 and 2.0.0-beta.4, the Discord OAuth provider does not check the verified field returned for an OAuth email before passing the address to Flarum core as trusted through provideTrustedEmail(). When Discord sign-in is enabled, an unauthenticated attacker who knows the email address of a Flarum user can configure a Discord account with that unverified address and a verified phone number, then sign in to cause Flarum to match the trusted address, link the attacker-controlled Discord identity to the existing user, and authenticate as the victim without a password or victim interaction. Exploitation requires that the victim's email address is not already associated with a Discord account, and it can compromise administrator accounts. Other bundled providers were not confirmed to be practically exploitable by this method because their relevant authentication flows return only verified or confirmed email addresses. This issue is fixed in versions 1.7.4 and 2.0.0-beta.4.

Affected: Packagist: fof/oauth

### Impact

An unauthenticated account takeover vulnerability exists in `fof/oauth` when the Discord OAuth provider is enabled.

Discord allows an account to use an unverified email address when its phone number has been verified. During OAuth authentication, Discord may return that email address with `"verified": false`.

Affected versions of `fof/oauth` did not validate this flag and passed the email address to Flarum core as trusted via `provideTrustedEmail()`. Flarum core could then match the trusted email address to an existing user, link the attacker-controlled Discord identity to that user, and authenticate the attacker as the victim.

An attacker who knows a user's email address can therefore take over the corresponding Flarum account without knowing its password or requiring any interaction from the victim. This includes administrator accounts.

#### Preconditions for exploitation

1. Discord sign-in is enabled on the forum. 2. The victim has a Flarum account with a known email address. 3. The victim's email address is not already associated with a Discord account. 4. The attacker creates or configures a Discord account using the victim's email address without verifying it and verifies a phone number instead. 5. The attacker authenticates to the forum using that Discord account.

The issue is specific to Discord's ability to return an unverified email address through OAuth. Other bundled providers were not confirmed to be practically exploitable through the same met…