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.

OAuth2 is a Ruby wrapper for the OAuth 2.0 and 2.1 authorization frameworks, including OpenID Connect (OIDC). From 0.4.0 to 2.0.21, a protocol-relative redirect Location returned to OAuth2::Client#request overrides the request authority, so the bearer Authorization header is sent to an attacker-controlled host, leaking the credential. This issue is fixed in version 2.0.22.