better-auth before 1.6.13 (and pre-release builds 1.7.0-beta.0 through 1.7.0-beta.3) fail to validate the scheme of redirecturis registered via the deprecated oidc-provider plugin and the mcp plugin (which wraps the same provider). An attacker can register an OAuth client with a javascript: redirecturi, which the authorization server later returns unchanged in the consent response. If the deployment's consent page navigates the browser to the returned redirectURI (e.g. assigning it to window.location.href), the attacker's JavaScript executes in the authorization-server origin, exposing the victim's session and enabling account takeover.

better-auth versions before 1.6.11 fail to delete cached sessions when removing users via admin, anonymous, or SCIM endpoints when secondaryStorage is configured and storeSessionInDatabase is false. Attackers can reuse deleted user session tokens to maintain authentication for up to seven days after account deletion.