WPMU DEV Dashboard <= 5.0.0 - Authentication Bypass to Arbitrary Plugin Installation (Remote Code Execution) via Forged WDP_AUTH HMAC on ?wpmudev-hub= Endpoint

The WPMU DEV Dashboard plugin for WordPress is vulnerable to Authentication Bypass in all versions up to, and including, 5.0.0. On sites not yet connected to the WPMU DEV Hub — the default state after installation — the site API key that keys the WDP-AUTH request signature is empty, making the signature verified by validate_hash() trivially forgeable; version 5.0.0 additionally removed the replay check in validate_nonce(), and the remote handler is bound to the public init hook with no capability check. This makes it possible for unauthenticated attackers to invoke privileged Hub actions — including installing and activating a plugin from an attacker-supplied URL (resulting in remote code execution), deleting plugins and themes, upgrading WordPress core, or logging in as an administrator via SSO. Sites connected to a WPMU DEV account, which have a non-empty 64-character API key, are not affected.