UpdraftPlus: WP Backup & Migration Plugin <= 1.26.4 - Unauthenticated Authentication Bypass via UpdraftCentral udrpc

The UpdraftPlus: WP Backup & Migration Plugin plugin for WordPress is vulnerable to Authentication Bypass in all versions up to, and including, 1.26.4 via the UpdraftPlus_Remote_Communications_V2::wp_loaded function. This is due to insufficient validation of the remote communications message format, where signature verification can be bypassed and unchecked decryption return values collapse to a predictable all-zero encryption key. This makes it possible for unauthenticated attackers to forge arbitrary RPC commands and run them as the connected administrator, such as uploading and activating a malicious plugin, which ultimately leads to remote code execution.