Invoice Ninja before 5.10.43 allows remote code execution from a pre-authenticated route when an attacker knows the APP_KEY. This is exacerbated by .env files, available from the products repository, that have default APP_KEY values. The route/{hash} route defined in the invoiceninja/routes/client.php file can be accessed without authentication. The parameter {hash} is passed to the function decrypt that expects a Laravel ciphered value containing a serialized object. (Furthermore, Laravel contains several gadget chains usable to trigger remote command execution from arbitrary deserialization.) Therefore, an attacker in possession of the APP_KEY is able to fully control a string passed to an unserialize function.
The product deserializes untrusted data without sufficiently ensuring that the resulting data will be valid.