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 verifying that the resulting data will be valid.
It is often convenient to serialize objects for communication or to save them for later use. However, deserialized data or code can often be modified without using the provided accessor functions if it does not use cryptography to protect itself. Furthermore, any cryptography would still be client-side security – which is a dangerous security assumption. Data that is untrusted can not be trusted to be well-formed. When developers place no restrictions on “gadget chains,” or series of instances and method invocations that can self-execute during the deserialization process (i.e., before the object is returned to the caller), it is sometimes possible for attackers to leverage them to perform unauthorized actions, like generating a shell.