The public-facing product registration endpoint server responds differently depending on whether the S/N is valid and unregistered, valid but already registered, or does not exist in the database. Combined with the fact that serial numbers are sequentially assigned, this allows an attacker to gain information on the product registration status of different S/Ns.
Workaround:
The product behaves differently or sends different responses under different circumstances in a way that is observable to an unauthorized actor, which exposes security-relevant information about the state of the product, such as whether a particular operation was successful or not.