Insecure direct object reference (IDOR) vulnerability in the MatchPoint API, which relies on the `actingasuser` HTTP header without verifying the requester's authorization for the specified user. Manipulation of sequential identifiers allows access to personal, banking, and financial data, private messages, and QR codes for physical access, as well as the ability to modify third-party accounts, change passwords, impersonate users, and transfer virtual balances.