DollarDollar through 4.4 Missing Authorization in /add_expense Balance Tampering

DollarDollar through 4.4 contains a missing authorization vulnerability in add_expense() that allows authenticated users to modify other users' account balances by supplying foreign account_id values. Attackers can submit sequential account_id, destination_account_id, and group_id values to /add_expense to tamper with balances and inject expenses into foreign groups with an arbitrary payer.