According to the Federal Trade Commission, a creditor can only garnish a bank account after the creditor receives a judgement against the debtor. To receive a judgement, the creditor must first sue the debtor, and a court must award the creditor the right to garnish the bank account.
Illinois Legal Aid notes that there are certain types of income that are not subject to bank garnishment even when a judgement is awarded to a creditor. These include state and federal unearned income, such as Social Security, unemployment benefits and welfare. Before these types of income can be garnished by a creditor, the bank must confirm that there is at least two months' worth of benefits in the account to serve as a lifeline for the debtor. If there are funds above two months of benefits, then those amounts may be garnished from a bank account by court order.