Confidentiality is one of your basic duties as an assistant. You will have access to company secrets, employee records, and financial information. Failure to keep information confidential will damage your reputation and the reputation of your organization. In certain circumstances, lack of confidentiality breaks the law. HR will have information on this topic. You need to treat all information that you come across at work as a confidential. Do not share information with friends, coworkers, clients, etc.; unless you are specifically told that it is part of the public domain.
Most breaches in confidentiality are not malicious. While it is true that some people are motivated by greed or revenge, a breach of confidentiality may be caused by attempts to impress rivals or clients. Extenuating circumstances may also cause breaches, meaning that accidental slips happen. You need to be careful handling confidential information. Never repeat anything that is related to your job, even when people tell you that they are cleared to know.