Whistle blower

one who reveals a wrongdoing or informs authorities about a misconduct, one who discloses the illegal or immoral acts of another person, informer

A whistleblower is an employee, former employee, or member of an organization, especially a business or government agency, who reports misconduct to people or entities that have the power and presumed willingness to take corrective action. Generally the misconduct is a violation of law, rule, regulation and/or a direct threat to public interest, such as fraud, health/safety violations, and corruption. One of the most publicized whistleblowing cases involved Jeffrey Wigand, who exposed the Big Tobacco scandal, revealing that executives of the companies knew that cigarettes were addictive while approving the addition of known carcinogenic ingredients to the cigarettes. Wigand's story was the basis for the 1999 movie The Insider. Another famous whistleblower is Dr. Frederic Whitehurst, who exposed irregularities at the Federal Bureau of Investigation's  FBI Crime Lab.
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Noun
1. an informant who exposes wrongdoing within an organization in the hope of stopping it; "the law gives little protection to whistleblowers who feel the public has a right to know what is going on"; "the whistleblower was fired for exposing the conditions in mental hospitals"
(synonym) whistle-blower, whistleblower
(hypernym) informant, source

Noun
1. an informant who exposes wrongdoing within an organization in the hope of stopping it; "the law gives little protection to whistleblowers who feel the public has a right to know what is going on"; "the whistleblower was fired for exposing the conditions in mental hospitals"
(synonym) whistle blower, whistleblower
(hypernym) informant, source

A person who has knowledge of fraudulent activities inside a firm or government agency, who is protected from the employer's retribution by federal law.