A warrant canary is a public statement posted by a social media platform to affirmatively state that the company has not received a national security letter (NSL) demanding data about the company's users.
The USA Patriot Act authorizes the federal government to issue national security letters without prior court approval. A company can challenge an NSL in court, but it is illegal for someone who has received an NSL to reveal it until after any court challenge is completed. This is the opposite of a tranditional search warrant which is an application to a court that is a matter of public record and can be discussed with anyone, including the object of the investigation. However, the law does not prohibit companies from stating that they have not received an NSL. So many companies are posting these statements (called "a warrant canary") with the understanding that if the company stops making the statement, then readers should assume that it has received an NSL. For example, And Reddit’s 2014 transparency report stated:
national security requestsThe failure to make a similar claim for 2015 indicates that Reddit received an NSL.
As of January 29, 2015, reddit has never received a National Security Letter, an order under the Foreign Intelligence Surveillance Act, or any other classified request for user information. If we ever receive such a request, we would seek to let the public know it existed.reddit supports reform of government surveillance programs and joined 86 other groups by signing an open letter to Congress in 2013.
The phrase is based on the way that coal miners protected against gas poisoning. They would bring a canary in a cage with them into a coal mine. The bird would stop chirping before any miners would notice symptoms of gas poisoning themselves, so the canary served as an indirect early warning system.