A warrant canary is a method by which a communications service provider informs its users that the provider has not been served with a secret United States government subpoena.
Secret subpoenas, including those covered under 18 U.S.C. §2709(c) of the USA Patriot Act, provide criminal penalties for disclosing the existence of the warrant to any third party, including the service provider’s users.
A warrant canary is posted by the provider to inform users of dates that they have not been served a secret subpoena. If the canary has not been updated in the time period specified by the host, users are to assume that the host has been served with such a subpoena.
This allows the provider to inform users of the existence of a subpoena passively without disclosing to others that the government has sought or obtained access to information or records under a secret subpoena.