A warrant canary is a method by which a communications service provider aims to implicitly inform its users that the provider has been served with a government subpoena despite legal prohibitions on revealing the existence of the subpoena. The warrant canary typically informs users that there has not been a court-issued subpoena as of a particular date. If the canary is not updated for the period specified by the host or if the warning is removed, users might assume the host has been served with such a subpoena. The intention is for a provider to passively warn users of the existence of a subpoena, albeit violating the spirit of a court order not to do so, while not violating the letter of the order.
Some subpoenas, such as those covered under 18 U.S.C. §2709(c) (enacted as part of the USA Patriot Act), provide criminal penalties for disclosing the existence of the subpoena to any third party, including the service provider's users.
National Security Letters (NSL) originated in the 1986 Electronic Communications Privacy Act and originally targeted those suspected of being agents of a foreign power. Targeting agents of a foreign power was revised in...