What is a warrant canary?

A warrant canary is a colloquial term for a regularly published statement that a service provider has not received legal process that it would be prohibited from saying it had received. Once a service provider does receive legal process, the speech prohibition goes into place, and the canary statement is removed.

Warrant canaries are often provided in conjunction with a transparency report, listing the process the service provider can publicly say it received over the course of a particular time period. The canary is a reference to the canaries used to provide warnings in coalmines, which would become sick before miners from carbon monoxide poisoning, warning of the danger.