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''Canaries'' or ''canary words'' are known values that are placed between a buffer and control data on the stack to monitor buffer overflows. When the buffer overflows, the first data to be corrupted will usually be the canary, and a failed verification of the canary data will therefore alert of an overflow, which can then be handled, for example, by invalidating the corrupted data. A canary value should not be confused with a [[sentinel value]].
The terminology is a reference to the historic practice of using [[animal sentinel#Toxic gases|canaries in coal mines]], since they would be affected by toxic gases earlier than the miners, thus providing a biological warning system. Canaries are alternately known as ''stack cookies'', which is meant to evoke the image of a "broken cookie" when the value is corrupted.
There are three types of canaries in use: ''terminator'', ''random'', and ''random [[XOR]]''. Current versions of StackGuard support all three, while ProPolice supports ''terminator'' and ''random'' canaries.
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