Stack Canary.

A stack canary is a secret value placed between a function's local buffers and its saved return address to detect stack buffer overflows before they hijack control flow. It belongs to the Application Security category of cybersecurity.

A stack canary is a secret value placed between a function's local buffers and its saved return address to detect stack buffer overflows before they hijack control flow.

Stack canaries, introduced by Crispin Cowan and the StackGuard team in 1998, are random values written between local variables and the saved return address on a function's stack frame. The compiler emits code on entry to set the canary and on exit to verify it; a mismatch aborts the program before the corrupted return address is used. Modern toolchains call this -fstack-protector or /GS, and they use a per-process random value plus a terminator byte (often 0x00) to defeat overflows that include the canary in their payload. Canaries complement but do not replace ASLR, DEP, CFI, and memory-safe languages.

Defences for Stack Canary typically combine technical controls and operational practices, as detailed in the full definition above.

Common alternative names include: StackGuard, Stack cookie, GS cookie.