The canary is determined when the program starts up for the first time which means that if the program forks, it keeps the same stack cookie in the child process. “Hey, can you send me 1000000 bytes? thx!”.A method around this would be to partially overwrite and then put the NULL back or find a way to leak bytes at an arbitrary stack offset.Ī few situations where you might be able to leak a canary: However Linux makes this slightly tricky by making the first byte of the stack canary a NULL, meaning that string functions will stop when they hit it. If we can read the data in the stack canary, we can send it back to the program later because the canary stays the same throughout execution. However, leaking the address and bruteforcing the canary are two methods which would allow us to get through the canary check. Stack Canaries seem like a clear cut way to mitigate any stack smashing as it is fairly impossible to just guess a random 64-bit value. Prior to a function return, the stack canary is checked and if it appears to be modified, the program exits immeadiately. Stack Canaries are a secret value placed on the stack which changes every time the program is started.
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