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Flaws with fork()
Some processes use
fork()to deal with multiple requests at once, most notably servers.
An interesting side-effect of
fork()is that memory is copied exactly. This means everything is identical - ELF base, libc base, canaries.
This "shared" memory is interesting from an attacking point of view as it allows us to do a byte-by-byte bruteforce. Simply put, if there is a response from the server when we send a message, we can work out when it crashed. We keep spamming bytes until there's a response. If the server crashes, the byte is wrong. If not, it's correct.
This allows us to bruteforce the RIP one byte at a time, essentially leaking PIE - and the same thing for canaries and RBP. 24 bytes of multithreaded bruteforce, and once you leak all of those you can bypass a canary, get a stack leak from RBP and PIE base from RIP.