undefined

points

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The binary "zip" isn't the exploit, it's the shellcode. The exploit is the rest, which changes the code of a SUID executable (su).

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I have a C translation here that should be pretty readable https://github.com/tgies/copy-fail-c

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The call to zlib basically overwrites a minimal ELF into a portion of the `su` binary, which exceve's /bin/sh.

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To be specific, the zlib'd binary basically does this (except that it directly uses Linux syscalls to do so rather then C wrappers):

    setuid(0);
    execve("/bin/sh", NULL, NULL);
    exit(0);