|Secure Programming for Linux and Unix HOWTO|
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Ensure that any call out to another program only permits valid and expected values for every parameter. This is more difficult than it sounds, because many library calls or commands call lower-level routines in potentially surprising ways. For example, many system calls are implemented indirectly by calling the shell, which means that passing characters which are shell metacharacters can have dangerous effects. So, let's discuss metacharacters.