Taint should be used when the caller of the script is different than the person who owns the script. setuid scripts, CGI scripts, etc. should use it. The environment, files, STDIN, command line arguments, etc are controlled by the caller and cannot be trusted. To allow the user to specify a file name (for example) could allow him to read and/or modify files to which he normally would not have access.

In reply to Re^5: Insecure dependency in open by ikegami
in thread Insecure dependency in open by argv

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