Be very wary of using the exec(), system(), passthru(), and popen() functions and the backtick (`) operator in your code. The shell is a problem because it recognizes special characters (e.g., semicolons to separate commands). For example, suppose your script contains this line:

If the user passes the value ” /tmp;cat /etc/passwd” as the $directory parameter, your password file is displayed because system() executes the following command:

In cases where you must pass user-supplied arguments to a shell command, use escapeshellarg() on the string to escape any sequences that have special meaning to shells:

Now, if the user passes ” /tmp;cat /etc/passwd“, the command that’s actually run is:

The easiest way to avoid the shell is to do the work of whatever program you’re trying to call in PHP code, rather than calling out to the shell. Built-in functions are likely to be more secure than anything involving the shell.

Here is the list of of Article in this Series:

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Thanks Kevin Tatroe, Peter MacIntyre and Rasmus Lerdorf. Special Thanks to O’Relly.