With the eval() function, PHP allows a script to execute arbitrary PHP code. Although it can be useful in a few limited cases, allowing any user-supplied data to go into an eval() call is just begging to be hacked. For instance, the following code is a security nightmare:

This page takes some arbitrary PHP code from a form and runs it as part of the script. The running code has access to all of the global variables for the script and runs with the same privileges as the script running the code. It’s not hard to see why this is a problem—type this into the form:

Never do this. There is no practical way to ensure such a script can ever be secure. You can globally disable particular function calls by listing them, separated by commas, in the disable_functions configuration option in php.ini. For example, you may never have need for the system() function, so you can disable it entirely with:

This doesn’t make eval() any safer, though, as there’s no way to prevent important variables from being changed or built-in constructs such as echo() being called.

Note that the preg_replace() function with the /e option also calls eval() on PHP code, so don’t use user-supplied data in the replacement string.

In the case of include, require, include_once, and require_once, your best bet is to turn off remote file access using allow_url_fopen.

Any use of eval() and the /e option with preg_replace() is dangerous, especially if you use any user-entered data in the calls. Consider the following:

It seems pretty innocuous. However, suppose the user enters the following value:

In this case, both the expected command and the one you’d rather avoid will be executed. The only viable solution is to never give user-supplied data to eval().

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.