Full Report
PHP is full of security footguns. Many of them have been fixed, such as 'abc' == 0. However, there are some that still remain. In the case of Craft CMS, a popular PHP based CMS, there are still some footguns lurking. PHP can be used both on the command line and as a web server. So, the global variables $_SERVER['argc'] and $_SERVER['argv']. If you run this as a web server then PHP will take argv from the query string, if you have enabled this! In the case of PHP, this functionality was on in the default Docker container. In the case of Craft CMS, there is some code that runs for parsing CLI functions. If the argv entry is empty, then it skips this though. Normally from a website, this would it would be empty. Like many programs, there is a lot of option processing when running the application. Here's the crazy bug though: since $_SERVER['argv'] can be controlled from web! This allows an attacker to control the configurations of the running website. For example, --configPath=/aaa will set the configuration path to a non-existent location, remotely. At this point, the vulnerability feels like than RCE but the path needs to be found. They tried some easy wins but no dice. The path that looked interesting was a request to get a file remotely. If a PHP file could be included then this would mean RCE. However, there was a file_exists check before this that prevented using HTTP, PHP or other many other types of files. Upon further inspection, the ftp:// URI supports file existence checks. Using a file here is blocked by the allow_url_include security feature. But, a template CAN be included! So, they created an FTP server with anonymous access and a file called index.twig with {{7*7}} in it. When craft loads the file, it gets evaluated! Craft CMS attempts to sandbox the Twig template renderer. So, simple calls to system are denied. They found that using {{ ['system', 'id'] | sort('call_user_func') }} bypassed the verification, which I don't really understand how it works. Understanding esoteric portions of languages and frameworks can be useful but it's hard to see the payoff until something like this happens sometimes. Great find!
Analysis Summary