Improper Control of Generation of Code ('Code Injection')
functions.php in WHMCompleteSolution (WHMCS) 4.0.x through 5.0.x allows remote attackers to trigger arbitrary code execution in the Smarty templating system by submitting a crafted ticket, related to improper handling of characters in the subject field.
CWE-94 - Code Injection
Code injection is a type of vulnerability that allows an attacker to execute arbitrary code. This vulnerability fully compromises the machine and can cause a wide variety of security issues, such as unauthorized access to sensitive information, manipulation of data, denial of service attacks etc. Code injection is different from command injection in the fact that it is limited by the functionality of the injected language (e.g. PHP), as opposed to command injection, which leverages existing code to execute commands, usually within the context of a shell.