MantisBT 2.28.3 and earlier contains a remote code execution vulnerability in the admin "Manage Configuration" feature (adm_config_set.php). When setting a configuration value with a non-string type (integer, float, complex), the value is passed through ConfigParser -> Tokenizer, which calls eval() with a "return;" prefix intended to prevent code execution. However, PHP hoists function and class declarations at compile time, even past a return statement. An attacker can define a class in the eval()'d code that hijacks a class loaded later via PHP's autoloader, achieving arbitrary code execution. This vulnerability requires administrator access to the web UI (adm_config_set.php). The REST API's ConfigsSetCommand does NOT use Tokenizer/eval() and is not affected.