Assuming you are seeing this error message on VaultPress? This is a false positive from Vault Press, it’s been known to happen for this particular class.
The indicated class is called Pimple and is a standard Dependency Injection container provided by Sensiolabs. It’s one of the top 5 most popular DI components available for PHP, in active use on millions of PHP-driven websites and applications. It’s also one of the few DI-containers that supports PHP 5.x which is a desirable feature when using within WordPress. According to Packagist, there are no security issues reported about Pimple. Please follow the links below to learn more about the Pimple DI library.
There are several similar support questions here on the WordPress forums where various plugins has had this issue reported by VaultPress, specifically for Pimple. Best practice seem to be to ask VaultPress to ignore this file and mark it as false positive.
Should you need any more help resolving this with VaultPress, don’t hesitate to get back to us. From now on, we have notifications enabled (sorry about the delay).