Networkmanager-libreswan: local privilege escalation via leftupdown

A flaw was found in the libreswan client plugin for NetworkManager (NetkworkManager-libreswan), where it fails to properly sanitize the VPN configuration from the local unprivileged user. In this configuration, composed by a key-value format, the plugin fails to escape special characters, leading the application to interpret values as keys. One of the most critical parameters that could be abused by a malicious user is the `leftupdown`key. This key takes an executable command as a value and is used to specify what executes as a callback in NetworkManager-libreswan to retrieve configuration settings back to NetworkManager. As NetworkManager uses Polkit to allow an unprivileged user to control the system's network configuration, a malicious actor could achieve local privilege escalation and potential code execution as root in the targeted machine by creating a malicious configuration.