LIFX bulbs act as a device on your existing Wi-Fi network. The security of each bulb is as strong as the security of your Wi-Fi network.
If someone has access to your Wi-Fi network, and the LIFX app on their device, they will have access to control your LIFX. This is only while there device is connected to your home Wi-Fi. When they leave your house, they no longer have access to your bulbs.
We recommend that you make sure your Wi-Fi network is secure. Use WPA or WPA2 security, and you don't share your Wi-Fi details with anyone you don't trust.