(CNN) - Travelers take note. Those free charging stations may not be as safe as one wound think.

The Federal Bureau of Investigation is warning people about "juice jacking" through public charging or USB ports in airports, hotels and malls. 

The FBI office in Denver tweeted Monday that "bad actors" have figured out how to use the ports to put malware and monitoring software on people's phones and other devices. 

Officials advise just carrying your own charger and using a regular electrical outlet instead.