The FBI warned that public charging stations with USB ports could download malware onto the devices of unsuspecting users.

Malicious actors can use USB ports, which are often available at locations such as airports and public transit for travelers running low on charge, as mechanisms to add malware to unsuspecting users’ phones or computers, thereby permitting criminals access to passwords or transactions.

“Avoid using free charging stations in airports, hotels or shopping centers,” the FBI’s field office in Denver warned on social media last week. “Bad actors have figured out ways to use public USB ports to introduce malware and monitoring software onto devices. Carry your own charger and USB cord and use an electrical outlet instead.”