A found flash drive is not a gift
Plugging in an unknown drive lets its owner run code on your computer. The same goes for free cables and public charging ports.
Why people leave drives lying around
Dropping a few flash drives in a car park or a lobby is an old and cheap way into an organisation. Curiosity does the rest: people plug the drive in to find the owner or to see what is on it.
Some drives carry infected files. Others only look like storage and behave like a keyboard that types commands the moment it is connected. Antivirus software does not see the second kind at all.
What to do with one
Do not connect it to any computer, including an old one at home. At work, hand it to IT. Anywhere else, give it to the staff of the building or leave it where it is.
Treat promotional drives from conferences and drives that arrive by post the same way. If you did not buy it, you do not know what is inside.
Your own drives, cables and chargers
Keep work and personal drives apart, and encrypt any drive that carries files you would not want a stranger to read. Small drives are lost more often than anything else.
A USB port carries data as well as power. Charge from your own adapter and a wall socket, or use a power bank. A cable that only charges, or a small data blocker, solves the problem with public charging ports.
This note is our own plain-language summary. For the full guidance, read the original: https://www.cisa.gov/news-events/news/using-caution-usb-drives
Next noteDevices on the road
If you did not buy it, do not plug it in.