Is it dangerous to share a USB flashdrive?
 

Hello!

Recently I've watched a movie "Blackhat" where just by inserting a USB flashdrive he could infect a host PC with some malicious program. Is it true that it's not safe to share USB flashdrives just like that? If yes, how come a "virus" can initiate itself in a host machine without being executed by a user? In other words, doesn't it need to be clicked on first?

On older Windows systems USB storage devices and optical media are scanned when inserted for the file autorun.ini. If it is found it is parsed and the program specified in it will be started by the OS. This feature can and has been abused by malware, so it should be disabled. Newer Windows systems ask the user if that program should be run.
I don't know how the large Linux DEs handle this, so if they have this feature you might want to disable it.

That was such a stupid movie.

However it was rumoured stuxnet was introduced via infected USB sticks strategically left lying around. And you don't get much more sophisticated than that little contribution from the government.

Yes, it's dangerous. The problem is that "U" in USB. It's universal, and supports a wide variety of devices. Those devices identify themselves through firmware contained on the device, and the firmware on a USB flash drive (there's quite a bit of it) can be reprogrammed to look like a different device, for example a keyboard, which, after being recognized as such by the system, starts sending commands which will be acted on just as though you had typed them at your keyboard. See Turning USB peripherals into BadUSB and other hits from https://www.google.com/search?q=usb+...rmware+exploit

These days, USB flash drives have to treated like hypodermic needles -- if it has touched anyone else, it's not safe for you any more.

Okay, thank you all for your replies!