The Weapon: Computer viruses
The Example: Independence Day
It's a strange fact, but true, that humanity's myriad of computer operating systems are - by some weird stroke of intergalactic fate - compatible with 90 percent of alien technologies. Yes, no matter how far advanced your hive ship is, no matter how superior to human technology it might be, it almost certainly run on the same operating system as Jeff Goldblum's 1996 laptop. And while you might think that your firewalls are secure, they will prove no match for any computer virus that humanity can create.
Of course, a large part of the reason for this is that no species but humanity would ever dream of creating anything as useless as a computer virus, but then they are not considered a plague on the galaxy for nothing. We advise protecting your network with passwords and generally adopting humanity's own security measures against any computer-based attacks before visiting Earth.