Artificial intelligence isn’t just for drafting essays and searching the web. It’s also a weapon. And on the internet, both the good guys and the bad guys are already using it.

Offense: Bots and algorithms perpetrate much of the world’s cybercrime. Con artists use them to generate deepfakes and phishing scams. Want malware to steal someone’s data? A chatbot can write the code. Bots also cook up disinformation. As Israel and Iran fired missiles at each other last month, they also flooded the internet with A.I.-powered propaganda.

Defense: Cybersecurity companies use A.I. to intercept malicious traffic and patch software vulnerabilities. Last week, Google announced that one of its bots had found a flaw in code used by billions of computers that cybercriminals wanted to exploit — likely the first time A.I. has managed such a feat.

Cybersecurity used to be slow and laborious. Human hackers would concoct new attacks, and then security companies would tweak their defenses to parry them. But now, that cat-and-mouse game moves at the speed of A.I. And the stakes couldn’t be higher: Cybercrime is expected to cost the world more than $23 trillion per year by 2027, according to data from the F.B.I. and the International Monetary Fund. That’s more than the annual economic output of China.

Today, I explain what the arrival of A.I. hacking means for the internet — and the billions who use it every day.

The siege

The newest cybercriminals are robots. They write with flawless grammar and code like veteran programmers. They solve problems in seconds that have vexed people for years.