WASHINGTON--The Supreme Court Thursday narrowed the scope of a federal anti-hacking law, ruling that it doesn't cover individuals who use their authorized access to obtain information for improper purposes. The decision came in the case of a police officer who ran a woman's license plate in exchange for cash from a man, something that "plainly flouted his department's policy," Justice Amy Coney Barrett wrote for a 6-3 court. But his action didn't violate the Computer Fraud and Abuse Act of 1986, which authorizes up to 10 years imprisonment for anyone who "intentionally accesses a computer without authorization or exceeds authorized access" to obtain computer information. In a 20-page opinion that, among other features, focused on the grammatical significance of the modifier "so," Justice Barrett drew a sharp distinction: The law covers people who, although they are authorized to use a computer system, obtain files that are off-limits to them. But it doesn't reach those who are entitled to access particular information--like Nathan Van Buren, a former Cumming, Ga., police sergeant who was authorized to use the motor-vehicle database--even if they misuse the data they pull.

Imagine you're cruising in your new Tesla, autopilot engaged. Suddenly you feel yourself veer into the other lane, and you grab the wheel just in time to avoid an oncoming car. When you pull over, pulse still racing, and look over the scene, it all seems normal. But upon closer inspection, you notice a series of translucent stickers leading away from the dotted lane divider. And to your Tesla, these stickers represent a non-existent bend in the road that could have killed you. In April this year, a research team at the Chinese tech giant Tencent showed that a Tesla Model S in autopilot mode could be tricked into following a bend in the road that didn't exist simply by adding stickers to the road in a particular pattern.