On Thursday, the Senate Judiciary Committee held a session to amend and then vote on the Open App Markets Act, a bipartisan bill designed to rein in the monopoly power of smartphone app stores—mainly those run by Apple and Google. Notably, the bill would require those companies to allow users of Android and iOS devices to download apps from places other than the Google Play store and Apple App Store, a practice called sideloading.

As you might imagine, Apple and Google and the lobby groups that represent them are trying hard (and spending big) to derail the antitrust bill. The bill may be particularly galling to Apple, which likes to keep tight control of the software on its devices, citing concerns over app security and user privacy. Google, by contrast, already allows users to install apps outside of its Play store.

The Judiciary Committee voted to send the bill on to the full Senate, where leadership will now decide whether to initiate debate. The bill has solid bipartisan support and has a real chance of passage. So it’s worth asking what Apple would do if it were required to allow apps on the iPhone from other app stores or marketplaces. What new security features could Apple introduce in iOS to prevent malicious apps from making it onto iPhones?

I asked some Apple pundits and security experts after the hearing Thursday.

“I guess they’d rely on sandboxing to isolate [malicious] apps,” says Charlie Miller, a veteran mobile security engineer who currently works for the autonomous car company Cruise. Sandboxing is a way of isolating a piece of software to prevent it from interacting with other apps or interfering with the operating system—a technique that can minimize the chances of an app doing intentional or unintentional harm.

But sandboxing is possible only after an app is already on the device. “You can install what you want, but iOS can ‘try to’ limit what it can do, i.e., it can’t read your Netflix password,” Miller said in a message. (Miller is coauthor with Dino Dai Zovi of The Mac Hacker’s Handbook.)

If the law passes, the experience of…