I'm curious about what happened too. Non custodial? Then how? Was the application FOSS? If not, I'd guess they were collecting some kind of data they shouldn't have been.

It's an app on a phone. It's only as self custodied as your front porch is. With entirety of the internet as your neighborhood.

Are you implying that the compromise happened due to an OS or app store level exploit/vulnerability?

If not, I don’t see how an app on a phone is worse than on a desktop.

It's worse because people use more phones then desktops, and they commonly use them for payments which makes them juicier target than desktops.

So the lesson is to use unpopular, yet still secure (i.e. audited and transparently and competently developed) software/hardware?

Yes.

Good luck finding something (lasting) checking all the boxes – sounds like an extremely unstable equilibrium to me.

It's been surprisingly easy for the last two decades. Just use about third most popular Linux.

So not your keys then. Thanks.

Well not anymore...

I don't get your response, though. It was an app where the keys were "yours". The "not your keys" is about custodial services, which this is not.

Well, then how can your coins be stolen without the keys?

If the keys were stolen using a cryptographic trick, how was it done at scale?

It sounds to me like at some moment the keys were exfiltrated.

So "not your keys, not your coins" stands. If you don't have strict control over the keys you don't have strict control over the coins.

Sounds like you're more advocating for cold storage. You could be right, but even the most secure software wallet is only as secure as the computer it is running on.