> 1. What you're describing would have to happen on the f-droid app, but the vulnerability seems to be on fdroidserver?
As far as I understand (I'm not an expert on F-Droid), this validation happens on the server side. The (repo) server verifies that the signature matches that of the first version it saw, the phone (when installing the APK) verifies that the signature matches that of the first version it saw.
Android keeps the fdroidserver honest for upgrades, fdroidserver provides an additional bootstrap point for Android's trust.
> 2. Even if this actually affected the f-droid app, what you described seems like a very modest increase in security. The attack this prevents (ie. a compromised server serving a backdoored apk with a different signature) would also raise all kinds of alarms from people who already have the app installed, so practically such an attack would be discovered relatively quickly.
Sure, it's the difference between "automated tooling sees the problem immediately and addresses it proactively" vs "hopefully someone will ring the alarm bell eventually".
> Doesn't f-droid perform the build themselves and sign the apk using their own keys? They might be pulling from the upstream repo, but that's in source form, and before apks are signed, so it's irrelevant.
According to the linked blog post, not anymore. Apparently, these days they serve the author's original APK, but after verifying that they can rebuild it (modulo the signature itself).