Comment Old-school methods (Score 1) 40
This whole method wouldn't work on any modern device following Google's recommendations for partition setup and security. On such a device you wouldn't be able to modify the system partition even with root and if you did it wouldn't boot.
install-recovery hasn't been supported since several Android versions, and it together with chattr are very old-school ways of root survival. Every non-malicious jailbreak app covering the "vulnerable" Android version uses them, they use it to keep the device jailbroken through OS updates.
This is root straight from the book. If it took "security researchers" this long to figure it out, they know preciously little about Android.
A full device reflash from the PC (including data wipe) would also nuke this, no questions asked. If the user installs the malicious app again though...