Facebook's questionable research tactics has led Apple to remind them who's boss.

After TechCrunch revealed Facebook misused Apple's Enterprise Developer Certificate program to spy on consumer phone and web activity through its "Research" app, Apple pulled the plug.

Facebook relies on Apple's program to run internal apps on iOS devices, from simple things like bus schedules to communication tools like Workplace and Messenger.

That led to plenty of frustration within Facebook's workplace, as employees were unable to get work done due to the shutdown, as reported in the New York Times.

"After Apple's revocation, employees inside Facebook became furious with the Onavo team, according to four people familiar with the company’s deliberations," the report read.

"Some said they would have to wait weeks to get app updates or changes approved through Apple’s App Store. Several employees in Facebook’s hardware division said they were considering quitting because they could not get any work done."

While Apple has a program for testing apps called TestFlight, Facebook skipped this by asking participants to sideload the "Research" app on their phones.

Apple's Enterprise Developer Certificate program gives permission for Facebook to install its apps on iOS devices, and allows for deeper access than usual. The program is intended for companies to distribute in-house apps among employees, and by using it to deliver apps to the public, Facebook broke Apple's rules.

Apple eventually restored Facebook's access to its Enterprise Developer Certificate program on Thursday afternoon.

Business Insider obtained a leaked memo from Facebook executive Pedro Canahuati, which offers more detail on Apple's power over the social media giant. The memo, however, stops short of admitting Facebook did wrong.

"Apple's view is that we violated their terms by sideloading this app, and they decide the rules for their platform, We've worked with Apple to address any issues; as a result, our internal apps are back up and running," the memo reads.

"Our relationship with Apple is really important — many of us use Apple products at work every day, and we rely on iOS for many of our employee apps, so we wouldn't put that relationship at any risk intentionally."