The issue was discovered by two security researchers bug hunting for security flaws in Intel's Management Engine. While digging around through the tens of ME configuration options, the two spotted a feature that they believed could lead to problems, if left enabled by accident on Intel chips.
Manufacturing Mode is an Intel ME option that desktop, server, laptop, or mobile OEMs can enable for Intel chips and use it for testing ME's remote management features. As the name implies, this configuration option should be enabled only on manufacturing lines to enable automated configuration and testing operations but disabled before shipping the end product. Leaving an Intel ME chip in Manufacturing Mode allows attackers to change ME settings and disable security controls, opening a chip for other attacks.
The two researchers said they only tested Lenovo and Apple laptops for the presence of Intel ME chips in Manufacturing Mode. Other laptops or computers may also be affected. Instructions on how to spot Intel ME chips in Manufacturing Mode and how to disable it are available. Apple fixed the bug in June, with the release of macOS High Sierra 10.13.5, and Security Update 2018-003 for macOS Sierra and El Capitan.
Apple did not make any noise about the matter, but the Tame Apple Press insists it has somehow been fixed.