The capability is named Intel Visualization of Internal Signals Architecture (Intel VISA) and it is a utility included in modern Intel chipsets to help with testing/debugging during manufacturing. It is included with Platform Controller Hub (PCH) chipsets, is a part of modern Intel CPUs, and functions much like a logic signal analyzer. It is able to collect signals sent from internal buses and peripherals to the PCH and CPU. Effectively this means unauthorized access to the VISA would expose ANY data to examination by an unscrupulous person to intercept and collect data from the computer memory and function at the lowest possible level.
The real question is: Is there a real threat? The researchers said they have several methods of enabling Intel VISA and capturing data, including the secretive Intel Management Engine (ME) which has been housed in the PCH since the release of the Nehalem processors and 5-Series chipsets. But there are caveats. On the positive side, Intel has not publicly disclosed the feature and is only shared with others under a non-disclosure agreement. Additionally, the feature is disabled by default, so attackers must first figure out how to enable it before exploiting it. On the negative side, the researchers found a way to disable Intel VISA using an older Intel ME vulnerability. Intel released a firmware patch that fixes that particular vulnerability in 2017 (INTEL-SA-00086), but unless there was an explicit update to the firmware (it’s not correctable via OS update) the CPU remains affected.
It’s worth noting that if the attacker has exploited the Intel ME vulnerability, they are well into your system and there is little additional capability offered via VISA that they don’t already have. But back on the negative side, if an attacker finds an alternate to enable VISA, that could indeed become a new attack vector.
The researchers indicated that they know three alternate ways to enable VISA, which they revealed in the presentation slides (link below). The bigger question remains: what other secret or undocumented modes/ features lie in Intel's CPUs? Intel may try to keep them secret from the public, but security through obscurity is no paradigm to follow.
As the researchers proved, people will uncover those secret features, and some will abuse them.