What is a Keystone?… From Vault 7:

Angelfire

31 August, 2017

Today, August 31st 2017, WikiLeaks publishes documents from the Angelfire project of the CIA. Angelfire is an implant comprised of five components: Solartime, Wolfcreek, Keystone (previously MagicWand), BadMFS, and the Windows Transitory File system. Like previously published CIA projects (Grasshopper and AfterMidnight) in the Vault7 series, it is a persistent framework that can load and execute custom implants on target computers running the Microsoft Windows operating system (XP or Win7).

Solartime modifies the partition boot sector so that when Windows loads boot time device drivers, it also loads and executes the Wolfcreek implant, that once executed, can load and run other Angelfire implants. According to the documents, the loading of additional implants creates memory leaks that can be possibly detected on infected machines.

Keystone is part of the Wolfcreek implant and responsible for starting malicious user applications. Loaded implants never touch the file system, so there is very little forensic evidence that the process was ever ran. It always disguises as "C:\Windows\system32\svchost.exe" and can thus be detected in the Windows task manager, if the operating system is installed on another partition or in a different path.