WikiLeaks has published the documentation manual for an alleged CIA tool that can track users of WiFi-capable Windows devices based on the ESS (Extended Service Set) data of nearby WiFi networks. The tool’s 42-page manual includes the following image to explain to CIA operatives how the tool works. ELSA works by scanning for nearby networks and collecting their ESS information, which is unique for each network. ESS data includes MAC address, SSID, signal strength, MAC address and signal strength.
Source: https://www.bleepingcomputer.com/news/security/vault-7-cia-malware-for-tracking-windows-devices-via-wifi-networks/