https://www.cnn.com/2018/06/01/politics/cell-phone-spying-dhs-wyden-study/index.html
In the 2017 study, DHS "did observe anomalous activity that appeared consistent with IMSI catcher technology within the [National Capitol Region], including locations in proximity to potentially sensitive facilities like the White House," but had "neither validated nor attributed such activity to specific entities, devices, or purposes," Krebs said.
"It is my understanding that relevant law enforcement and counterintelligence agencies conducted further investigation and determined some detected signals were emanating from legitimate cell towers," he continued.
ISMI devices act essentially as fake cellphone towers and, as mobile devices connect to them, they are able to snoop on the traffic that goes through. Their legal use in the US is limited to official law enforcement and public safety entities, but the technology poses significant benefit for foreign espionage. Counterintelligence experts and DHS officials have conceded that the technology could pose privacy and national security concerns if employed by foreign adversaries.
In a statement Friday, Wyden demanded telephone companies and the Federal Communications Commission act to curtail the potential targeting.
https://www.livescience.com/62215-what-are-cell-site-simulators.html
People who want to protect their communications can use end-to-end encryption with apps like Signal or WhatsApp, but people can do little to protect themselves against the location tracking of cell-tower simulators, Quintin said.
While law enforcement agencies are the primary lawful users of these devices, embassies in Washington, D.C., which are on "sovereign soil," can also lawfully install a spying device.
Every embassy "worth their salt" has a cell-tower simulator installed, Aaron Turner, president of the mobile security consultancy IntegriCell, told the AP. The Russians have simulators that can track people from a mile (1.6 kilometers) away, Turner told the AP