Meet Rayhunter:
A New Open Source Tool from EFF to Detect Cellular Spying
At EFF we spend a lot of time thinking about Street Level Surveillance technologies
—the technologies used by police and other authorities to spy on you while you are going about your everyday life
—such as automated license plate readers,
facial recognition,
surveillance camera networks,
and cell-site simulators (. #CSS ).
Rayhunter is a new open source tool we’ve created that runs off an affordable mobile hotspot that we hope empowers everyone,
regardless of technical skill,
to help search out CSS around the world.
CSS
(also known as #Stingrays or #IMSI #catchers)
are devices that masquerade as legitimate cell-phone towers,
tricking phones within a certain radius into connecting to the device rather than a tower.
CSS operate by conducting a general search of all cell phones within the device’s radius.
Law enforcement use CSS to pinpoint the location of phones
often with greater accuracy than other techniques such as cell site location information (CSLI)
and without needing to involve the phone company at all.
CSS can also log International Mobile Subscriber Identifiers (IMSI numbers) unique to each SIM card,
or hardware serial numbers (IMEIs) of all of the mobile devices within a given area.
Some CSS may have advanced features allowing law enforcement to intercept communications in some circumstances.
What makes CSS especially interesting, as compared to other street level surveillance, is that
so little is known about how commercial CSS work.
We don’t fully know what capabilities they have
or what exploits in the phone network they take advantage of to ensnare and spy on our phones, though we have some ideas.
We also know very little about how cell-site simulators are deployed in the US and around the world.
There is no strong evidence either way about whether CSS are commonly being used in the US to spy on First Amendment protected activities
such as protests, communication between journalists and sources, or religious gatherings.
There is some evidence
—much of it circumstantial
—that CSS have been used in the US to spy on protests.
There is also evidence that CSS are used somewhat extensively by US law enforcement,
spyware operators, and scammers.
We know even less about how CSS are being used in other countries,
though it's a safe bet that in other countries CSS are also used by law enforcement.
Much of these gaps in our knowledge are due to a lack of solid, empirical evidence about the function and usage of these devices.
Police departments are resistant to releasing logs of their use,
even when they are kept.
The companies that manufacture CSS are unwilling to divulge details of how they work.
Until now, to detect the presence of CSS, researchers and users have had to either rely on Android apps on rooted phones,
or sophisticated and expensive software-defined radio rigs.
Previous solutions have also focused on attacks on the legacy 2G cellular network, which is almost entirely shut down in the U.S.
Seeking to learn from and improve on previous techniques for CSS detection we have developed a better, cheaper alternative that works natively on the modern 4G network.
https://www.eff.org/deeplinks/2025/03/meet-rayhunter-new-open-source-tool-eff-detect-cellular-spying
