From electromagnetic snooping, absolutely they can. The TEMPEST teams checked our installations often from DC to light.
Depends on what you define as "secured" and how much you are willing to spend to achieve it.
Would they admit what if they couldn't what?
Definitely interested in the answer.
The low frequency magnetic fields can be easily stopped with magnetic shielding that's required in the TEMPEST spec. You use very high permeability metals ( MuMETAL) to shunt magnetic field in the ranges that a Faraday cage is ineffective.
That's a completely different question from what you asked originally. And now we need to know whose "they" is.
To some degree this is true by definition. But the attacks are often discovered by non-malicious actors, frequently the people that designed the system in the first place. Often the existence of attack strategies are known long before anyone, on either side, figures out a way to implement a practical attack using that strategy.
While security analyses could be kept private, they often (usually?) aren't. Most things WILL get out, if only because a third party discovers/reinvents the attack and publishes it. Then the original company gets caught with egg on their face when, invariably, it becomes known that they knew about the risk and did not inform their customers in a reasonable amount of time.
