cnet.com says:
Even in Snowden's room, the group took precautions not to be overheard. Greenwald and Poitras would remove the batteries from their mobile phones and put them in refrigerator of Snowden's minibar (...)
Is there any security reason they would do that?
With its insulated walls and rubber door seals, a refrigerator is the most soundproof box commonly found in ordinary living spaces. If it's running, even better, it provides white noise. If one was worried about listening devices, then a fridge would be a reasonable and available place to stash them. And Snowden has alleged the NSA can do lots of things even with a phone that's turned off.
Minor update: Out of curiosity, I put my phone in the fridge, and later in the freezer, with batteries still inserted. In both cases I was able to call it quite easily - the refrigerator did not act as a Faraday cage - but the ringtone volume was noticeably subdued. So I do think the fridge in the quoted story was a soundproofing issue, not a Faraday Cage as has been suggested.
I know it's just an experiment of one, but it was a fun experiment :)
Even more minor update: I was curious and checked... my microwave, which is a Faraday Cage, doesn't block cell signals either (good explanation here).
I suspect that the passage is intended to say that they took the battery out of their phone, and then put both the battery and the phone in the fridge.
Assuming that's right, most likely there are two threats this is defending against:
Taking the battery out of the phone ensures that the phone is powered off. This defends against a malicious application or malicious code in the phone's firmware. (The concern is that someone might have attacked the software running on the phone and then installed malicious software or a malicious application that runs in the background and uses the microphone to record conversations. Taking the battery out of the phone powers off the phone's processor and prevents any such malicious code from running.)
Putting the items in the fridge provides soundproofing. This defends against a bug that was surreptitiously added to the phone. (The concern here is that an attacker might have, at some point in the past, planted a bug inside their phone. Such a bug might have its own battery and its own circuitry and its own microphone, and be recording their conversations. Taking the battery out of the phone will not stop such a bug. However, putting the components into the fridge will provide sound isolation, because the fridge is relatively soundproofed and also because the fan in the fridge provides a bit of additional noise.)
In other words, the first is about malicious software; the second is about malicious hardware. If you were only concerned about malicious software, it would be enough to take the battery out of the phone without putting it into the fridge -- but putting both of them into the fridge covers both bases. In their setting, this kind of paranoia makes sense.
Note that it wouldn't be enough to just put the battery into the fridge, but not the phone. The problem with that is that it wouldn't stop a bug that was surreptitiously planted inside their phone. That's why I suspect the quote meant to say that they took the battery out of their phone and then put both the battery and the phone into their fridge. As it stands, the wording of the quote is a bit ambiguous, so I realize I'm making a particular interpretation of the quote -- but given the two kinds of threats, I think my interpretation makes sense.
Because they thought the refrigerator would act as a Faraday cage. Completely powering off their phones and placing them in a thick metal box gave them the peace of mind that those devices were not transmitting anything they were saying, either through the stock phone, or a bug placed by a government agency on the battery pack or the phone.
Stainless steel is quite effective at blocking cell tower signals, and many other frequencies used to transmit data across distances. Heather Murphy writes:
It is the materials that make up refrigerator walls that could potentially turn them into anti-eavesdropping devices...Refrigerators made from metal with thick insulation could potentially [block noise and cell transmissions].
On a less serious note: Placing your battery in the fridge helps it retain a charge better because it reduces the chemical reactions that cause passive drain. Could they have been environmentally minded? :-)
Its not just malware. It has been claimed that mobile telephones are subject to an attack in which they can be remotely activated from the cell tower to relay audio back to the tower but does not ring or change the appearance of the phone.
Their concern was that their phones may have been infected with malware which would record sound and/or video of their conversations. Fridges are insulated and have ambient noise from the machinery running, so would effectively block sound from conversations being recorded.
I can't think of anything security-related with using fridge as battery storage, however, I do this myself. The reason for this is that Li-ion batteries live longer when they are kept reasonably cold:
Table 3: Estimated recoverable capacity when storing Li-ion for one year at various temperatures
Temperature 40% charge 100% charge
0°C 98% 94%
25°C 96% 80%
40°C 85% 65%
60°C 75% 60% (after 3 months)
Of course, this only makes sense if the battery is unused for a long period of time (a month or more), storing the battery in the fridge overnight only to use it again in the morning will do more harm than good due to moisture.
