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On a long haul flight, I imagine that charging a phone (in flight mode) with the inbuilt USB port on the head rest would be a security risk.

Could I mitigate that risk by taking a regular USB cable and cutting the data (but not the power) cables? Or does the USB protocol need a data handshake to begin charging? Or is there another better solution?

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  • 81
    I can already go and buy power-only USB cables which have no data connection. They are already on the market.
    – Michael Hampton
    May 25, 2016 at 10:11
  • 3
    Brace yourself, DRM (aka protection against malicious chargers) is coming: androidauthority.com/…
    – Noir
    May 25, 2016 at 13:07
  • 35
    @Noir DRM is never the correct answer :/
    – cat
    May 25, 2016 at 15:37
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    @MasonWheeler That is assuming that it is telling the truth or bad guy has not found a zero day flaw that gets around that.
    – DarcyThomas
    May 25, 2016 at 21:21
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    @MasonWheeler I am guessing that it may be possible to finger print a device even in charge mode. A bit paranoid perhaps, but still a (small) security risk.
    – DarcyThomas
    May 25, 2016 at 21:25

9 Answers 9

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Could I mitigate that risk by taking a regular usb cable and cutting the data (but not the power) cables? Or does the usb protocol needs a data handshake to begin charging?

Such a cable does exist, so a data handshake must not be required.

Such cords are discussed on some Stack Exchange sites:

So yes, using such a cable or making one using a DIY approach could mitigate a risk that depends on the 2 data pins. Of course, a different kind of attack where unexpected power is sent, possibly with the intent to damage the device, would still be possible.

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    In my scenario (on a commercial plane) you would need to have a pretty malicious attacker, who if they can't hack you they resort to breaking your toys. Unlikely if they are trying to otherwise keep a low profile.
    – DarcyThomas
    May 25, 2016 at 4:31
  • Usually shorting the two data pins on the phone's side of the cable is enough to allow it to charge.
    – Hitechcomputergeek
    May 25, 2016 at 17:28
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    @called2voyage: That is not quite enough, fuses react very slowly, and if you shove 1kV into it, no fuse will protect you. You need some heavier gear like MOVs and all the standard overvoltage (and under) protection
    – PlasmaHH
    May 25, 2016 at 21:39
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    @called2voyage: It won't work at all. The time a fuse reacts everything is destroyed already. Even with 10V likely the fuse does nothing but the chipset is fried.
    – PlasmaHH
    May 25, 2016 at 21:41
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    @PlasmaHH is right (and probably knows more about these things than I do): You could presumably do something with a Zener diode to shunt the high current caused by the overvoltage, combined with a fuse. But a transorb would probably be better. This has been discussed in How to decide between a TVS or Zener diode for overvoltage protection? at electroni.cs.se
    – Chris H
    May 26, 2016 at 8:28
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USB devices can negotiate the provided current from the host device. If power negotiation is not possible (because you cut the data wires), a maximum of 100 mA is supplied. This means you can charge devices, but it may be slower than usual.

Devices such as LockedUSB do the power negotiation on both sides, while not allowing data to pass between devices. This charges your devices faster, but arguably may be less secure than cut wires.

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    @spectras It's less secure in the sense that you have to trust the creators of the cable to a) be honest and b) have done their job right.
    – Pepijn Schmitz
    May 25, 2016 at 9:46
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    In practice, quite a lot of dumb chargers don't do the negotiation and will happily let you pull 2A with no data lines.
    – pjc50
    May 25, 2016 at 9:53
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    Shorting out the data pins should trick the phone into believing it's connected to a 2 amp charger and is quite effective. If a host device really can't supply 2A its overcurrent protection would just shut it down preventing any damage. So it's a really good, simple and cost-effective solution compared to making a "firewall" that allows power negotiation to pass through while denying everything else.
    – André Borie
    May 25, 2016 at 12:00
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    Note that most hosts ignore the spec and supply at least 500mA when no negotiation is done. On the other hand, some of the high power charging standards require specific resistance between the data wires to indicate the power capability of the supply. Thus shorting the lines incorrectly can cause damage. Better just leave the pins floating.
    – billc.cn
    May 25, 2016 at 12:30
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    @PepijnSchmitz> in the device he linked, there is not data wire at all. Seems dubious that they could wire one accidentally. And for the trust part, that's true with any usb cable anyway, a usb controller with some micro chip easily fits inside the plug itself.
    – spectras
    May 25, 2016 at 17:57
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Although not entirely in line with the OP's question, I wanted to share a different solution.

You could take a powerbank with you. Small powerbanks capable of charging your phone at least 1 time don't take up too much space. You can charge the powerbank at the airport terminal, in your hotel, or even on the flight (as there's no risk for malicious data affecting the powerbank). When you need to charge your phone, you can use your freshly recharged powerbank to fill it up.

Another advantage of this solution is that you have a powerbank with you on your trip, should you ever need to recharge your phone in an area where no power outlet is available, or when you don't have your charging cable with you. You can even use the powerbank in other areas where you don't trust a USB outlet.

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    Of course, if you are really paranoid, you wouldn't charge your powerbank from an untrusted USB port, since a compromised powerbank could potentially compromise any device plugged into it.
    – MJeffryes
    May 25, 2016 at 10:05
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    @MJeffryes If you are really paranoid, you don't use a mobile phone.
    – gerrit
    May 25, 2016 at 10:55
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    @gerrit - If you've gotten that paranoid, holing up in an embassy or hiding in Russia may be your only hope...
    – Richard
    May 25, 2016 at 12:24
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    @MJeffryes you can buy powerbanks with no capability to run code - they're the cheap ones.
    – Chris H
    May 26, 2016 at 8:30
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    Just be aware that airlines and airport security are getting picky about power banks. Last time I flew they were not allowed in the checked luggage, but ok in the carry-on (in limited numbers). Badly designed ones are a potential fire hazard.
    – Tor Klingberg
    May 26, 2016 at 13:10
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Modern smart devices are power hungry machines that require frequent charging. This problem is usually solved by bringing along portable power chargers that are expensive and bulky. As a solution to the power problem, many public facilities or events are starting to provide booths with USB ports for free charging. However, charging smart devices on unknown / public USB ports will make your device vulnerable to “juice jacking”, which allows unauthorized users access to data on your phone. Additionally, smart devices can be infected with malware through these unidentified USB ports.

The Solution  A “neutered” USB cable that allows the charging current to pass through but physically disabling all forms of data transmission. This essentially blocks out all forms of communication between the smart device and the source of power, regardless whether it is a computer or an adapter plugged into a wall socket. This can be achieved by physically disconnecting the data transmission wires between the device and the source of power. Doing this on a standard USB cable requires some technical knowledge and makes for an unsightly cable. This is where a USB condom comes in. Putting one on at the end of a standard USB cable will block the data transmission lines between the device and the power source.

How is it Made?  Solder the first and fourth pins of the male drive to the female drive and you have a USB condom that works with most devices. //For the device to work with iPhones, you’ll have to include an additional circuit board to fake a voltage charge.

Note: The current design shown above works for all devices, even iPhones!

http://m.instructables.com/id/Making-a-USB-Condom/?download=pdf

Maker Credit: Tan Guo Wei, Creative Technologist at Metalworks by Maxus

http://m.instructables.com/id/Making-a-USB-Condom/

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  • I think the only part of that which wasn't in the OP is "an additional circuit board is necessary".
    – JDługosz
    May 28, 2016 at 2:08
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A slightly different take, but you can use a trusted wall-to-USB charger. This is the approach I take when charging from a public/shared facility.

Most (if not all) flights I've been on with power, offer both a wall-compatible outlet as well as USB.

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Yes, there is a good solution. The Portpilot is a USB power analyzer which gives you control over locking down data and checking modes, http://portpilot.net/. Shoutout to Hak5, it is available at http://hakshop.myshopify.com/products/portpilot. Not affiliated in any way, but it is on my wish list.

Ok, I read the question again. If you want to dissect the cable and cut parts of it I guess that would work. If you want a solution that works with any USB cable and any device, I would vote for Portpilot.

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Most (all?) phones will not have any "data" connection without specifically having been authorised by an affirmative action by the user, e.g. unlocking the phone and accepting a dialog.

So the use of some special (expensive) "secure" USB charging cable is not required.

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    I didn't downvote, but I disagree. Perhaps this is the trend on newer and more expensive devices, but no phone I have ever owned had this feature. If a users phone has this feature, and they frequently attach their phone to their laptop to transfer data, they might turn this feature off to avoid the hassle of answering "allow" all the time, and then promptly forget the phone had that feature. Also, some phones that may have this feature may have a setting to control this that is defaulted to off and the user may not know the feature is even available.
    – Kevin Fegan
    May 28, 2016 at 0:24
  • It's a per-device nag and as far as I know you can't turn it off, but I suppose on Android there would no doubt be some way to disable it.
    – Coxy
    May 28, 2016 at 7:08
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    @Coxy Your phone can still be fingerprinted with USB. No bueno.
    – Navin
    May 30, 2016 at 6:50
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    This feature comes standard in the stock ROM on Android.
    – Ajedi32
    Jun 1, 2016 at 18:59
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TL;DR: yes, but connect a small resistor between the data lines at the host end.

USB protocols do support power delivery without data lines. According to the Wikipedia article,

The charging device identifies the type of port through non-data signalling on the D+ and D− signals immediately after attach. A [dedicated charging port] simply has to place a resistance not exceeding 200 Ω across the D+ and D− signals.

Without the connection between the data pins, the charging port would supply only 100 mA (a low-speed single unit load - the initial non-negotiated supply), but with this modification (at the power supply end), you should get at least 1.5 A and (at the port's discretion) up to 5A without further action.

For further information, you'll want to consult USB Battery Charging Specification (sorry, that's a Zip archive containing PDFs). There are higher-current modes that may be enabled by FSK signalling on the power lines themselves (USB Power Delivery), but you can safely ignore those.

Note that in this answer, I'm only addressing data risks, that might subvert your device without your knowledge. You are still subject to the usual Denial of Service risks such as over-voltage and reverse voltage, but you will obviously at least know if your device is destroyed by the outlet. I'm assuming you have decent backups of your device (including any removable media you were foolish enough to leave inserted), and that you're not someone who is likely to be specifically targeted - if a charging port is indiscriminately destroying users' devices, it will get noticed!

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I've seen one Nokia phone that would charge via USB only when connected to PC, but in most cases it would just work with data wires cut
But there are other risks involved: unstable power source may damage your device, even if it is not intended to be malicious.
To be 100% sure I'd recommend using the powerbank solution, effectively having safety buffer between the pricey device and untrusted power (and possibly data) source

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  • Surely an unstable powersupply would be a risk (money, not security) even when using with the data cables intact. I.e., what muggle would normally use. (And I am not aware of that being a major problem). A power brick could also have stability problems (although probably less so )
    – DarcyThomas
    May 25, 2016 at 21:16
  • @DarcyThomas well yes, but you could always test your powerbank beforehand, and also it protects you from intentionally malicious power sources, or at least the more valuable hardware, as powerbank could get fried or even explode (but that's better than explosion of your flagship phone I think) if connected to something designed to damage stuff
    – zakius
    May 27, 2016 at 8:18
  • While the personal loss from the powerbank's explosion might be less than from that of the phone, the explosion itself is likely to be far worse as its size will generally be proportional to the capacity of the power cells, and most powerbanks have more capacity than most smartphone batteries.
    – Dan Henderson
    May 27, 2016 at 13:13

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