Are NFC hardware tokens (Yubikey, NitroKey) less secure than USB-only ones? If so, how to mitigate?

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Yubikey and NitroKey offer NFC and non-NFC versions of their flagship hardware security tokens (HSTs).

NFC is convenient, but can under some circumstances send e.g. challenge-response exchanges in clear text.

Smartcards using RFID, a similar though not identical protocol, can be queried from ~100cm away.

  • Are there other ways are NFC HSTs are known to be more risky than their non-NFC counterparts?
  • Should users store NFC HSTs in RFID-blocking pouches, like those used for wireless car keys or contactless bank cards?
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You can disable the NFC or USB transport per protocol on yubikeys with the yubikey manager. I can imagine that the nitrokey can do that too. So if you don’t want to use NFC just disable it.

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In absolute terms, there is probably some small increase in risk due to the NFC interface. As a practical attack, it’s probably not worth worrying about. Directly from the paper you linked:

NB the term skimming for such an attack can be misleading if one thinks of well-known mag-stripe skimming of bank cards. When skimming mag-stripes, the attacker secretly reads the information on the mag-stripe, and can then make copies to create clones of the original bank card that cannot be distinguished from the original. For RFID tags, an attacker will not be able to create clones

with a skimming attack, as long as the protocol uses some form of challenge-response.

Which, the YubiKey, at least, used a challenge-response. While there is a known vulnerability which can be used to extract the internal secret and therefore clone a YubiKey 5, it requires long term physical access and is not trivial.

The bigger concern for the NFC tag would be its use in a relay attack. And with a big enough antenna, an attacker might be able to do that. However, I would again point to the paper you linked to, their results were “long” for an RFID activation and read. But, not really all that long:

a gate with a width of 100 cm, which can communicate with a tag held near the center between the two antenna

And their setup required a couple large antennas that the victim would need to stand in between. Not impossible, but you’d notice with each side being half a meter away.

So yes, it’s a technical risk, but not one that I’d bother putting much effort into avoiding. And the being able to use the key via NFC is probably worth the risk.

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OP edited

their setup required a couple large antennas that the victim would need to stand in between. Not impossible, but you’d notice with each side being half a meter away.

So yes, it’s a technical risk, but not one that I’d bother putting much effort into avoiding. And the being able to use the key via NFC is probably worth the risk.

This was my conclusion, too, but I didn’t want to prejudice the discussion. Thanks for corroborating.

IMO, using a Faraday pouch isn’t “much effort”, and is therefore worth doing if the HST is being carried in unfamiliar/non-secure locations.

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The N in NFC is Near. For someone to exploit it they need to be within centimeters of your device. Even if you can do it at 1m, thats close enough that it can just be stolen from you.

Also, your link to rfidgate appears broken.

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OP

Even if you can [exploit NFC] at 1m, thats close enough that it can just be stolen from you.

Stealing the HST should not give the user a false sense of security. Not so dangerous.

Silently exfiltrating the private key (or data for a replay attack), OTOH, would leave the user with a false sense of security. Dangerous.

your link to rfidgate appears broken

Wfm. Here’s an archive link.

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The setup they have requires a pair of huge antennas, so pulling that off surreptitiously feels unlikely. They also had a controlled lab environment, so who knows how it would work in a less controlled RF space.

Definitely cool research, but this isnt a viable attack vector IMO.

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not an expert on this but i would imagine a Faraday case for it, perhaps with a little flap for ease of access would work?

I would also hope a security company making hardware keys would be aware of and avoid the sending in clear text, as it would be a major vulnerability. Then again, ive never trusted a company to do anything beyond what is absolutely necessary to stay afloat.

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