A recent attack against Bluetooth, called KNOB, has
been making waves last week. In essence, it allows an attacker to
downgrade the security of a Bluetooth so much that it's possible for
the attacker to break the encryption key and spy on all the
traffic. The attack is so devastating that some have described it as
the "stop using bluetooth" flaw.
This is my attempt at answering my own lingering questions about "can
I still use Bluetooth now?" Disclaimer: I'm not an expert in Bluetooth
at all, and just base this analysis on my own (limited) knowledge of
the protocol, and some articles (including the paper) I read on
the topic.
Is Bluetooth still safe?
It really depends what "safe" means, and what your threat model is. I
liked how the Ars Technica article put it:
It's also important to note the hurdles—namely the cost of equipment
and a surgical-precision MitM—that kept the researchers from
actually carrying out their over-the-air attack in their own
laboratory. Had the over-the-air technique been easy, they almost
certainly would have done it.
In other words, the active attack is really hard to do, and the
researchers didn't actually do one at all! It's a theoretical flaw,
at this point, and while it's definitely possible, it's not what the
researchers did:
The researchers didn't carry out the man-in-the-middle attack over
the air. They did, however, root a Nexus 5 device to perform a
firmware attack. Based on the response from the other device—a
Motorola G3—the researchers said they believe that both attacks
would work.
This led some researchers to (boldy) say they would still use a
Bluetooth keyboard:
Dan Guido, a mobile security expert and the CEO of security firm
Trail of Bits, said: "This is a bad bug, although it is hard to
exploit in practice. It requires local proximity, perfect timing,
and a clear signal. You need to fully MitM both peers to change the
key size and exploit this bug. I'm going to apply the available
patches and continue using my bluetooth keyboard."
So, what's safe and what's not, in my much humbler opinion?
Keyboards: bad
The attack is a real killer for Bluetooth keyboards. If an active
attack is leveraged, it's game over: everything you type is visible to
the attacker, and that includes, critically, passwords. In theory, one
could even input keyboard events into the channel, which allows
basically arbitrary code execution on the host.
Some, however, made the argument that it's probably easier to implant
a keylogger in the device than actually do that attack, but I
disagree: this requires physical access, while the KNOB attack can be
done remotely.
How far this can be done, by the way, is still open to debate. The
Telegraph claimed "a mile" in a click-bait title, but I think
such an attacker would need to be much closer for this to work, more
in the range of "meters" than "kilometers". But it still means "a
black van sitting outside your house" instead of "a dude breaking into
your house", which is a significant difference.
I'm not sure mice and other input devices are such a big deal,
however. Extracting useful information from those mice moving around
the screen is difficult without seeing what's behind that
screen.
So unless you use an on-screen keyboard or have special input devices,
I don't think those are such a big deal when spied upon.
They could be leveraged with other attacks to make you "click
through" some things an attacker would otherwise not be able to do.
Speakers: okay
I think I'll still keep using my Bluetooth speakers. But that's
because I don't have much confidential audio I listen to. I listen to
music, movies, and silly cat videos; not confidential interviews with
victims of repression that should absolutely have their identities
protected. And if I ever come across such material, I now know that I
should not trust that speaker..
Otherwise, what's an attacker going to do here: listen to my (ever
decreasing) voicemail (which is transmitted in cleartext by email
anyways)? Listen to that latest hit? Meh.
Do keep in mind that some speakers have microphones in them as well,
so that's not the entire story...
Headsets and microphones: hum
Headsets and microphones are another beast, as they can listen to
other things in your environment. I do feel much less comfortable
using those devices now. What makes the entire thing really iffy is
some speakers do have microphones in them and all of a sudden
everything around you can listen on your entire life.
(It seems like a given, with "smart home assistants" these days, but I
still like to think my private conversations at home are private, in
general. And I generally don't want to be near any of those "smart"
devices, to be honest.)
One mitigating circumstance here is that the attack needs to happen
during the connection (or pairing? still unclear) negociation, which
doesn't happen that often if everything works
correctly. Unfortunately, this happens more than often exactly with
speakers and headsets. That's because many of those devices stupidly
have low limits on the number of devices they can pair with. For
example, the Bose Soundlink II can only pair with 8 other
devices. If you count three device by person (laptop, workstation,
phone), you quickly hit the limit when you move the device around. So
I end up repairing that device quite often.
And that would be if the attack takes place during the pairing
phase. As it turns out, the attack window is much wider: the attack
happens during the connexion stage (see Figure 1, page 1049 in the
paper), after devices have paired. This actually happens way more
often than just during pairing. Any time your speaker or laptop will
go to sleep, it will disconnect. Then to start using the device again,
the BT layer will renegociate that keysize, and the attack can happen
again.
(I have written the authors of the paper to clarify at which
stage the attack happens and will update this post when/if they
reply. Update: Daniele Antonioli has confirmed the attack
takes place at connect phase.)
Fortunarely, the Bose Soundlink II has no microphone, which I'm
thankful of. But my Bluetooth headset does have a microphone, which
makes me less comfortable.
Bluetooth, finally, is also used to transfer stuff other than audio of
course. It's clunky, weird and barely working, but it's possible to
send files over Bluetooth, and some headsets and car controllers will
ask you permission to list your contacts so that "smart" features like
"OK Google, call dad please" will work.
This attack makes it possible for an attacker to steal your contacts,
when connecting devices. It can also intercept file transfers and so
on.
That's pretty bad, to say the least.
Unfortunately, the "connection phase" mitigation described above is
less relevant here. It's less likely you'll be continuously connecting
two phones (or your phone and laptop) together for the purpose of file
transfers. What's more likely is you'll connect the devices for
explicit purpose of the file transfer, and therefore an attacker has a
window for attack at every transfer.
I don't really use the "contacts" feature anyways (because it creeps
me the hell out in the first place), so that's not a problem for
me. But the file transfer problem will certainly give me pause the
next time I ever need to feel the pain of transfering files over
Bluetooth again, which I hope is "never".
It's interesting to note the parallel between this flaw, which will
mostly affect Android file transfers, and the recent disclosure of
flaws with Apple's Airdrop protocol which was similarly believed
to be secure, even though it was opaque and proprietary. Now, think a
bit about how Airdrop uses Bluetooth to negociate part of the
protocol, and you can feel like I feel that everything in security
just somewhat keeps crashes down and we don't seem to be able to make
any progress at all.
Overall: meh
I've always been uncomfortable with Bluetooth devices: the pairing
process has no sort of authentication whatsoever. The best you get is
to enter a pin, and it's often "all zeros" or some trivially easy
thing to bruteforce. So Bluetooth security has always felt like a
scam, and I especially never trusted keyboards with passwords, in
particular.
Like many branded attacks, I think this one might be somewhat
overstated. Yes, it's a powerful attack, but Bluetooth implementations
are already mostly proprietary junk that is undecipherable from the
opensource world. There are no or very few open hardware
implementations, so it's somewhat of expected we find things like
this.
I have also found the response from the Bluetooth SIG is
particularly alarming:
To remedy the vulnerability, the Bluetooth SIG has updated the
Bluetooth Core Specification to recommend a minimum encryption key
length of 7 octets for BR/EDR connections.
7 octets is 56 bits. That's the equivalent of DES, which was broken
in 56 hours back, over 20 years ago. That's far from enough. But
what's more disturbing is that this key size negociation protocol
might be there "because 'some' governments didn't want other
governments to have stronger encryption", ie. it would be a
backdoor.
The 7-byte lower bound might also be there because of Apple
lobbying. Their AirPods were implemented as not-standards-compliant
and already have that lower 7-byte bound, so by fixing the standard to
match one Apple implementation, they would reduce the cost of their
recall/replacements/upgrades.
Overally, this behavior of the standards body is what should make us
suspicious of any Bluetooth device going forward, and question the
motivations of the entire Bluetooth standardization process. We can't
use 56 bits keys anymore, and I can't believe I need to explicitely
say so, but it seems it's where we're at with Bluetooth these days.
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