Root shell on a credit card terminal

by stgl • 1 year ago

840 251 1 year ago

Liftyee• 1 year ago

Bravo!

I love thinking of ways to exploit and circumvent hardware restrictions like the extensive tamper protection here, but it seems I'd assumed that once they were triggered it was game over. Apparently not so - still plenty of interesting bits left over to poke around with. Makes sense that the secure part gets properly disabled though, otherwise I'd lose all confidence in their designers.

lxgr• 1 year ago

That's possibly still true for the hardened processor: As TFA notes, that's not what was compromised here.

[...] only text strings seem to be passed to a binary (display_tool), that issues some inter-processor messages. The same goes for the key pad or the card reader itself. I could not find any evidence that these peripherals could be accessed directly from Linux.
Instead, there is an entirely separate processor, refered to as mp1, that seems to handle all the “secure” stuff, like handling the card, getting the pin and showing information on the screen. The “insecure” Linux, running on the second processor, mp2, only handles the networking, the updating, and the business logic.

fredthestair• 1 year ago

From the description it sounded like the linux side may play some role in tamper event handling, but hopefully it can just also see it has occurred, otherwise getting a root shell first may lead to an opportunity to prevent the tamper event from clearing security keys.

mrbluecoat• 1 year ago

For those easily excited:

the exposed root shell does not seem to be as big of a risk as initially feared. ... I could not find any evidence that sensitive data, such as card details, could become compromised this way

A good read for security designers, though.

LunaSea• 1 year ago

I highly doubt that having root a physical access to a terminal doesn't let you read credit card numbers.

In security, physical access and to a lesser extent root access are equivalent to a guaranteed hack.

kbolino4• 1 year ago

Which is why modern card technologies like chip cards and tap-to-pay don't expose the sensitive numbers at all. The card reader can't steal a number that doesn't exist. Only magnetic stripe cards are so insecure, but the card reader exploited here doesn't even have a magnetic stripe reader.

That having been said, this isn't perfect security. While a chip/tap card is in contact with the reader, it can still be used for fraudulent purposes. And physical access can open the door to other exploits, like trying to break the card's own security or installing a camera to capture images of the card.

IshKebab• 1 year ago

Read the article. The actual work is done by another processor that only runs signed images.

pelorat• 1 year ago

These are all over the Netherlands, this particular model. They don't accept any credit cards, only debit.

__MatrixMan__• 1 year ago

If this terminal is anything like the ones I've worked with, the device he had a root shell on was only the "same device" as whatever handles the card details in the sense that it lives in the same housing and on the same PCB. They are otherwise two separate computers.

In my case, the "outer" system was Android, so we could use ADB to control it however we liked--with relatively lax security. The "secure board" only took over for the part where it had to handle payment details. In order to test the payment flows we ended up revamping a bunch of 3D printers to hold a stylus instead because the inbuilt android tools couldn't see or touch anything that secure board was doing except for a relatively narrow set of actions like "start payment flow for $X" and "user canceled" and "payment for $X complete".

emidln2• 1 year ago

If you take a look at the binary that decides whether to boot the secure firmware or the tamper screen, it's probably trivial to patch to get the secure firmware running for more inspection. If the point of the linux system is networking and updates, that implies a method for updating the firmware of the secure portion which isn't ideal. If their check for whether it's tampered or not is in the linux userland, I'd be awfully suspect of their firmware update.

zzzeek• 1 year ago

is the rooted mp2 subsystem responsible for acquiring the tamper state from the hardware that's passed off to the mp1 system? the diagram seems to indicate this is the case. Why not then try to disable the tamper signal and get mp1 to boot up with the device opened?

stgl10• 1 year ago

No, I don't think so. I think the tamper logic is implemented in hardware and cannot be easily fooled. It seems like both mp1 and mp2 access memory-mapped registers of the tamper subsystem to check its status (and other hardware system stuff like reset reason etc.)

However, I am assuming that there is a way to gain write access to the hardware registers from Linux. After all, the manufacturer has the ability to "un-tamper" devices and there is this nor_update tool in Linux that might be able to do it. But my guess would be that first a key has to be loaded through some authenticated interface in order to unlock that functionality.

tomsonj• 1 year ago

Reminds me of "Every 10-ft wall can be defeated with an 11-ft ladder"

qmarchi• 1 year ago

Disc: Former Visa Employee

Generally, these devices will use the mp1 to do all of the cryptographic operations around the devices.

The biggest part of this is the keys defined between the terminal and the acceptance gateway (something like CyberSource or Authorize.net).

When the temper protection is tripped the keys that are used are immediately dropped from RAM and you can't recover them, they have to manually be input into the device again to reset the tamper protection.

(Side Note: keys are specific to a merchant. If you're able to extract them, it limits the blowback.)

dedicate• 1 year ago

I'm curious: with that level of access, even if you can't directly grab PANs, what are the practical limitations? Could they, say, intercept user input before encryption, or cause targeted denials of service?

stgl10• 1 year ago

I cannot tell for sure. I didn't have time to really look at the Linux applications and what they do exactly. My guess is that most of the sensitive stuff is done on mp1 (reading the card, verifying the pin etc.) and the Linux just acts as untrusted relay to the network. Denial of service should be possible in the sense that it could just put the system in a boot loop.

robocat• 1 year ago

Denial of service should be possible

That's sounds like an overcomplicatedengineer solution.

The 10x engineer uses a hammer to deny service.

Aside: I'm an artist-engineer. I like the performance art of using an oversized hammer to solve a problem that isn't obviously solved using a hammer - and I love the discovery process when my aims fail.

Hilift• 1 year ago

Devices such as this are cloned and used for fraud. It can be done without triggering any of the sensors. Secret service confiscated multiple cloned devices in an enforcement operation recently in NYC.

"Recent incidents in the U.S. include criminals committing fraud through processing fraudulent return transactions. As part of the fraud scheme, criminals obtain Point-of-Sale (POS) devices—either from an acquirer or agent while posing as a merchant, from online resellers or auctions, or through theft—and program the POS devices with the credentials of a legitimate merchant, thus effectively cloning the unsuspecting merchant’s actual POS device. Criminals use the cloned POS devices to..."

https://cardsystems.com/point-of-sale-clone-fraud-activity/

halpow• 1 year ago

If you want to try easy mode, check out those newfangled android-based credit card terminal. I bet they're much more rewarding, especially since you tap your pin on the screen. Juicy.

jeroenhd• 1 year ago

That'd get you the PIN quite easily, but if they're designed the same way (with all the important bits being handed off to a secure secondary processor) you still wouldn't be able to do much with the card as modern cards do a whole load of cryptography on-card to prevent stuff like this.

The attack would only work on terminals where every payment option but the magnetic card reader is broken, but those should give off skimmer alert alarm bells before you ever see a PIN prompt.

user_78322• 1 year ago

I'm not sure which type of android terminals you have where you are, but in India they seem to be running Android Oreo (support ended in Jan '21). Yummy!

user_78322• 1 year ago

Also, it is possible to open other apps and the notification centre. And unsurprisingly the entire device is terribly laggy.

_djo_3• 1 year ago

The PIN data is still encrypted even when displayed on a touch pad, using user interfaces controlled by firmware running in the trusted zone.

So the applications in between, that would be accessible in an attack like this, can't view the PIN.

bmurray7jhu• 1 year ago

The touch controller is generally connected to a MUX controlled by the security processor. When entering sensitive data (PIN/PANs), the touch controller output is routed directly to the security processor, bypassing any Android-derived OS responsible for the GUI.

tgsovlerkhgsel3• 1 year ago

And as a user, I have absolutely no way of distinguishing this from a device that had all secure features removed, and is running a random Android that proxies the NFC or chip data to a real reader, siphoning off what they can, while my PIN gets proxied by a human typing it into the real reader in real time. All I'd notice is a second or so of latency.

thenthenthen• 1 year ago

These are everywhere in Europe. Not sure about Switzerland, but credit cards are not really owned or used in much of the Europe I know. I would call it a POS, point of sale (system), these things can read all kinda of cards. Nevertheless, nice write up!

jajko• 1 year ago

Oh yes they are. I detest having to juggle all those cards in my wallet, already have tons for various reasons (not just payment stuff), so no room for ie debit ones.

Dont see the appeal of putting even more stuff into phone or ewatches (prefer good mechanical ones), losing it would be catastrophe enough as it is for privacy. But thats me.

pjerem2• 1 year ago

Actually, putting your card on your phone is a better protection against theft : they cannot be used without biometric identification, can be remotely revoked and you can still keep your plastic cards as backup.

I could easily live without it and I wouldn’t even consider it to be an important gesture for my next phone but it’s pretty useful when you have it.

Also, idk for Android but on iPhone it’s faster than plastic cards and more secure.

But I hope plastic cards are there to stay because that’s yet another thing locking us into the iOS/Android duopoly.

ComputerGuru2• 1 year ago

The (compromised) Linux decides whether to load the “compromised mode” code or the mp1 secure system? Sounds like an avenue to explore. It says the bootloader itself is secure, but that doesn’t mean much if it’s being loaded into a compromised environment, depending on where it is actually being executed. I guess the coprocessor could be considered a Secure Enclave of Sorts, but the fact that Linux could load a separate bootloader and run that (somehow) is of concern.

stgl10• 1 year ago

No, it cannot load a separate bootloader. I tried to tamper with the loadercode (the "secure" bootloader), but it wouldn't boot. So I am guessing there is some third party (boot ROM) that verifies it.

Also, I think Linux always loads loadercode + mp1.img, regardless of the tamper state. The different code paths depending on tamper state are taken within the (integrity protected) loadercode.

ComputerGuru2• 1 year ago

Keep in mind that no tamper mode will be set if you use the external debug interface. If Linux is used for networking then maybe you could MITM payments.

londons_explore• 1 year ago

It's also possible that the root shell is opened up when the tamper seal is triggered.

Ie. The system is either in secure mode (with all necessary crypto keys for operation), or it is in insecure mode with a root shell open for debugging and failure analysis, but that transition also deleted the critical private keys.

fp643• 1 year ago

Was my guess as well, maybe it's even possible to use it to flash new keys so the device can be used again?

Now I am curious if I can find a terminal myself, if they are actually getting phased out it might not be too difficult to find a used one...

stgl10• 1 year ago

I had the same idea, but no, I tried with a second, untampered one and I also got a working shell. So it does not seem to be dependent on the tamper state.

lelanthran10• 1 year ago

Was my guess as well, maybe it's even possible to use it to flash new keys so the device can be used again?

What keys would you flash them with? Anything encrypted with your "new" keys can't be decrypted on the other end of the transaction anyway, so what would be the point?

avipars• 1 year ago

Can it run doom?

blooalien• 1 year ago

Can it run doom?

Almost certainly yes.

Bad_CRC• 1 year ago

I expect an updated post with it running doom as the author is not strange to it: https://stefan-gloor.ch/voip-phone-hack

joshstrange3• 1 year ago

Aside from the fact that I wouldn't know what I was looking at, I've been tempted to crack open one of the Stripe M2 readers I have and look inside.

Unfortunately, out of 36 readers that I bought, 7 have "died" (2 won't hold a charge, 1 won't scan NFC, and 4 report "tampered"). That attrition rate is pretty bad on the surface but it doesn't tell the whole story of course, how often were they used? How old are they?

The answer to those questions is much more damning. The devices are 1-3 years old (I bought them over time) and have been used a max of 9 days total [0]. Yes, 9 days _total_ of use and 7 of 36 readers failed in some fashion. Oh, and the readers are all kept in hardshell cases with foam inserts (1 slot per reader) whenever they are moved.

Needless to say, I'm not a huge fan of the M2 readers but they are still the best option for me :/

[0] Some background, my company handles festival payments. We travel to events and handle the in-person payments (most happens on web/app) with iPads+M2 Readers. That's why there are so few "days of use" over 3 years.

mrbuttons454• 1 year ago

I'd be sure to charge them before storing them for the next show. Most batteries don't like being stored for long periods in a low SoC state. And I'm sure the tamper requires a functional battery.

joshstrange3• 1 year ago

All readers are charged for a day minimum after a festival before I pull the power. I then normally power them up at least a week before the next event so I can update them all and check that no more have randomly died on me. I keep a healthy buffer of devices so that I can replace them without paying for express shipping from Stripe which saved me for the last event I did where I had 6 of them die (before that pre-event check I only had the 1 bad NFC reader from the previous year).

I have to assume that one of the hard-shell cases got thrown around a little too much which caused 4 readers to go into tamper mode. 3 of the tamper mode readers were in the same case but the other was in my nicest case and still had the issue ("nicest" = custom made case vs pick-and-pluck-style).

The tampers I can somewhat understand (still just so odd to have 4 die "at once"), maybe they did get banged up, but the failed NFC and not holding a charge issues are less acceptable to me given the time/use.

absurdo• 1 year ago

For the young players: this is what hacker in “Hacker News” stands for. This is 101 and it’s very simply explained which makes it a great step by step example of a typical journey. Hack-a-day is full of these if you want more.

The author is clearly curious and leads in knowing a lot to begin with.

The work-behind-the-work is looking up data sheets for the chips involved, desoldering them without damaging them, in the case of memory resoldering with hookup wire and hopefully its access is slow enough that it can work fine over the length of the wire, following hunches, trying things, and knowing (for next time) the possibility of using a pinhole camera or something of the sort when drilling shallow holes and looking through for tamper traces to avoid in further drills, if so desired be.

As others have mentioned, it would be interesting if the author stuck in and got past the tamper checks to see if it would work as normal. Oh well!

zidoo• 1 year ago

Amen for the first sentence. One more LLM wrapper today, and I would die.

account-5• 1 year ago

It's so tiresome and is slowly ruining HN for me personally.

lucianbr• 1 year ago

Hear hear.

paulddraper• 1 year ago

Or UI pontifications

belter• 1 year ago

Yeah, this has been LLM News for a while...

airstrike3• 1 year ago

It does feels like a good use of AI (ML, really) would be to write a "disaggregator" for HN that tags submissions by category and lets users browse the bits they care about. Wish I had time to do it....

ajsnigrutin3• 1 year ago

It's either LLM or "<existing software> written in <languge of the week>"

Pulcinella• 1 year ago

"Prompt Kiddie" should be the new "Script Kiddie"

suobset• 1 year ago

Thank you so much. Been a long time lurker, only recently got an account to start commenting. Posts like this make my entire day: concise, well thought/researched/executed, and they put the "hacker" in HN.

AaronAPU• 1 year ago

I think this might have been the first post I actually upvoted. I do upvote comments often, but this post itself is actually .. about hacking!

goodpoint• 1 year ago

"Hacker" news was never about that tho. It came out from SV...

aag• 1 year ago

The term "hacker," even in the computer field, originally had a larger scope than computer security. It had a more philosophical definition, too. I host a copy of the Jargon File[1], compiled by Guy Steele et al., on my web site. It defined "hacker" as[2]:

  HACKER [originally, someone who makes furniture with an
  axe] n. 1. A person who enjoys learning the details of
  programming systems and how to stretch their capabilities,
  as opposed to most users who prefer to learn only the
  minimum necessary. 2. One who programs enthusiastically,
  or who enjoys programming rather than just theorizing
  about programming. 3. A person capable of appreciating
  hack value (q.v.). 4. A person who is good at programming
  quickly. Not everything a hacker produces is a hack. 5. An
  expert at a particular program, or one who frequently does
  work using it or on it; example: "A SAIL
  hacker". (Definitions 1 to 5 are correlated, and people
  who fit them congregate.) 6. A malicious or inquisitive
  meddler who tries to discover information by poking
  around.  Hence "password hacker", "network hacker".

I'm guessing that PG had this broader definition in mind when Hacker News was started.

No history of the term "hacker," however brief, would be complete with a reference to The UNIX-HATERS Handbook[3].

[1] https://speechcode.com/jargon/

[2] https://speechcode.com/jargon/jargon.info.Hacker.html

[3] https://web.mit.edu/~simsong/www/ugh.pdf

bonoboTP• 1 year ago

Or as RMS defines it, playful cleverness.

BuildTheRobots• 1 year ago

The term "hacker," even in the computer field, originally had a larger scope than computer security.

I've been helping run a Hackspace for a few years; I can't tell you how many times (sometimes per week or even day depending on what I'm doing) I have to have this conversation.

I've gotten quite used to going through the patter with non-tech people - try dealing with the local council, or applying for insurance or even personal jobs when it's on your CV. Thankfully a lot of the older people remember the term "hack" meaning "amateur" which eases explanations in a workshop context.

I always assumed IT people would realise the non-negative connotations, but that really isn't the case.

echelon_musk• 1 year ago

would be complete with a reference

's/with/without/'

CooCooCaCha• 1 year ago

Given that this website comes from a VC startup incubator I highly doubt it’s meant to refer to hacking security.

I think it’s meant to mean the “move fast and break things” style hacking. Which basically means churning out code fast and pushing through problems quickly instead of getting stuck on perfectionism.

wkat42424• 1 year ago

As others have mentioned, it would be interesting if the author stuck in and got past the tamper checks to see if it would work as normal. Oh well!

That's the kind of thing a lot of guys in suits would take very seriously. After all you have to connect it to the banking network to see if it works as normal. Not advised.

It could also lead to lowlife types putting pressure on you to help them do that. Not the kind of thing to brag about.

wkat42424• 1 year ago

Ps: like the screenshot says the private keys are already gone so there's no way to get it reactivated on the same account.

kragen• 1 year ago

For the young players: this is what hacker in “Hacker News” stands for.

No, it is not, though there is a connection. As it happens, the founder of Hacker News wrote a very inspiring essay shortly before setting up the site on what the word "hacker" means to him, and it's not breaking into computers: https://www.paulgraham.com/gba.html

To the popular press, "hacker" means someone who breaks into computers. Among programmers it means a good programmer. But the two meanings are connected. To programmers, "hacker" connotes mastery in the most literal sense: someone who can make a computer do what he wants—whether the computer wants to or not.
To add to the confusion, the noun "hack" also has two senses. It can be either a compliment or an insult. It's called a hack when you do something in an ugly way. But when you do something so clever that you somehow beat the system, that's also called a hack. The word is used more often in the former than the latter sense, probably because ugly solutions are more common than brilliant ones.

...

He goes on to explain in detail how he thinks about the connections between the different senses of the word.

Because it requires no mastery or indeed any special level of ability, finding a passwordless root shell on an exposed serial port does not rise to the level of being a hack in this sense, only in the popular-press sense.

Disposal8433• 1 year ago

You can make fake debit/credit card transactions on a $2 USB card reader. All the specs are here and the protocol is public and documented (IIRC 5000 pages in a lot of PDF, it's a pain in the ass to read).

But to validate those transactions, you must send them to the bank over the internet, and you'll get a visit from the feds/FBI/whatever if you do it.

There is no real protection on card readers (most use Linux with a small shitty password). The protection comes from the contracts and regulations between the shops and the banks.

csinode• 1 year ago

I'd be more worried about someone compromising a card reader in the field and reading cached/stored real CC details, or installing some kind of intercepting malware. (That does seem to be difficult/impossible in this specific case, but it means research in this area is relevant.)

christina97• 1 year ago

There are much easier ways to skim cards than hacking the terminal.

rockbruno• 1 year ago

Aren't credit cards nowadays basically physical private keys? IIRC transactions are one-time payloads signed specifically for that operations, so intercepting that won't help you if I'm not mistaken about how cards work nowadays.

reaperducer• 1 year ago

I'd be more worried about someone compromising a card reader in the field and reading cached/stored real CC details, or installing some kind of intercepting malware.

That's happened at least several times already.

I believe breached PoS terminals were what happened in the big Target hack.

adolph• 1 year ago

This story about a physical card reader checker to detect skimmer devices was interesting and posted here a couple years back.

https://tech.target.com/blog/cybersecurity-easysweep

https://news.ycombinator.com/item?id=36788831

nine_k3• 1 year ago

Someone with a root access to a card reader could just make it collect CC details with every transaction, no caches needed. It could also make certain transactions "temporarily fail", while siphoning a certain amount of funds to another, legit-looking, merchant under the hood.

Toritori12• 1 year ago

I thought most POS devices stop accepting "offline" payments.

sgjohnson2• 1 year ago

Maybe today, when the line between a Credit and a Debit card has gotten significantly blurrier (except in the US).

I definitely remember making an _offline_ payment with an AMEX card at a restaurant in the UK some 10 years ago.

Also, most airlines that take payments on board also run the terminals in offline mode.

There must be some mapping of BIN codes and whether to allow an offline transaction.

glitchc3• 1 year ago

There is no real protection on card readers (most use Linux with a small shitty password). The protection comes from the contracts and regulations between the shops and the banks.

I'm afraid that's not true. Merchant terminals have secure hardware embedded inside to store the bank and interchange keys. If those keys leaked, someone could spoof legitimate transactions.

kuschku• 1 year ago

If those keys leaked, someone could spoof legitimate transactions.

You mean, whenever those keys leak. It's not that hard to do, see e.g. https://media.ccc.de/v/32c3-7368-shopshifting#t=2207

stef252• 1 year ago

You can make fake debit/credit card transactions on a $2 USB card reader

Not asking "teach me how to do this" but could you explain in a little more detail ?

quesera4• 1 year ago

I believe GP is confused.

You can read a card (stripe) with one of those cheap readers. The magnetic stripe is not encrypted and you can extract the card number (PAN), expiration date, and cardholder name. Some other less important bits too. You cannot extract the CVV this way, but that is not required for transactions.

Most cards are EMV now. That data is encrypted and it's read/write. These keys can leak, putting you back into a similar state as if you'd read the card via magnetic stripe.

But no amount of card reading gives you the ability to submit transactions to the network. For that, you'd need merchant credentials for their gateway or processor, and you'd usually need to have a presence on the merchant's network (to get through the upstream firewall).

These are all achievable things. Doing so gets you the ability to create transactions against the card. These transactions will be submitted to the network and approved or declined. If approved, funds will settle from the issuing bank to the merchant bank, possibly in multiple steps.

I'm oversimplifying a bit, but the essential point is that funds will settle to the merchant's bank account, not yours. You can cause some headaches, but you cannot steal money.

The compromise of a credit card terminal is only interesting because it gives an attacker the ability to steal the card details for all cards that are subsequently used at the compromised terminal. They can be saved and retrieved later, or sent out to a C&C server, etc. Then these card details can be used for all the usual types of credit card fraud.

charcircuit• 1 year ago

and you'll get a visit from the feds/FBI/whatever if you do it

Is there any proof to this statement or are you just trying to scare people? I think it's possible there are bigger groups that would have their attention and a single fraudulent transaction would just be noise.

Nextgrid2• 1 year ago

In his scenario I don't understand where he'd send the transaction to begin with?

In a typical scenario my understanding is that you get the terminal from your acquirer - this is your broker to the card networks. When the terminal makes a transaction, it does some crypto magic using its own keys (that identify it to the acquirer), sends that to the card which does more crypto magic using its own keys, and finally the result of that is sent to the acquirer.

If you do this flow yourself with fake keys you'd get the card to sign a transaction for your fake terminal's key (assuming you know the card's PIN of course - unless you're happy to forego any CVM), but you have no acquirer that would accept said transaction, so I don't see how you could commit a crime here even if you wanted to? You just got some meaningless bytes back.

And of course, if you have an actually valid terminal key that is trusted by an acquirer and do all this, you've effectively just made a normal payment - if the person was willingly paying you then no crime either, and otherwise it's no different than using a legit terminal to bill someone without their knowledge.

lelanthran10• 1 year ago

There is no real protection on card readers (most use Linux with a small shitty password).

Sure there is: only signed binaries can run, executable filesystem is read-only, data filesystem has noexec bit set, root login is disabled, crippled busybox misses a lot of functionality, keys are loaded from a secure area on bootup, master key injection only available when loading at the factory, bootup itself is more or less secure, tamper detection blanks the chip, etc.

Sure, if you have a cheap non-EMV certified Android terminal imported from Asia it'll probably use a standard Linux, with a rw root filesystem, with root login enabled *and* sudo enabled for the username used to execute applications, tamper-detection is non-existent, screen-casting not locked down, ports are all openable and busybox is more or less complete.

Source: Me. I developed (and still sometimes do) EMV applications for card acquisition for a few years. Even in dev mode (which requires the vendor to provide IDs of the developers), these things are very much locked down solidly.

timewizard• 1 year ago

Sure there is: only signed binaries can run, executable filesystem is read-only, data filesystem has noexec bit set, root login is disabled, crippled busybox misses a lot of functionality, keys are loaded from a secure area on bootup, master key injection only available when loading at the factory, bootup itself is more or less secure, tamper detection blanks the chip, etc.

Sounds good. Has anyone been hired to attempt to attack the device and find vulnerabilities?

cluckindan• 1 year ago

The OP device tried to mount a USB volume and would have started dropbear if it had been found (presumably in PATH). Maybe maybe maybe…

pabs32• 1 year ago

Have you considered using dm-verity instead of signed binaries?

Y_Y3• 1 year ago

If I buy a device it sure as shit better give me root.

If a payment system is relying on my local privilige on some random-ass device to authenticate a payment it deserves every garbage request it gets.

account4210• 1 year ago

Even in dev mode (which requires the vendor to provide IDs of the developers), these things are very much locked down solidly.

Probably different vendors but this has not been my experience.

TacticalCoder• 1 year ago

There is no real protection on card readers (most use Linux with a small shitty password). The protection comes from the contracts and regulations between the shops and the banks.

Wait wait wait... Aren't these EMV cards smart cards using the Java smart card technology? I thought these were heavily using encryption, including PKCS.

They're using challenge/response protected by the PIN no!? If I am not mistaken if you insert a card in a totally compromised reader you cannot clone the card: that alone is already quite an amazing feature compared to where we're coming from with the magnetic stripes ones (and the date to sunset for good magnetic stripes in the EU has already been set IIRC).

A fully compromised reader could trick you in stealing your PIN (so that an accomplice could then later on steal the card physically) and a compromised reader could also trick you into signing a 2 EUR transaction while it's actually wiring 2 000 EUR out of your account but I think that's about it!?

It comes to this: either the card can be cloned from a compromised reader or it cannot. I don't think that someone inserting it's card and getting its PIN intercepted means: "Bad guy can do countless transactions for days on until the account is empty".

If I'm right, that's a very far cry from "protection comes from the contracts and regulations between the shops and the bank".

AIUI a compromised reader can fake one big transaction or maybe a few transaction in quick succession but as soon as the owner pulls its card, they cannot do anything anymore?

bogantech• 1 year ago

But to validate those transactions, you must send them to the bank over the internet

Not how it works at all, banks don't have some open API on the internet for processing card transactions

nine_k3• 1 year ago

It's over the Internet, because you're not going to run a dedicated fiber to every card reader. But it's not over the unprotected internet; your card reader will establish a VPN connection of sorts, or at least talk via an encrypted channel (think TLS) is you use e.g. a Square terminal.

Not that a random person can hit these endpoints, unauthenticated, and try to run a transaction.

tialaramex2• 1 year ago

I agree that this isn't how it works.

The first thing to understand at an even higher level about payment cards is that they have always had two separate and barely related components, Authorisation and Settlement.

Authorisation is concerned with whether this specific transaction has been approved in some sense by a card issuer. Authorization today is relatively high tech, there's somewhat decent cryptography, tamper resistance, uniqueness = they really care - and that's because when Authorization problems occur the banks might lose money, which they hate.

Settlement is "just" moving the money from one customer to another. $123.45 from Jim Smith to Terrible Goose Inc, done. This is very mid-late-20th century technology, we're not talking pieces of paper and scribbly hand writing, but fixed width ASCII fields on magnetic tape is fine - it's the customer's money so the banks don't care more than legally required.

Settlement replays are how you get "accidents" where a big store's customers all get charged twice for a whole day - the associated Authorizations can't be replayed, that's the banks money at risk - but the settlements aren't protected.

Merchants can, and some do, choose not to care about Authorization. In a huge business it could make sense to eat say 2% of sales as undetected fraud (ie you never receive payment) rather than have any transactions fail. If you operate a food truck using a terminal to take $1000 per day on your iPhone the people who supply your terminal may not let you opt out because that's risk they don't want. But if Jeff Bezos or Doug McMillon makes more without Auth he's turning it off.

NoahZuniga2• 1 year ago

I mean, maybe they don't have an open API, but they sure do have an API on the internet. Surely the payment terminals are communicating with the issuing bank in someway, perhaps over an interface of some kind.

I believe what this comment is getting at is that making fake transactions is useless without a connection to a bank that will execute them.

miki1232112• 1 year ago

The protection comes from the contracts and regulations between the shops and the banks.

While the comment is not quite true (see sibling replies), this part is spot on.

This is also why crackpot theories about people walking around with portable card readers and stealing money from contactless cards are false. Yes you can walk around and make those transactions, what comes after (and the setup you had to do before) is the problem. I'm not even sure whether you could get the money out before being caught and shut down. With how many people these days have push notifications for their transactions, I highly doubt that.

girvo2• 1 year ago

Yes you can walk around and make those transactions, what comes after (and the setup you had to do before) is the problem

I mean with the amount of stolen card details routinely traded and used successfully (at least for a while) and with how little crime like that is investigated or punished in some jurisdictions, I dunno...

Still it's not quite as simple as "taking the money from your card" like said crackpots think.

stephen_g2• 1 year ago

I did somehow have two different cards (an Amex and a Visa) compromised at the same time about two months ago, and I do wonder if it was some kind of skimmer setup - if it was just one of them then I'd assume it was just some online store I'd used it at that had been hacked, but I've not used both those cards on the same sites.

I got a notification asking me to confirm a transaction on the Visa and then looked in my app and found they'd actually got another transaction pending at a hotel. I called them up and the hotel said they would "kick out the guests" and refund me. Not sure why they didn't want to call the police on them, because when I called later they said I should report it to the police, but all of the transactions had been refunded so I literally had zero loss and there was nothing really to report... It was them who'd provided the services and suffered the loss, the hotel should have had the police remove them!

Anyway, on the same day the Visa had been used at the hotel, I also had some fraudulent transactions on the Amex, although most of them seemed to be automatically refunded by the vendor themselves (so maybe it was flagged by the vendor's anti-fraud mechanisms and refunded to avoid a chargeback from American Express) before I cancelled the card. They'd tried three times with a similar amount and they'd all refunded.

The other weird thing was that the hotel that the Visa was used at claimed that it had to be a card-present transaction or in a digital wallet, but I didn't get any notification about it being enrolled in a digital wallet and I always had the physical card with me. So not sure if that was mistaken or BS or if they managed to somehow fake the digital wallet.

But yeah it didn't work out for them because I caught the transaction the same day they'd checked in to a hotel with the card and then both were cancelled that day...

jojobas• 1 year ago

Didn't people manage to present a remote card (i.e. in a mark's pocket) to a legitimate terminal through an NFC tunnel of sorts? Limited to no pin required amounts, but still.

champtar• 1 year ago

I remember when contactless was introduced in France, someone from the CB bank card group (https://en.m.wikipedia.org/wiki/CB_Bank_Card_Group) said that contactless was secure because you are insured. At that time France was already using chip+pin for a while.

At the end of the day the money only goes from one bank account to another, account can be frozen, charge reversed, ... So you just need to secure the POS enough that user feel safe to use it and there is a low number of people that can hack them and are willing to risk prison.

mihaaly• 1 year ago

Aren't the contracts and regulations holding back the honest people only? But not those who violate contracts and regulations, like the dishonest ones?

rcxdude• 1 year ago

Well, the issue is that you need a merchant account to send the money to, and it's quite hard to get one without identifying yourself, and it takes a bit of time after the transaction to actually get the money because of chargebacks and the like. So you can't just pull the money out directly. But that's true of basically any credit card fraud: what you do instead is buy something from someone who takes credit cards. Which is entirely possible with contactless, you can e.g. proxy a connection from a reader to a victim card, it's just that the limits and difficulty of lining up the time of the transaction with someone walking past make it not particularly useful (compared to just stealing or skimming the card details).

_1tem• 1 year ago

What if you live in a country where Visa/Mastercard works but is way out of the jurisdiction of the feds?

rswail2• 1 year ago

How exactly are you going to get the diversified IPEK to generate the necessary keys to create the HMAC for the transaction that will be authorized by your merchant acquirer?

The BDK for your merchant acquirer is held by them in an HSM or equivalent. The IPEK is device specific, derived from the BDK based on the terminal ID.

The individual keys for the HMAC are generated for each transaction and are cryptographically linked to a transaction ID and the IPEK, so that the acquirer can determine whether there are missing messages.

Card readers that comply with the latest EMV chip/contactless standards require a secure element that maintains the crypto information and only allows specific requests from the non-secure element and only provides encrypted blobs for transmission between the acquirer backend and the secure element.

https://en.wikipedia.org/wiki/Derived_unique_key_per_transac...

ofjcihen• 1 year ago

The look into these is neat but…why immediately open it up and trigger the tamper state? Did they not know most readers would have one?

Any real testing happening in the tamper state might be meaningless. Perhaps the shell is available after the tamper state triggers for resetting purposes.

It just seems like opening it would be the last thing you would try.

stgl10• 1 year ago

Well, I felt like I first had to get a feeling for what I am working with. Hardware, what SoC, interfaces, flash etc... Otherwise I am too much in the dark. But sure, in hindsight I could've just tapped the debug connector and could have been done with it.

No, I got a shell on second, untampered one, as well.

allenrb• 1 year ago

Specifics of this hack aside, I wish I could solder like that!

_trampeltier• 1 year ago

I know they run Linux, once a such terminal updated and rebooted when I would pay. It was in the evening rush hour on in a shop. The other person before me paid, then the salesperon scanded my things and suddenly an update was on the screen.

bill_mcgonigle• 1 year ago

What a nice writeup! Just curious - what kind of wire do you use to solder on the pins - enameled?

I wonder if they offer their customers source to keep the Busybox folks happy?

stgl10• 1 year ago

Yes, 0.1 mm diameter enameled copper wire. Chasing GPL violations sounds like a fun hobby :)

zoobab3• 1 year ago

I think US based SFLC has budgets if you request it to enforce the GPL.

Pre-OpenWRT days I was maintained a Linux distro ISL3893 that ended up in court in Germany, the first GPL enforcement:

http://isl3893.wikidot.com/

"17 apr 2004 — GPL testing in court by the Netfilter/Iptables team, due to refuse to give source code of the Sitecom WL-122 (isl3893 based!). In the same time, some source code has appeared on the webserver of Sitecom."

jdefr89• 1 year ago

UART always the first place I look…

Sakthimm• 1 year ago

My thoughts exactly. Why desolder the flash chip before probing for UART? Still, impressive work.

stgl10• 1 year ago

Thanks! Yep, I feel foolish. I just could not imagine it would be that easy on a locked-down terminal. But I learned for next time :)

Lord_Zero• 1 year ago

Any way to reset the tamper protection from software?

reliablereason• 1 year ago

Hopefully the important private keys that is used to identify the terminal are erased when the tamper is activated.

drdaeman• 1 year ago

I don’t know much about those terminals, but the screenshot hints at that (key=zeroes part), and then according to the diagram it waits for USB to flash new secure firmware and keys in case the terminal had to be serviced.

This root shell is probably not a security issue - and maybe even meaningfully user-accessible (for some fancier network configurations or diagnostics) - as, I suspect, it’s nothing more than a glorious modem for a stream of encrypted and authenticated data, plus a firmware updater (likely useless without a valid signature) - it could be potentially pretty much the same as hacking the upstream router. Sensitive (so there’s probably a sticker intended) but not something that breaks device’s security.

SamuelAdams• 1 year ago

Alright sorry if this is dumb but how does the OP know about all this?

tialaramex2• 1 year ago

Light Blue Touchpaper https://www.lightbluetouchpaper.org/ might well be interested in this work, the card payment tech is something they spent lots of time looking at and they tend to have a more realistic assessment of where the weaknesses might be that the industry which tends to see itself as infallible.

calltrace• 1 year ago

Great story, how long did this take you?

wkat42424• 1 year ago

Very interesting. These terminals are super common in the Netherlands also. The exact same ones. Kudos to the author, rigging up a BGA chip is no small feat. I love this kind of curiosity (one of the reasons I like HN)

kriro• 1 year ago

Excellent article. Reminds me of a CCC talk in the early 2000s where an enthusiast checked ATMs of different banks across Germany and most where running unpatched Windows 98 (I think). If my memory is correct it was also very easy to get physical access. I believe removing a couple of normal screws got you to an ethernet port or something similar.

israrkhan• 1 year ago

Once he got the root shell on "insecure" processor, that could have been used as an attack point for the secure processor. It would be much more interesting to pwn the secure processor from insecure Linux OS.

grishka4• 1 year ago

These are (were?) popular in Finland and confused me a lot every time I visited. The NFC reader in them is on the side, which is very unusual compared to the kinds of terminals I'm used to, which have it in the screen so you just tap your card/phone on the screen.

davedx• 1 year ago

I used to work with set-top-boxes for a big media corporation, those also had the debug serial port. I remember it being fiddly but also absolutely vital to be able to do any useful work with the STB's. It's quite eye opening now reading this to see that they're also a huge potential security hole (in the right hands), unless everything is locked down with the second secure processor like in this setup.

ttkari• 1 year ago

Reading about all the tamper detection on the device makes me wonder what would be the easiest way to trigger the tamper mode. After all, being able to do that on just a handful of devices would be an efficient denial of service attack on a retail location when the majority - and sometimes all - of payments go through these things.

neop1x• 1 year ago

Dropping it on the floor or pouring water on it.

anemic• 1 year ago

I once had the (dis)pleasure of working with these Yomani terminals. I got a development unit (with red text "DO NOT PAY" on the side). I plugged it in my home internet which has a public ip with dhcp just to get it quickly online and keep it out of my internal home network. The next day I got a call from my ISP saying I had a compromised machine in my network with malware. I was like WTF?! and they gave me the mac address and it was the Yomani terminal! I promptply unplugged it from the network and started investigating. Indeed, this development unit had a telnet(!) port open and root login without password was possible. So, having a wide open telnet port on a public ip and it's just a matter of minutes until someone uploads a generic arm malware onto it. I returned the terminal to the vendor with explanation but never got a followup. Lesson learned: never attach anything to public internet, even if it looks secure.

I guess Atos Worldline really doesn't like root passwords.

stgl10• 1 year ago

Very interesting!

rswail2• 1 year ago

So a few things:

1. Under the current EMV standards for terminals, the NFC and/or chip connection is via a secure element that protects the data in the transaction both in transit and at rest.

2. The transaction is protected in transmission not only by TLS encryption, but with an individual HMAC generated using a key that is derived from a base key that is unique to the terminal and merchant.

The interaction with the card is relatively read-only. There is a "dynamic CVV" that is generated on the card in chip/contactless transactions that adds some security, however, the data on the card is mostly open (PAN, expiry date, CC name etc).

The CHI (cardholder information) has to be maintained in an encrypted state at all times.

The business processes (where this article found the root shell) is not involved in the actual card or transaction security. It communicates with the secure element via a protocol that allows it to specify the amount and other details for a transaction, but the authorization (PIN or otherwise) and encrypted messages are entirely within the secure element.

Root shell on a credit card terminal

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