hacking Archivi - Page 2 of 3 - Pasquale 'sid' Fiorillo

The affected vendor did not answer to my responsible disclosure request, so I’m here to disclose this “hack” without revealing the name of the vendor itself.

The target machine uses an insecure NFC Card, MIFARE Classic 1k, that has been affected by multiple vulnerabilities so should not be used in important application.
Furthermore, the user’s credit was stored on the card enabling different attack scenarios, from double spending to potential data tamper storing an arbitrary credit.

Useful notes from MIFARE Classic 1K datasheet:

EEPROM: 1 kB is organized in 16 sectors of 4 blocks. One block contains 16 bytes.
The last block of each sector is called “trailer”, which contains two secret keys and programmable access conditions for each block in this sector.

Manufacturer block: This is the first data block (block 0) of the first sector (sector 0). It contains the IC manufacturer data. This block is read-only.
Data blocks: All sectors contain 3 blocks of 16 bytes for storing data (Sector 0 contains only two data blocks and the read-only manufacturer block).
The data blocks can be configured by the access conditions bits as:
- Read/Write blocks: fully arbitrary data, in arbitrary format
- Value blocks: fixed data format which permits native error detection and correction and a backup management.
  A value block can only be generated through a write operation in value block format:
  - Value: Signifies a signed 4-byte value. The lowest significant byte of a value is stored in the lowest address byte. Negative values are stored in standard 2´s complement format. For reasons of data integrity and security, a value is stored three times, twice non-inverted and once inverted.
  - Adr: Signifies a 1-byte address, which can be used to save the storage address of a block, when implementing a powerful backup management. The address byte is stored four times, twice inverted and non-inverted.

Value block example for value 0x0012D687

Let’s start hacking:

In this post I did not show you how to crack the MIFARE Classic Keys needed to read/write the card, ’cause someone else has already disclosed it some time ago, so google is your friend.
At last, please, use this post to skill yourself about the fascinating world of reverse engineering, and not for stealing stuffs.

In order to start the analysis I need some dump to compare.
The requirements of this task are nfc-mfclassic tool included in libnfc, a NFC hardware interface like ACR122U, and a binary compare (aka binarydiff) tool like dhex.

Dumps:

Dump 0: Virgin card (not included in the screenshot below ’cause all data bytes were 0x00, except for the sector 0 that has UID and manufacturer information. These sector is read only, so these bytes are the same across dumps)
Dump 1: Card charged with single 0.10€ coin (Note that vending machine displays the balance with 3 decimals, 0.100€)
Dump 2: 0.00€ after spending the entire balance with 4 transactions of 0.025€ each
Dump 3: 0.10€ recharged with one single coin

Dump 1 compared to Dump 2, *yellow bytes differ*

Dump 2 compared Dump 3, yellow bytes differ

Blurred bytes are the MIFARE keys A and B, except for the 32 bytes at 0xE0 offset of which I don’t know their purpose.
The 4 bytes between the keys are Access Condition and denotes which key must be used for read and write operation (A or B key) and the block type (“read/write block” or “value block”).

The tool mfdread is useful to decode the Access Condition bytes rapidly, and, in general, to display MIFARE Classic data divided by sectors and blocks:

Early analysis:

Note: from now on I will refer to the offsets with a [square parenthesis] and a value with no parenthesis.

Blocks 8, 9, 10, 12 and 13 can be used also as “value block”
Except for bytes between offsets [0x80] and [0x9F], only few bytes differ between dumps
Some data are redundant, for example [0x60 … 0x63] has the same values of [0xA0 … 0xA3]
Values at [0xC0], [0xD0], [0xC8], [0xD8] differ by 4 between 1st and 2nd dump (eg: 0xFE – 0xFA = 0x4) and differ by 1 between 2nd dump and 3rd dump (eg: 0xFA – 0xF9 = 0x1)
Values at [0xC4], [0xD4] differ by 4 between 1st and 2nd dump (eg: 0x05 – 0x01 = 0x4) and differ by 1 between 2nd and 3rd dump (eg: 0x06 – 0x05 = 0x1)
- 4 is the number of spent transaction made the first time, and 1 is the number of recharge transaction made the second time
Sum between yellow squared and red squared offsets has 0xFF value. In other words red squared is inverse (XOR with 0xFF) of yellow squared. For example:
- 0xFE ⊕ 0xFF = 0x01
- 0xFF ⊕ 0xFF = 0x00
- …
- 0x7F ⊕ 0xFF = 0x80
Values at [0x60 … 0x63] are a UNIX TIMESTAMP in little endian notation:
- Dump 1: 0x4F9E2C27 -> 0x272C9E4F = 657235535 = 10/29/1990 @ 9:25pm
- Dump 2: 0x71B62C27 -> 0x272CB671 = 657241713 = 10/29/1990 @ 11:08pm
- Dump 3: 0x18592D27 -> 0x272D5918 = 657283352 = 10/30/1990 @ 10:42am
  - Ok, we are not in the 90ies, but the time difference between transactions is correct, maybe the vending machine doesn’t have an UPS 🙂

Early findings:

Timestamp of the last transaction was stored as 32 bit integer at MIFARE block 6 and redundant at at MIFARE block 10
Only MIFARE blocks 12 and 13 has “Value block” format, and they are used to store the counter of remain transaction in 32 bit format.
This counter starts from 0x7FFFFFFF (2.147.483.647) and is decreased at each transaction
Blocks 1, 4, and 14 contains some data that are fixed between dumps
Blocks 8 and 9 changes entirely at each transaction

The credit:

If there is credit stored on the card, it was encoded at blocks 8 and 9, and the number of bytes involved between small credit difference (for example between 0.00€ and 0.10€) could indicate that some cryptographic function is involved.

At this time, a double spending attack could confirm if the credit is really stored on the card.
So, after spending all the credit, I have rewritten a previous dump on the card and I went to test it at the vending machine. The card was fully functional with the previous credit stored in that dump. Now, I’m certain that the credit is encoded (and probably encrypted) in the blocks 8 and 9.

Conclusion:

Even if the encoding format of the credit is still unknown, a double spending attack was possible.

This means that the vendor’s effort to obfuscate the credit is nullified 🙁

Adding some unique token on the card that are invalidated into back-end after each transaction, means that this token needs to be shared between all the vending machines of the vendor, but, if we add internet connection to the vending machine, there is no longer reason to store the credit on the card.

So, after all, the only remediation action that makes sense is: DO NOT STORE THE CREDIT ON THE CARD! And, more generally: DO NOT TRUST THE CLIENT!

Road to arbitrary credit:

Spending 1€ infinite times isn’t the scope of that hack. The only real scope is FUN!
To continue this analysis I need to collect a large number of dumps to advance some hypothesis so, when I have other material I will make another post.

An example of easier card:

Some vendor has more easier approach by using the MIFARE “Value block” to store the credit without obfuscation or encryption.

The above screenshot made with “MIFARE Classic Tool” on Android smartphone, represents a Value Block used to store the credit:

0x00000CE4 = 3300 is the value in Euro thousandths (3.30€).

This particular vendor do not use key A and the Key B is a default key 0xFFFFFFFFFFFFFFFF, so the attacker doesn’t need to crack anything.

Reverse engineering and cracking of a Vending Machine is always funny 🙂

Versione italiana sul sito dell’associazione LILIS.

The RS232 standard implements an hardware flow control mechanism. While TxD and RxD pins are used to send and receive data, DCD, DTR, DSR, RTS, CTS and RI allow devices which support hardware control flow to maintain a reliable data connection between a transmitter and a receiver.

Signal Pin  Pin  Direction  Full
Name   (25) (9)  (computer) Name
-----  ---  ---  ---------  -----
FG      1    -      -       Frame Ground
TxD     2    3      out     Transmit Data
RxD     3    2      in      Receive  Data
RTS     4    7      out     Request To Send
CTS     5    8      in      Clear To Send
DSR     6    6      in      Data Set Ready
GND     7    5      -       Signal Ground
DCD     8    1      in      Data Carrier Detect
DTR    20    4      out     Data Terminal Ready
RI     22    9      in      Ring Indicator

From the computer side the output pins are: RTS, DTR and obviously TxD.

Both RTS and DTR pins are usable to our scope, I have chosen the RTS one.

The RS-232 standard defines the voltage levels that correspond to the logical “one” in the range of +3V to +15V and the logical “zero” in the range of -15V and -3V but, through a TTL/CMOS converter, we can use 0V and +3.3V/+5V.
If you want to use a real RS232 port (DB9 connector) you can use the MAX232 IC to use a TTL or CMOS voltage level.

In a modern computer without a RS232 port you can use a cheap USB/TTL-232 interface like this, which already has the TTL voltage values.

Not all of these interfaces have all control flow pins so, if you want to buy the new one, check if RTS and/or DTR are present. On my old USB-to-TTL232 interface neither of those was there, but it uses a PL2303 driver that exposes RTS on pin 3 and DTR on pin 2 so I had to solder a wire directly on IC pin.

The output of RTS (or DTR) pin needs to be connected to the Base of a BJT transistor that controls the current flow through a relay:

If you don’t want to build that part you can use one of these: https://amzn.to/2QDpRQF or https://amzn.to/2QycYHM.

This is the code to SET the RTS pin:

#include <fcntl.h>
#include <sys/ioctl.h>
#define PORT "/dev/ttyUSB0"
main() {
  int fd;
  fd = open(PORT, O_RDWR | O_NOCTTY);
  ioctl(fd, TIOCMBIS, TIOCM_RTS);
  close(fd);
}

and to CLEAR the RTS pin:

#include <fcntl.h>
#include <sys/ioctl.h>
#define PORT "/dev/ttyUSB0"
main() {
  int fd;
  fd = open(PORT, O_RDWR | O_NOCTTY);
  ioctl(fd, TIOCMBIC, TIOCM_RTS);
  close(fd);
}

To use DTR instead of RTS change TIOCM_RTS to TIOCM_DTR.

Compile it with gcc:

gcc set-rts.c -o set-rts

or run in with tcc:

tcc -run set-rts.c

Here’s mine:

Tag: hacking

Approaching the Reverse Engineering of a RFID/NFC Vending Machine