Resources

People often ask me "How did you learn how to hack?" The answer: by reading. This page is a collection of the blog posts and other articles that I have accumulated over the years of my journey. Enjoy!

Radio-frequency identification (RFID) cards are common for scanning for identity reasons, such as school cards or at hospitals. RFID cards do not have power themselves; the reader gives the card the power. So, what happens if a user tears the card away from the reader? Writes may be unfinished, leaving the card in an insecure state!

In a previous article, the same authors discussed this as a whole. They discuss the theoretical nature of the attack with EEPROM writes, then move into RFID specific attacks.

The original article attacks the ATA5577C RFID writer. This device attempts to protect devices from read/write operations with a password. Using a few different techniques, this security protection was able to be bypassed! In particular, by tearing the RFID card AFTER a clearing operation, only SOME of the bits were flipped. Because of this, some of the security had been defeated.

The actual linked above looks into a Monotonic counter that is implemented by MIFARE Ultralight EV1. In this mode, there are three operations: increment count, read the count and read the validity flag. The goal of the attack is to bypass the anti-tearing feature in order to corrupt the counter.

The validity flag is presumably written at the same time as a new counter value is written. But, the slot containing the highest value is return for the counter value, as there are multiple slots. The flag determines which slot to take.

After doing some testing, the authors noticed that at the limit of a valid flag and reading the counter multiple times that the value being read would be different! But why? One of the bits could be weakly programmed, resulting in a different value being read at different points. If this weakly programmed bit is used in conjunction with a tearing event (corrupted tearing flag), this could cause trouble!

The attack works as follows:

• Have to have a weakly programmed bit at a power of 2. This is done by executing a value that will ROLE this over from 0x100. Hopefully, this will be 0x?00, where the ? is the weakly programmed bit.
• Increment by a value of 0. At the end of the write operation, we tear this off in order to get the second slot corrupted.
• Now, the second slot needs to be recovered. Because the second slot is corrupted and part of the first slot is weakly programmed, we hope for a read of 0x000 instead of 0x100. When we increment by 0, this operation happens.
• With both values stable, the attack has reset

The EEPROM volatility via weakly programmed bits, along with a logic bug allowed for this to be possible. The proxmark now has the capability to perform this attack automatically, which is pretty awesome! This is a super interesting piece of research that is hard to pull of but impactful if it works.