RFID: Monotonic Counter Anti-Tearing Defeated

Tear-off techniques to the next level.

Introduction

For this second post in collaboration with Iceman, we will briefly present how the generic tear-off tools presented in the previous blog post RFID: New Proxmark3 Tear-Off Features and New Findings [2] were used to defeat a secure monotonic counter implementation present in some models of MIFARE Ultralight and NFC cards from NXP.

Bypassing this security feature can have different impact depending on how the vulnerable cards and the specific feature is used in different systems. This blog post will only describe the technical matters of the issue and not analyze the possible impact on various systems. The issue was disclosed to NXP and the vendor has published Application Notes (referenced below) describing possible mitigations.

We won't repeat what was explained in length in the previous blog posts [1] and [2]. The main results we'll use are that each bit gets erased at its own pace depending on stochastic manufacturing process, and that some bits can be weakly programmed, i.e. they are not fully programmed and when being read back, they are sometimes interpreted as 1 and sometimes as 0.

MIFARE Ultralight EV1 Monotonic Counters

The MIFARE Ultralight EV1 [3] contains three 24-bit monotonic counters with anti-tearing support, which means one can increment a counter by an arbitrary value but never decrement it.

The available commands related to these counters are the following:

• INCR_CNT to increment by 0 or a positive value;
• READ_CNT to read the current counter value;
• CHECK_TEARING_EVENT to read an anti-tearing 8-bit validity flag.

The commands accept a counter number as parameter as there are three counters, but to keep things simple we'll assume there is only one counter.

The validity flag is presumably written at the same time as a new counter value is written and it should be equal to 0xBD. Any other value means that an increment operation was interrupted and, internally, the new counter value could not be fully written. If such event occurs, the counter keeps its previous value.

Under the hood, the counter is saved in two memory slots, similar to the Protection Words of the EM4305 we defeated in our previous blogpost. But here, the memory slots are not directly made available and READ_CNT implements some logic to return the highest of the slots containing a valid value (i.e. associated to a valid 8-bit flag). The same logic is applied for INCR_CNT to decide which slot to read from and in which slot to store the new counter value. On the other side, CHECK_TEARING_EVENT returns either the corrupted flag if one is corrupted, or the flag of the slot containing the lowest value.

Thanks to these anti-tearing mechanisms, the counter should never be corrupted or decreased.

Experiments and strategy

We observed that by tearing an INCR_CNT by 1 at the limit of getting a valid flag, then reading the counter several times, sometimes a READ_CNT returns the previous counter value, while CHECK_TEARING_EVENT returns a valid flag.

A possible explanation is that while the flag was properly written, one bit of the new counter was weakly programmed and when it's read as a 0, the counter value is lower than the old counter value still present in the other slot and yet READ_CNT returns the old counter value as being the current one.

Based on this observation, we developed a strategy to reset completely the counter back to zero.

Imagine we set the counter to a power of 2, e.g. 0x000100 and the only bit set is a bit we can program weakly. This counter slot could be read as 0x000100 or 0x000000. When it's seen as 0x000000, the other slot with the previous counter value will become the current counter and, as in our first experiment, we only manage to rollback the counter. But if we invalidate the other slot by tearing an INCR_CNT sooner to corrupt the flag, then 0x000000 will become the current counter value.

Let's define the two slots as A and B and let's assume that when both slots have the same content and have a valid flag, B gets selected as the current one, i.e. when one reads the counter, slot B value is returned and when one increments the counter, the new value is written in A.

Our strategy is the following:

• Increase the counter to the closest $$2^{N}-1$$ value. If e.g. the counter is equal to 0x00008F, execute an INCR_CNT(0x70) to reach 0x0000FF;
• Execute an INCR_CNT(0) so both slots are equal, e.g. A:0x0000FF, B:0x0000FF;
• Execute an INCR_CNT(1) and tear near the end of the write operation. Hope for a counter set to $$2^{N}$$ with a weak bit. e.g. A:0x000?00, B:0x0000FF with both flags still valid;
• Execute an INCR_CNT(0) and tear before the write operation. Hope A is read as $$2^{N}$$ so B gets corrupted. e.g. A:0x000?00 and B flag indicates B is corrupted;
• Execute an INCR_CNT(0). Hope A is read as 0x000000 so A is copied to B, e.g. A:0x000?00, B:0x000000 and both flags are again valid;
• Execute an INCR_CNT(0). Hope A is read as 0x000000 so B is copied to A and they become both stable, e.g. A:0x000000, B:0x000000;
• If it fails, try again. If the counter increases slightly with attempts, it's not a problem as once the attack is successful we can bump the counter back to $$2^{N}-1$$ and try again;
• If after a while, there is no indication that bit $$N$$ can be weakly programmed, move to the next bit and bump the counter to $$2^{N+1}-1$$.

The strategy must be complemented to adjust automatically the timings and to cover all the possible outcomes, including corrupted flags and weakly programmed flags.

Note that the probability of getting a 0 or a 1 when reading a weak bit can be influenced by controlling the distance between the tag and the reader. Closer to the reader you'll get more often a 1 and further away a 0.

Results

We have demonstrated the possibility to reset completely a MIFARE Ultralight EV1 counter despite its anti-tearing features and we reported the issue to NXP. The vendor had reproduced our findings and acknowledged it as a vulnerability.

According to NXP, the list of affected products is the following.

In MIFARE Ultralight family:

• MIFARE Ultralight EV1, MF0UL;
• MIFARE Ultralight C, MF0ICU;
• MIFARE Ultralight NANO, MF0UN.

In NTAG 21x family:

• NTAG 210(µ)/212: NT2L1, NT2H10, NT2H12;
• NTAG 213 (TT/F) /215 /216 (F): N2H13, NT2H15, NT2H16.

None of these products are Common Criteria certified.

Other security features likely based on hidden slots might be affected by tearing events and weak bits too, such as OTP bits or Lock bits. But it seems much harder to corrupt them and if the attack succeeds, it will probably clear only a few bits.

Mitigations

An update of Application Notes AN11340 [5] and the new AN13089 [6] propose mitigations, such as doubling writes on OTP and Lock bits to update all internal slots and using the upper range of the counters. For example, if an application needs a counter from 0x000000 to 0x000100, use a counter from 0xfffeff to 0xffffff. And when it reaches 0xffffff, issuing a dummy increment by zero to update the other internal slot will prevent any further attempt to affect the counter as it will only affect slot A while slot B will always have priority when the counter is read.

Adding a MAC (Message Authentication Code) may help too but beware of rollback attacks.

The described attack has been realized with the generic tear-off tooling (hw tearoff combined with hf 14a raw) we added to the Proxmark3 RRG code repository as explained in the previous blogpost [2].

Conclusion

This third blogpost on the EEPROM tearing topic shows that designing a security function while taking care of all the possible weird effects of weak bits is quite challenging. This leaves a number of opportunities for attackers to defeat security features based on EEPROM, even in the presence of anti-tearing countermeasures.

So far, we analyzed only RFID EEPROMs but the same type of issues might probably be found on other non-RFID EEPROMs as well. So, look around you and try to find other interesting targets!

We will present an overview of our tearing techniques, their consequences and their use to defeat various security features at SSTIC 2021 [4] the 3rd of June at 15:00 CET. The talk will be mainly in French but a (long!) article in the proceedings will be published in English at https://www.sstic.org/2021/presentation/eeprom_it_will_all_end_in_tears/. We will cover this attack in greater detail as well.

We want to highlight that we appreciate the cooperation of NXP leading to this coordinated disclosure.