On June 25th, we released a firmware update for Trezor One devices, bringing new features and adapting the device to the structural changes of several cryptocurrencies. The firmware version 1.6.2 also contains two security fixes for issues discovered on May 25th, the details of which are going to be introduced in this article. We have opted to delay the release of the details for a number of reasons, but the main rationale was that the major aspects of these issues affect outdated firmware (1.6.0 and earlier) of Trezor One. The existing firmware 1.6.1, released on March 21st (prior to discovery of these bugs) had already partly mitigated these issues.

There were two security fixes included in this firmware update. Both bugs were discovered and responsibly disclosed by Christian Reitter during the research of his master’s thesis under the supervision of Dr. Jochen Hoenicke (also a security researcher at SatoshiLabs) and Dr. Daniel Dietsch. The first one is a buffer overflow bug, affecting devices with firmware 1.6.0 and earlier. The second one is a stack overflow bug in the dry-run recovery, affecting devices up to firmware 1.6.1.

There is no evidence that either of the vulnerabilities has been used in practice.

Two important security issues were fixed in 1.6.2. All Trezor One devices should be updated to the latest firmware version.

The Trezor Model T is not affected by this vulnerability.

The security issues were discovered by Christian Reitter with the help of extensive fuzz testing during his academic research about Trezor One. As a result he discovered the two following bugs.

The first issue that Christian found is a buffer overflow. The bug can be exploited by an attacker who floods the device with data packets of a specific type. After a certain number of packets, the buffer overflows, allowing the attacker to write up to 60 bytes of data into a protected part of the memory. Depending on the memory layout the flaw can be escalated to arbitrary code execution (firmware 1.5.x), or has no consequences (1.6.0 and 1.6.1)

This bug affects mainly Trezor One devices with firmware version 1.5.2 and below. Since firmware 1.6.0 can be downgraded, it indirectly affects devices with this version as well. However, physical access to the device is required to perform this downgrade. Devices with firmware 1.6.1 are unaffected due to other changes to the firmware, which moved the protected part of the memory into a different section of the device.

The attack method requires the attacker to install malware on a host device (computer) of an unsuspecting Trezor owner to obtain sufficient access to the USB interface. The bug also cannot be triggered by a malicious, fake Trezor Wallet, because the Wallet does not have sufficient low-level access. The malware would have to be delivered via the Trezor Bridge, Chrome plugin, or other software.

Christian also found that there is a sufficiently large window in the dry-run recovery during which recursive packet handling behavior can be triggered, leading to a stack overflow. This bug affects all Trezor One devices with firmware version 1.6.1 and earlier. However, the exploitation of this bug is made more difficult by several additional safeguards that have been put in place during previous firmware releases.

To exploit this bug, the attacker would have to coordinate the stack overflow with the right data fields to overwrite important data. This is difficult as USB communication data is the first data that would be corrupted by a stack overflow, preventing an attacker from sending more USB packets. Since firmware version 1.6.1 the MPU is activated, and therefore SRAM is not executable (the device cannot run commands from the SRAM). Moreover, there is no easy way to debug the attack method on regular hardware, making it more difficult to create and test the exploit.

Both issues affect devices with firmware version 1.5.2 and earlier. The buffer overflow can also be exploited on Trezor One with firmware version 1.6.0 but requires a forced firmware downgrade.

The stack overflow bug can be triggered on Trezor One with firmware version 1.6.1 and earlier.

The bug was reported via the responsible disclosure program. After determining that 1.6.1 was not affected by the more serious buffer overflow bug, we made the update to this version mandatory.

Both bugs were fixed by adding proper bounds checking and avoiding unbounded recursion. The fix was kept in a private repository until the new firmware 1.6.2 was ready.

First of all, please make sure you have your recovery seed with you when you perform the update. (Link to manual)

Go to Trezor Wallet and follow the update instructions shown on the screen. When prompted, replug your Trezor One device with both buttons plugged to start it in bootloader mode. Confirm the update procedure, and you will have a new firmware on your device.

Please keep in mind that if your Trezor is running firmware 1.6.1, it will be wiped during an update, due to an unrelated bug. If you do not have your recovery seed, make sure to retrieve it before starting the update process. If you are unsure about the correctness of your recovery seed backup, verify it using dry-run recovery.

If you are running firmware earlier than 1.6.1, please start the update process as soon as possible. Keep in mind that you might still need your recovery seed, as the memory might be erased due to failures (cable, data, computer) during the update process. You should always have your recovery seed available during the update process.

There is no evidence that either of the vulnerabilities has been used in practice. However, this may change after the vulnerabilities were disclosed and we urge our customers to update the firmware for all their Trezor One devices.

If your Trezor One is running firmware 1.6.2, your device is safe against all known attack vectors.

The Trezor Model T is not affected by this vulnerability.

No, Trezor devices are shipped without firmware preloaded, therefore you will install the latest version.

If your device is not initialized yet, then please update the firmware first.

Even though the vulnerabilities disclosed in this article have not been used in practice, we still recommend keeping your devices up-to-date at all times. Regular firmware updates are the key to a secure product.

Please, go to Trezor Wallet. If the Wallet tells you your firmware is outdated, please run the update process. The firmware update will update the bootloader as well.

Firmware: 1.6.2

Bootloader: 1.5.0

An US-based Trezor-clone hardware wallet was affected by a different but related vulnerability. We disclosed the issues to their team and cooperated with them to resolve the bugs.

All other hardware wallets based on the Trezor One design are most likely vulnerable.

Trezor Model T is the next-generation hardware wallet, designed with experiences of the original Trezor in mind, combined with a modern and intuitive interface for improved user experience and security. It features a touchscreen, faster processor, and advanced coin support, as well as all the features of the Trezor One.

Trezor One is the most trusted and ubiquitous hardware wallet in the world. It offers unmatched security for cryptocurrencies, password management, Second Factor, while maintaining an absolute ease-of-use, whether you are a security expert or a brand new user.

SatoshiLabs is the innovator behind some of the most pivotal and influential projects with Bitcoin and cryptocurrencies, mainly Trezor, the world’s first cryptocurrency hardware wallet, or CoinMap.org, the primary resource for bitcoin-accepting venues.

Details about the security updates in Trezor One firmware 1.6.2 was originally published in Trezor Blog on Medium, where people are continuing the conversation by highlighting and responding to this story.

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