Ledger Nano S MCU firmware verification bypass

The Ledger Nano S has two chips: a Secure Element that holds the keys, and a non-secure STM32 microcontroller (MCU) that runs the display, buttons and USB. The Secure Element checked the MCU firmware by asking the MCU to report back the contents of its flash memory. Security researcher Saleem Rashid showed that this check could be fooled, so a device could pass verification while running modified MCU firmware.

Ledger’s security bulletin LSB-002 states that the verification “was not robust enough”. Rashid’s proof of concept, a supply-chain scenario, made the device show recovery words chosen by the attacker instead of generating a new seed; he also described an evil-maid scenario in which modified firmware captures the PIN, seed and passphrase after setup. Ledger states that the Secure Element itself was not affected.

Rashid reported the issue to Ledger in November 2017. Ledger released a Nano S firmware update on 6 March 2018 and the write-up was published on 20 March 2018. Ledger states that firmware 1.4.1 fully secures the Nano S; at the time it said the Ledger Blue was also affected and a fix was in progress.

What Ledger advises:

• Buy directly from Ledger or an authorized reseller.
• Update the firmware before use; Ledger states a successful update proves the device was not compromised.