Simos18 Supplier Bootloader (SBOOT) Exploit: Reading Boot Passwords

• Simos18_SBOOT

  5 84 0.0 Python

  Documentation and tools about Simos18 SBOOT (Supplier Bootloader), including a Seed/Key bypass and Tricore boot password recovery tool.

I have glossed over all of the actual data details here for brevity. For details including exact messages (and code!), please visit https://github.com/bri3d/Simos18_SBOOT

• TC1791_CAN_BSL

  1 47 0.0 C

  CAN Bootstrap Loader (BSL) for Tricore AudoMAX (TC1791 and friends), including arbitrary read/write as well as compressed read functionality.

In addition to the RSA signature, all Simos18 code blocks are also checked against a simple CRC checksum. And, in this case, SBOOT tries to validate the CRC checksum before it validates the RSA signature. It turns out the CRC validation is performed using an address header, and the address header has weak bounds checking. So, we can ask the ECU to checksum the boot passwords, and send us the checksum back. Since CRC is not a cryptographic hash (it is reversible), we can use the CRC values to back-calculate the boot passwords. Unfortunately, we can't do this cleanly, because the high side of the address range IS correctly bounds-checked. So instead, we have to start the CRC checksum process, then rapidly reboot into the Tricore Bootstrap Loader and dump the contents of RAM, to see the intermediate/temporary CRC values that have been calculated for only the boot passwords. https://github.com/bri3d/TC1791_CAN_BSL/blob/main/bootloader.py#L113

• WorkOS

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• ghidra_tc1791_registers

  1 12 1.8

As for my process: I loaded up SBOOT in Ghidra using a register map I built using available TriCore toolchain information: https://github.com/bri3d/ghidra_tc1791_registers . Then I literally just, well, read the code. The GPT timing comparisons were easy to find from access to the GPTA registers, but it took a long time to realize the base frequency the PLLs were locked at (8.75Mhz) to translate the cycle-timing back into real timing.

• crchack

  1 180 6.7 C

  Reversing CRC for fun and profit

After the Seed/Key process was figured out, the CRC exploit was very obvious as the bounds checking is blatantly broken. https://github.com/resilar/crchack came in handy to take the CRC + unknown data and back-generate the correct data used to produce the CRC.

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