Simos18_SBOOT VS ME7Sum

I did find an older VW "emergency start" product that claims to only work with Bosch MED17 and MED9, and I suspect it's using a memory-access primitive (either UDS or CCP) to release the immobilizer.

It's trivial to disable an immobilizer in software by re-flashing the ECU, yes, but modern ECUs have two strong protections against this:

* Cryptographic signature checking against update/re-flash payloads (I've done extensive research on these on VW Continental ECUs - https://github.com/bri3d/VW_Flash )

and an even better and more obvious protection:

* The ECU application software won't descend into the re-flash software (Customer Bootloader) unless the immobilizer is free (a valid key is present).

This is a lot of what helps to reduce surface area from an "emergency start" style attack to an AKL attack - now that the Customer Bootloader won't start without the Immobilizer being unlocked, an attacker needs to remove the control unit to flash it with a Supplier Bootloader exploit ( https://github.com/bri3d/simos18_sboot ) or physical access (BDM/JTAG).

SIMOS18 SBOOT: https://github.com/bri3d/simos18_sboot Illustrates common security vulnerabilities in modern control units (inadequate RNG entropy, reset exploits). Illustrates common "SBOOT recovery mode break-in" / "TSW Mode" concept that many control units have.

That's pretty cool! I wonder how properly they were really signed - there are _so many_ mistakes even in systems that at least don't use an example key off the Internet.

The most common ones I know of are:

* Out-of-bounds write issues allowing "signature was validated" flags to be overwritten in Flash memory, like https://github.com/jglim/UnsignedFlash

* State machine mistakes, like https://github.com/bri3d/VW_Flash/blob/master/docs/docs.md - allowing Flash to be written again after it was already written, without an erase first.

* Filesystem parsing mistakes, like those in a number of VW AG head units: https://github.com/jilleb/mib2-toolbox/issues/122

* The use of RSA with E=3 and inadequate padding validation, like https://words.filippo.io/bleichenbacher-06-signature-forgery... .

* Failure to understand the system boundaries, like in the second part of https://github.com/bri3d/simos18_sboot where "secret" data can be recovered by halting the system during a checksum process.

* Hardware fault injection issues, as used in https://fahrplan.events.ccc.de/congress/2015/Fahrplan/system... .

Fundamentally this is of course, a very hard problem, since in the "protect against firmware modification" case, the attacker has physical access. But, compared to the state of the art in mobile devices and game consoles, automotive stuff is still way behind.

My writeups and JG Lim's cover three of the common mistakes in modern modules (supplier backdoor bugs in Simos supplier bootloader, state machine issues in Simos VW bootloader, and block buffer validity confusion / bounds check issues in Mercedes instrument cluster).

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

ME7Sum: https://github.com/nyetwurk/ME7Sum . Reads, analyzes, and fixes the complex proprietary checksum system used in old Bosch ECUs. Checksums in newer control units have mostly gotten simpler as more RAM and CPU were available and "multipoint" schemes were less necessary. Also can correct the very silly ME7.5 RSA signature system, where firmware was signed but self-checked using a public key contained... inside of the firmware. So the key could just be replaced and the firmware re-signed. Interesting read to understand the often arcane proprietary checksum routines manufacturers love to use.