Here's an Intel Whitepaper detailing one such vulnerability of moderate severity (CVE-2019-0090, CVSS 4.4) [1]. From the 'Potential Consequences' table:
> Unauthorized BIOS compromises OS loader and OS integrity... All use cases / user secrets protected by Intel TPM are compromised.
Other bits and pieces of firmware including AMT have been similarly plagued with vulns [2], such as CVE-2017-5689, with a CVSS score of 10!
These are often difficult to patch, and even more difficult to convince manufacturers to spend the effort testing and distributing updates for their products. My own laptop is still running a version of CSME vulnerable to many of these attacks.
[0] https://www.cvedetails.com/vulnerability-list/vendor_id-238/...
[1] https://www.intel.com/content/dam/www/public/us/en/security-...
[2] https://www.cvedetails.com/vulnerability-list/vendor_id-238/...
Intel ME and the equivalent AMD PSP.
> I'm of course not talking about social engineering or fooling the user into typing their password on a fake system, since this is preventable.
As other pointed out when the atacker has physical access it is game over. Also network access is insecure due to management engines.