Days after being announced, Tenable reverse engineered the Intel AMT Vulnerability. According to a blog post [tenable.com], the vulnerability is a backdoor dream. The AMT web interface uses HTTP Digest Authentication [wikipedia.org], which uses MD5. The problem is that partial matches of the hash are also accepted. Therefore, Tenable decided to experiment and while doing so:
[W]e reduced the response hash to one hex digit and authentication still worked. Continuing to dig, we used a NULL/empty response hash (response="" in the HTTP Authorization header).
Authentication still worked. We had discovered a complete bypass of the authentication scheme.
Long story short, for over five years, a complete and trivial bypass of AMT authentication has existed. If this wasn't an intentional backdoor, it is a monumental mistake in security and coding best practices. Regardless, the "backdoor" is now public. With Shodan showing thousands of unpatchable computers (as no patch is currently available, assuming they would ever be patched) exposed to the Internet, some poor IT sod is bound to show up to work some bad news on Monday.