There's only so much an API can do with garbage input. If this function took a pointer and a length, that's not a magic fix, you could still pass it a bad pointer and/or a bad length..
The problem here is that binary input is valid according to the spec [1]. It's not malicious input in the sense that a programmer is using an interface deliberately wrong but rather in the sense that a counterparty could send you a non-garbage certificate that contains that data - which would be valid, but still break this code. That's not comparable to passing a bad pointer or a bad length.
[1] at least according to the post. The fact that gnutls added a binary interface later seems to support that reading.
Yes, the spec says the field can be string or binary. The API only handles string fields. The API should be (and was) updated with a function that handles binary fields, but there's nothing wrong with the original function.
If this was C++, and the function took a std::string, would you say it was horribly broken because you serialized a 4 byte IP address into a 4 byte std::string buffer and the function didn't handle it correctly?
Seriously, if the function deals with untrusted user input and pretends to conform to a spec, weasel wording around by saying that the function does only partially conform to the spec and will blow up in the users face when passed other, spec compliant input and then deferring all responsibility to the user, yes, that counts as horribly broken in my books (even though you were the one to introduce those words). That's what we have libraries for, so that me and you don't have to deal with this mess that x509 cert parsing is - I've seen enough of it that I know I don't want to go down this particular hell hole - and I've just been standing at the sideline and watching others wrestle with it.
functions don't conform to specs, the API and library as a whole should conform to the spec. It's perfectly valid to have one function that supports strings and another function that supports binary data.
They do now, and the first one is still using strlen(). Does the existence of the second function mean it's now ok for the first to use strlen()? You can still crash the program if you send binary data to the string function instead of the binary function..
