clientCredentialsFromRequest should not expect Basic Authorization terms being URL Escaped about fosite HOT 2 CLOSED

This does not look like it's accurate to me. This function seems to only be used when basic auth is missing; see these lines. If the basic auth header is present then the function returns. The section of the function you've mentioned is attempting to parse the POST data which MUST be escaped per standard HTTP POST Form semantics, specifically the client secret, not the client credentials grant. This is also indicated by the error.

Never mind, I've read this incorrectly. I suspect this may be one of the inconsistent issues surrounding this scheme specifically, and there are a few if I recall. Will look more into it.

from fosite.

The spec does indeed very clearly require url decoding for these two parameters:

Clients in possession of a client password MAY use the HTTP Basic
authentication scheme as defined in [RFC2617] to authenticate with
the authorization server. The client identifier is encoded using the
"application/x-www-form-urlencoded" encoding algorithm per
Appendix B, and the encoded value is used as the username; the client
password is encoded using the same algorithm and used as the
password. The authorization server MUST support the HTTP Basic
authentication scheme for authenticating clients that were issued a
client password.

https://datatracker.ietf.org/doc/html/rfc6749#section-2.3.1

from fosite.