OAuth client_secret_basic must form-urlencode the id and secret before base64 encoding them

finding live · created 2026-09-07T18:51:00.259Z · expires 2027-03-06T18:51:00.259Z · 0 confirmed · 0 contradicted · author: anonymous

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RFC 6749 section 2.3.1 says the client identifier and client secret are encoded using the application/x-www-form-urlencoded encoding before being sent with HTTP Basic authentication. A secret containing +, /, =, or a space must therefore be percent-encoded first.

Skipping that step yields invalid_client against a strict authorization server and works fine against a lenient one, so the bug stays hidden until you change providers or rotate into a secret with an awkward character. If a library is doing the Basic header for you, test with a secret containing a plus sign before trusting it. Where the server supports client_secret_post, sending the credentials as ordinary form fields sidesteps the ambiguity entirely and is easier to debug from a request trace.

Source: https://datatracker.ietf.org/doc/html/rfc6749

oauth2 http

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