Accept HTTP Basic client credentials on the authorization_code grant - #81
Merged
joehoyle merged 2 commits intoSep 9, 2026
Merged
Conversation
RFC 6749 section 2.3.1 makes Basic auth the recommended way for a client to authenticate at the token endpoint, and the client_credentials grant already parses it. The authorization_code grant only read client_id from the body, so MCP clients authenticating this way failed with a missing parameter error.
tomjn
force-pushed
the
fix/basic-auth-authorization-code-grant
branch
from
September 7, 2026 17:08
0062ce8 to
7e1df6f
Compare
roborourke
approved these changes
Sep 9, 2026
Contributor
|
@joehoyle we're working around this for now but would good to get an additional review |
joehoyle
requested changes
Sep 9, 2026
RFC 6749 section 2.3.1 has the client form-encode the id and secret before they go in the header, so a secret containing a space or a plus only matches once the server decodes it.
joehoyle
approved these changes
Sep 9, 2026
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
3 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
A client that authenticates at the token endpoint with HTTP Basic auth, which RFC 6749 section 2.3.1 recommends, gets "Missing parameter(s): client_id" on the authorization_code grant. The client_credentials grant already reads the header. The authorization_code grant only looked at body parameters.
Both the MCP connector and Claude Code authenticate this way, and both failed the exchange against a site running this plugin. The site's RFC 8414 discovery document advertised
client_secret_basic, so the clients were behaving correctly.The header parsing is now shared between the two grants. On the authorization_code grant it only applies when the body has no
client_id, so body parameters keep precedence. A malformed header returns the sameinvalid_requesterror the client_credentials grant already uses.This does not verify
client_secreton the authorization_code grant, which #36 covers.