oauthex: require S256 PKCE support in GetAuthServerMeta - #1327
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guglielmo-san merged 2 commits intoOct 2, 2026
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GetAuthServerMeta rejected an authorization server that advertises no PKCE method, but accepted one whose code_challenge_methods_supported lists only "plain". The SDK's clients always send an S256 challenge (oauth2.S256ChallengeOption in auth and auth/extauth), so with such a server the flow failed late, when the authorization endpoint rejected the request in the user's browser, rather than during discovery with a clear error. Require "S256" in the advertised methods. The MCP authorization spec requires clients to use S256 (OAuth 2.1 section 7.5.2), and the TypeScript SDK refuses such a server during discovery in the same way. Servers that offer both methods, such as Google's (see testdata/google-auth-meta.json), are unaffected. The new server_with_only_plain_pkce case of TestGetAuthServerMetaPKCESupport fails without the change; server_with_plain_and_s256_pkce checks that a mixed list is accepted. Fixes modelcontextprotocol#1326 Signed-off-by: Akshita kumari <110122283+akshita317@users.noreply.github.com>
guglielmo-san
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Oct 2, 2026
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GetAuthServerMeta rejected an authorization server that advertises no
PKCE method, but accepted one whose code_challenge_methods_supported
lists only "plain". The SDK's clients always send an S256 challenge
(oauth2.S256ChallengeOption in auth and auth/extauth), so with such a
server the flow failed late, when the authorization endpoint rejected
the request in the user's browser, rather than during discovery with a
clear error.
Require "S256" in the advertised methods. The MCP authorization spec
requires clients to use S256 (OAuth 2.1 section 7.5.2), and the
TypeScript SDK refuses such a server during discovery in the same way.
Servers that offer both methods, such as Google's (see
testdata/google-auth-meta.json), are unaffected.
The new server_with_only_plain_pkce case of
TestGetAuthServerMetaPKCESupport fails without the change;
server_with_plain_and_s256_pkce checks that a mixed list is accepted.
Fixes #1326