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Validate the authorization server metadata issuer on every discovery path (#3398)

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Max 2026-09-02 16:51:22 +01:00 • committed by GitHub
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@ -2499,7 +2499,9 @@ metadata's `issuer` exactly matches the authorization server URL advertised in t
resource metadata, as required by [RFC 8414](https://datatracker.ietf.org/doc/html/rfc8414)
section 3.3 ([SEP-2468](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2468)).
The comparison is a simple string comparison ([RFC 3986](https://datatracker.ietf.org/doc/html/rfc3986)
section 6.2.1), so even a trailing-slash disagreement counts as a mismatch. v1 accepted the
section 6.2.1), so even a trailing-slash disagreement counts as a mismatch. (For an older server
that publishes no protected resource metadata the expected value is the MCP server's own origin,
and there a root issuer with a trailing slash is accepted too.) v1 accepted the
metadata without checking, so a server pairing whose two values disagree authenticated fine
under v1 and now fails the entire flow. For example, when the MCP server's protected resource
metadata advertises
@ -2517,7 +2519,7 @@ OAuthFlowError: Authorization server metadata issuer mismatch: https://as.exampl
There is no client-side override. Fix the deployment instead: make the authorization server's
`issuer` string-equal the URL in the protected resource metadata's `authorization_servers`
list. See [OAuth metadata URLs no longer gain a trailing slash](#oauth-metadata-urls-no-longer-gain-a-trailing-slash)
list (or the MCP server's origin, without protected resource metadata). See [OAuth metadata URLs no longer gain a trailing slash](#oauth-metadata-urls-no-longer-gain-a-trailing-slash)
for how v2 preserves the exact string form of these URLs.
### OAuth client requests `offline_access` and adds `prompt=consent` when the authorization server supports it ([SEP-2207](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2207))