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Fix: Relax resource URI validation to accept base URL (#1517)

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Co-authored-by: Jérémy Ballan <jballan@chapsvision.com>
Co-authored-by: Tarek Mahmoud Sayed <10833894+tarekgh@users.noreply.github.com>
Co-authored-by: Tarek Mahmoud Sayed <tarekms@ntdev.microsoft.com>
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Jérémy 2026-06-01 18:49:23 +02:00 • committed by GitHub
commit 168c37179d
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1 changed files with 122 additions and 16 deletions
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29
src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs
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@ -774,15 +774,19 @@ internal sealed partial class ClientOAuthProvider : McpHttpClient
}
/// <summary>
/// Verifies that the resource URI in the metadata exactly matches the original request URL as required by the RFC.
/// Per RFC: The resource value must be identical to the URL that the client used to make the request to the resource server.
/// Verifies that the resource URI in the metadata matches the original request URL.
/// Accepts either an exact match with the full request URL, or a match with the base URL
/// (authority only, path discarded) as allowed by the MCP spec, which derives the authorization
/// base URL by discarding the path component from the MCP server URL.
/// </summary>
/// <param name="protectedResourceMetadata">The metadata to verify.</param>
/// <param name="resourceLocation">
/// The original URL the client used to make the request to the resource server or the root Uri for the resource server
/// if the metadata was automatically requested from the root well-known location.
/// </param>
/// <returns>True if the resource URI exactly matches the original request URL, otherwise false.</returns>
/// <returns>
/// True if the resource URI exactly matches the original request URL or its authority-level base URL, otherwise false.
/// </returns>
private static bool VerifyResourceMatch(ProtectedResourceMetadata protectedResourceMetadata, Uri resourceLocation)
{
if (protectedResourceMetadata.Resource is null)
@ -790,14 +794,22 @@ internal sealed partial class ClientOAuthProvider : McpHttpClient
return false;
}
// Per RFC: The resource value must be identical to the URL that the client used
// to make the request to the resource server. Compare entire URIs, not just the host.
// Normalize the URIs to ensure consistent comparison
string normalizedMetadataResource = NormalizeUri(protectedResourceMetadata.Resource);
string normalizedResourceLocation = NormalizeUri(resourceLocation);
return string.Equals(normalizedMetadataResource, normalizedResourceLocation, StringComparison.OrdinalIgnoreCase);
// Accept exact match with the full MCP endpoint URI
if (string.Equals(normalizedMetadataResource, normalizedResourceLocation, StringComparison.OrdinalIgnoreCase))
{
return true;
}
// Per the MCP spec's "Canonical Server URI" section, both the path-specific URI (e.g. https://mcp.example.com/mcp)
// and the authority-only URI (e.g. https://mcp.example.com) are valid canonical URIs for identifying an MCP server.
// Accept a match with the base URL (authority only, path discarded) to support servers that use the less specific form.
string normalizedBaseUrl = NormalizeUri(new Uri(resourceLocation.GetLeftPart(UriPartial.Authority)));
return string.Equals(normalizedMetadataResource, normalizedBaseUrl, StringComparison.OrdinalIgnoreCase);
}
/// <summary>
@ -916,7 +928,8 @@ internal sealed partial class ClientOAuthProvider : McpHttpClient
// https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization#protected-resource-metadata-discovery-requirements
metadata.WwwAuthenticateScope = wwwAuthenticateScope;
// Per RFC: The resource value must be identical to the URL that the client used to make the request to the resource server
// Validate that the resource URI in metadata corresponds to the server we're connecting to.
// VerifyResourceMatch accepts both an exact URI match and an authority-level (base URL) match per the MCP spec.
LogValidatingResourceMetadata(resourceUri);
if (!isLegacyFallback && !VerifyResourceMatch(metadata, resourceUri))