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Enforce PKCE metadata compliance in OAuth client (#730) (#1700)

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Pranav Senthilnathan 2026-07-21 08:24:25 -07:00 • committed by GitHub
commit 7369a265a8
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1 changed files with 230 additions and 10 deletions
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68
src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs
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@ -366,8 +366,15 @@ internal sealed partial class ClientOAuthProvider : McpHttpClient
private async Task<AuthorizationServerMetadata> GetAuthServerMetadataAsync(Uri authServerUri, string? resourceUri, CancellationToken cancellationToken)
{
// Tracks whether at least one well-known endpoint returned a structurally valid metadata document.
// This distinguishes "no metadata document was found" (eligible for the 2025-03-26 legacy fallback)
// from "a metadata document exists but failed PKCE validation" (must not fall back to synthesized defaults).
var metadataDocumentFound = false;
McpException? pkceValidationFailure = null;
foreach (var wellKnownEndpoint in GetWellKnownAuthorizationServerMetadataUris(authServerUri))
{
AuthorizationServerMetadata? metadata;
try
{
var response = await _httpClient.GetAsync(wellKnownEndpoint, cancellationToken).ConfigureAwait(false);
@ -377,7 +384,7 @@ internal sealed partial class ClientOAuthProvider : McpHttpClient
}
using var stream = await response.Content.ReadAsStreamAsync(cancellationToken).ConfigureAwait(false);
var metadata = await JsonSerializer.DeserializeAsync(stream, McpJsonUtilities.JsonContext.Default.AuthorizationServerMetadata, cancellationToken).ConfigureAwait(false);
metadata = await JsonSerializer.DeserializeAsync(stream, McpJsonUtilities.JsonContext.Default.AuthorizationServerMetadata, cancellationToken).ConfigureAwait(false);
if (metadata is null)
{
@ -395,23 +402,63 @@ internal sealed partial class ClientOAuthProvider : McpHttpClient
ThrowFailedToHandleUnauthorizedResponse($"AuthorizationEndpoint must use HTTP or HTTPS. '{metadata.AuthorizationEndpoint}' does not meet this requirement.");
}
metadata.ResponseTypesSupported ??= ["code"];
metadata.GrantTypesSupported ??= ["authorization_code", "refresh_token"];
metadata.TokenEndpointAuthMethodsSupported ??= ["client_secret_post"];
metadata.CodeChallengeMethodsSupported ??= ["S256"];
return metadata;
// A structurally valid metadata document was discovered. Even if it fails PKCE validation
// below, its existence disqualifies the legacy fallback that would otherwise synthesize S256.
metadataDocumentFound = true;
}
catch (Exception ex)
{
LogErrorFetchingAuthServerMetadata(ex, wellKnownEndpoint);
continue;
}
try
{
// Per the MCP spec, clients MUST verify PKCE support via authorization server metadata and refuse
// to proceed without it. Rather than failing on the first document that lacks it, skip to the next
// discovery endpoint: different well-known endpoints (OAuth 2.0 vs OpenID Connect) can return
// different documents for the same server, and OpenID Connect metadata commonly includes the field.
var codeChallengeMethods = metadata.CodeChallengeMethodsSupported;
if (codeChallengeMethods is null)
{
ThrowFailedToHandleUnauthorizedResponse(
$"Authorization server metadata from '{wellKnownEndpoint}' does not include 'code_challenge_methods_supported'. MCP clients require PKCE support and must refuse to proceed.");
}
if (!codeChallengeMethods.Contains("S256"))
{
var advertisedMethods = codeChallengeMethods.Count > 0
? string.Join(", ", codeChallengeMethods)
: "<none>";
ThrowFailedToHandleUnauthorizedResponse(
$"Authorization server metadata from '{wellKnownEndpoint}' does not advertise required PKCE method 'S256' in 'code_challenge_methods_supported'. Advertised methods: {advertisedMethods}.");
}
}
catch (Exception ex)
{
LogAuthServerMetadataMissingPkceSupport(ex, wellKnownEndpoint);
pkceValidationFailure = ex as McpException ?? new McpException(ex.Message, ex);
continue;
}
metadata.ResponseTypesSupported ??= ["code"];
metadata.GrantTypesSupported ??= ["authorization_code", "refresh_token"];
metadata.TokenEndpointAuthMethodsSupported ??= ["client_secret_post"];
return metadata;
}
// A discovered metadata document that failed PKCE validation must not be replaced with synthesized
// defaults. Surface the specific PKCE failure regardless of whether PRM was available.
if (metadataDocumentFound && pkceValidationFailure is not null)
{
throw pkceValidationFailure;
}
if (resourceUri is null)
{
// 2025-03-26 backcompat: when PRM is unavailable and auth server metadata discovery
// also fails, fall back to default endpoint paths per the 2025-03-26 spec.
// 2025-03-26 backcompat: when PRM is unavailable and no auth server metadata document was
// discovered, fall back to default endpoint paths per the 2025-03-26 spec.
return BuildDefaultAuthServerMetadata(authServerUri);
}
@ -1155,6 +1202,9 @@ internal sealed partial class ClientOAuthProvider : McpHttpClient
[LoggerMessage(Level = LogLevel.Error, Message = "Error fetching auth server metadata from {Endpoint}")]
partial void LogErrorFetchingAuthServerMetadata(Exception ex, Uri endpoint);
[LoggerMessage(Level = LogLevel.Warning, Message = "Authorization server metadata from {Endpoint} does not satisfy PKCE requirements; skipping it and trying the next discovery endpoint.")]
partial void LogAuthServerMetadataMissingPkceSupport(Exception ex, Uri endpoint);
[LoggerMessage(Level = LogLevel.Information, Message = "Performing dynamic client registration with {RegistrationEndpoint}")]
partial void LogPerformingDynamicClientRegistration(Uri registrationEndpoint);