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Merged
anandh8x merged 4 commits into from
Aug 28, 2026
Merged

fix(mcp): discover OAuth through protected resource metadata #991

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a9e9476
fix(mcp): discover OAuth through protected resource metadata
anandh8x Aug 28, 2026
66543a4
fix(mcp): refuse OAuth discovery redirects
anandh8x Aug 28, 2026
1079939
fix(mcp): harden OAuth metadata discovery
anandh8x Aug 28, 2026
975eaee
fix(mcp): protect discovered OAuth endpoints
anandh8x Aug 28, 2026
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114 changes: 100 additions & 14 deletions internal/mcp/oauth.go
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Original file line number Diff line number Diff line change
Expand Up @@ -63,8 +63,9 @@ func tokenToStored(t oauth.Token) StoredToken {
}

// OAuthConfig describes how to authenticate to a remote MCP server using OAuth.
// Endpoints may be discovered from the server's metadata document; explicit
// values here override or fill in anything discovery cannot provide.
// Endpoints may be discovered through RFC 9728 protected-resource metadata and
// RFC 8414 authorization-server metadata; explicit values override or fill in
// anything discovery cannot provide.
type OAuthConfig struct {
ClientID string
ClientSecret string
Expand All @@ -85,6 +86,11 @@ type authServerMetadata struct {
TokenEndpoint string `json:"token_endpoint"`
RegistrationEndpoint string `json:"registration_endpoint"`
ScopesSupported []string `json:"scopes_supported"`

protectedResourceURL string
protectAuthorizationEndpoint bool
protectTokenEndpoint bool
protectRegistrationEndpoint bool
}

// pkceParams holds a PKCE verifier/challenge pair.
Expand Down Expand Up @@ -120,7 +126,7 @@ type authorizationFlow struct {
// discoverAuthorizationServer fetches the RFC 8414 authorization server metadata
// at the well-known path under baseURL, via the shared engine.
func discoverAuthorizationServer(ctx context.Context, client *http.Client, baseURL string) (authServerMetadata, error) {
meta, err := oauth.DiscoverAuthorizationServer(ctx, client, baseURL)
meta, err := oauth.DiscoverAuthorizationServer(ctx, withoutDiscoveryRedirects(client), baseURL)
if err != nil {
return authServerMetadata{}, err
}
Expand All @@ -133,10 +139,9 @@ func discoverAuthorizationServer(ctx context.Context, client *http.Client, baseU
}, nil
}

// validateResolvedEndpoints applies the shared https/loopback endpoint rule to
// every non-empty endpoint in the resolved metadata — configured or discovered
// alike — so discovery metadata can never downgrade an MCP login to an insecure
// or attacker-controlled authorization, token, or registration endpoint.
// validateResolvedEndpoints applies the shared https/loopback scheme rule to
// every non-empty endpoint in the resolved metadata. Protected-resource
// discovery adds its stricter provenance-aware network rule below.
func validateResolvedEndpoints(metadata authServerMetadata) error {
for _, ep := range []struct{ kind, url string }{
{"authorization", metadata.AuthorizationEndpoint},
Expand All @@ -153,9 +158,43 @@ func validateResolvedEndpoints(metadata authServerMetadata) error {
return nil
}

// resolveAuthorizationServer discovers metadata and applies explicit config
// overrides. Configured endpoints take precedence over discovered ones, and act
// as a fallback when discovery fails or omits a value.
func validateProtectedDiscoveredEndpoints(ctx context.Context, metadata authServerMetadata) error {
if metadata.protectedResourceURL == "" {
return nil
}
policy, err := newOAuthDiscoveryNetworkPolicy(metadata.protectedResourceURL)
if err != nil {
return err
}
for _, endpoint := range []struct {
kind string
url string
protected bool
}{
{kind: "authorization", url: metadata.AuthorizationEndpoint, protected: metadata.protectAuthorizationEndpoint},
{kind: "token", url: metadata.TokenEndpoint, protected: metadata.protectTokenEndpoint},
{kind: "registration", url: metadata.RegistrationEndpoint, protected: metadata.protectRegistrationEndpoint},
} {
if !endpoint.protected || strings.TrimSpace(endpoint.url) == "" {
continue
}
parsed, parseErr := url.Parse(endpoint.url)
if parseErr != nil || parsed.Hostname() == "" {
return fmt.Errorf("mcp oauth: %s endpoint: %w", endpoint.kind, errUnsafeOAuthDiscoveryTarget)
}
if err := policy.validateLiteralHost(parsed.Hostname()); err != nil {
return fmt.Errorf("mcp oauth: %s endpoint: %w", endpoint.kind, err)
}
if _, err := policy.resolve(ctx, parsed.Hostname()); err != nil {
return fmt.Errorf("mcp oauth: %s endpoint: %w", endpoint.kind, err)
}
}
return nil
}

// resolveAuthorizationServer follows protected-resource metadata when the MCP
// endpoint advertises it, falls back to direct authorization-server discovery
// for legacy providers, then applies explicit config overrides.
func resolveAuthorizationServer(ctx context.Context, client *http.Client, baseURL string, cfg OAuthConfig) (authServerMetadata, error) {
// When the config supplies both the authorization and token endpoints
// directly, skip network discovery entirely: there is nothing to discover,
Expand All @@ -174,17 +213,40 @@ func resolveAuthorizationServer(ctx context.Context, client *http.Client, baseUR
return metadata, nil
}

discoveryClient := withoutDiscoveryRedirects(client)
discoveryBase := strings.TrimSpace(cfg.IssuerURL)
protectedResourceDiscovery := false
if discoveryBase == "" {
discoveryBase = baseURL
issuer, found, protectedErr := discoverProtectedResourceAuthorizationServer(ctx, discoveryClient, baseURL)
if protectedErr != nil {
return authServerMetadata{}, protectedErr
}
if found {
discoveryBase = issuer
protectedResourceDiscovery = true
discoveryClient, protectedErr = newAdvertisedOAuthDiscoveryClient(discoveryClient, baseURL)
if protectedErr != nil {
return authServerMetadata{}, protectedErr
}
} else {
discoveryBase = baseURL
}
}

metadata, err := discoverAuthorizationServer(ctx, client, discoveryBase)
metadata, err := discoverAuthorizationServer(ctx, discoveryClient, discoveryBase)
if err != nil {
// Discovery failures are non-fatal when the config supplies the endpoints
// directly; otherwise surface the discovery error.
metadata = authServerMetadata{}
}
if protectedResourceDiscovery {
if strings.TrimSpace(metadata.Issuer) == "" {
return authServerMetadata{}, errors.New("mcp oauth: authorization server metadata is missing its required issuer")
}
if metadata.Issuer != discoveryBase {
return authServerMetadata{}, errors.New("mcp oauth: authorization server metadata issuer does not match protected resource metadata")
}
}

if endpoint := strings.TrimSpace(cfg.AuthorizationEndpoint); endpoint != "" {
metadata.AuthorizationEndpoint = endpoint
Expand All @@ -195,6 +257,12 @@ func resolveAuthorizationServer(ctx context.Context, client *http.Client, baseUR
if endpoint := strings.TrimSpace(cfg.RegistrationEndpoint); endpoint != "" {
metadata.RegistrationEndpoint = endpoint
}
if protectedResourceDiscovery {
metadata.protectedResourceURL = strings.TrimSpace(baseURL)
metadata.protectAuthorizationEndpoint = strings.TrimSpace(cfg.AuthorizationEndpoint) == ""
metadata.protectTokenEndpoint = strings.TrimSpace(cfg.TokenEndpoint) == ""
metadata.protectRegistrationEndpoint = strings.TrimSpace(cfg.RegistrationEndpoint) == ""
}

if strings.TrimSpace(metadata.AuthorizationEndpoint) == "" {
return authServerMetadata{}, errors.New("no authorization endpoint discovered or configured")
Expand All @@ -205,6 +273,9 @@ func resolveAuthorizationServer(ctx context.Context, client *http.Client, baseUR
if err := validateResolvedEndpoints(metadata); err != nil {
return authServerMetadata{}, err
}
if err := validateProtectedDiscoveredEndpoints(ctx, metadata); err != nil {
return authServerMetadata{}, err
}
return metadata, nil
}

Expand Down Expand Up @@ -363,10 +434,25 @@ func Login(ctx context.Context, options LoginOptions) (StoredToken, error) {
}
defer listener.Close()
redirectURI := fmt.Sprintf("http://%s/callback", listener.Addr().String())
protectedEndpointClient := httpClient
if metadata.protectRegistrationEndpoint || metadata.protectTokenEndpoint {
protectedEndpointClient, err = newAdvertisedOAuthDiscoveryClient(httpClient, metadata.protectedResourceURL)
if err != nil {
return StoredToken{}, err
}
}
tokenClient := httpClient
if metadata.protectTokenEndpoint {
tokenClient = protectedEndpointClient
}

if strings.TrimSpace(cfg.ClientID) == "" {
if registration := strings.TrimSpace(metadata.RegistrationEndpoint); registration != "" {
clientID, clientSecret, regErr := registerClient(loginCtx, httpClient, registration, redirectURI, cfg.Scopes)
registrationClient := httpClient
if metadata.protectRegistrationEndpoint {
registrationClient = protectedEndpointClient
}
clientID, clientSecret, regErr := registerClient(loginCtx, registrationClient, registration, redirectURI, cfg.Scopes)
if regErr != nil {
return StoredToken{}, regErr
}
Expand All @@ -390,7 +476,7 @@ func Login(ctx context.Context, options LoginOptions) (StoredToken, error) {
}

flow := &authorizationFlow{
httpClient: httpClient,
httpClient: tokenClient,
metadata: metadata,
config: cfg,
pkce: pkce,
Expand Down
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