oidc.go

/*
Copyright 2017-2019 Gravitational, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package auth

import (
	"context"
	"encoding/json"
	"fmt"
	"io/ioutil"
	"net/http"
	"net/url"

	"github.com/gravitational/teleport"
	"github.com/gravitational/teleport/lib/defaults"
	"github.com/gravitational/teleport/lib/events"
	"github.com/gravitational/teleport/lib/services"
	"github.com/gravitational/teleport/lib/utils"

	"github.com/coreos/go-oidc/jose"
	"github.com/coreos/go-oidc/oauth2"
	"github.com/coreos/go-oidc/oidc"
	"github.com/gravitational/trace"
	"golang.org/x/oauth2/google"
	"golang.org/x/oauth2/jwt"
)

func (s *AuthServer) getOrCreateOIDCClient(conn services.OIDCConnector) (*oidc.Client, error) {
	client, err := s.getOIDCClient(conn)
	if err == nil {
		return client, nil
	}
	if !trace.IsNotFound(err) {
		return nil, trace.Wrap(err)
	}
	return s.createOIDCClient(conn)
}

func (s *AuthServer) getOIDCClient(conn services.OIDCConnector) (*oidc.Client, error) {
	s.lock.Lock()
	defer s.lock.Unlock()

	clientPack, ok := s.oidcClients[conn.GetName()]
	if !ok {
		return nil, trace.NotFound("connector %v is not found", conn.GetName())
	}

	config := oidcConfig(conn)
	if ok && oidcConfigsEqual(clientPack.config, config) {
		return clientPack.client, nil
	}

	delete(s.oidcClients, conn.GetName())
	return nil, trace.NotFound("connector %v has updated the configuration and is invalidated", conn.GetName())

}

func (s *AuthServer) createOIDCClient(conn services.OIDCConnector) (*oidc.Client, error) {
	config := oidcConfig(conn)
	client, err := oidc.NewClient(config)
	if err != nil {
		return nil, trace.Wrap(err)
	}

	ctx, cancel := context.WithTimeout(context.Background(), defaults.WebHeadersTimeout)
	defer cancel()

	go func() {
		defer cancel()
		client.SyncProviderConfig(conn.GetIssuerURL())
	}()

	select {
	case <-ctx.Done():
	case <-s.closeCtx.Done():
		return nil, trace.ConnectionProblem(nil, "auth server is shutting down")
	}

	// Canceled is expected in case if sync provider config finishes faster
	// than the deadline
	if ctx.Err() != nil && ctx.Err() != context.Canceled {
		var err error
		if ctx.Err() == context.DeadlineExceeded {
			err = trace.ConnectionProblem(err,
				"failed to reach out to oidc connector %v, most likely URL %q is not valid or not accessible, check configuration and try to re-create the connector",
				conn.GetName(), conn.GetIssuerURL())
		} else {
			err = trace.ConnectionProblem(err,
				"unknown problem with connector %v, most likely URL %q is not valid or not accessible, check configuration and try to re-create the connector",
				conn.GetName(), conn.GetIssuerURL())
		}
		s.EmitAuditEvent(events.UserSSOLoginFailure, events.EventFields{
			events.LoginMethod:        events.LoginMethodOIDC,
			events.AuthAttemptSuccess: false,
			events.AuthAttemptErr:     trace.Unwrap(ctx.Err()).Error(),
			events.AuthAttemptMessage: err.Error(),
		})
		// return user-friendly error hiding the actual error in the event
		// logs for security purposes
		return nil, trace.ConnectionProblem(nil,
			"failed to login with %v, please contact your system administrator to check audit logs",
			conn.GetDisplay())
	}

	s.lock.Lock()
	defer s.lock.Unlock()

	s.oidcClients[conn.GetName()] = &oidcClient{client: client, config: config}

	return client, nil
}

func oidcConfig(conn services.OIDCConnector) oidc.ClientConfig {
	return oidc.ClientConfig{
		RedirectURL: conn.GetRedirectURL(),
		Credentials: oidc.ClientCredentials{
			ID:     conn.GetClientID(),
			Secret: conn.GetClientSecret(),
		},
		// open id notifies provider that we are using OIDC scopes
		Scope: utils.Deduplicate(append([]string{"openid", "email"}, conn.GetScope()...)),
	}
}

func (s *AuthServer) UpsertOIDCConnector(connector services.OIDCConnector) error {
	return s.Identity.UpsertOIDCConnector(connector)
}

func (s *AuthServer) DeleteOIDCConnector(connectorName string) error {
	return s.Identity.DeleteOIDCConnector(connectorName)
}

func (s *AuthServer) CreateOIDCAuthRequest(req services.OIDCAuthRequest) (*services.OIDCAuthRequest, error) {
	connector, err := s.Identity.GetOIDCConnector(req.ConnectorID, true)
	if err != nil {
		return nil, trace.Wrap(err)
	}
	oidcClient, err := s.getOrCreateOIDCClient(connector)
	if err != nil {
		return nil, trace.Wrap(err)
	}
	oauthClient, err := oidcClient.OAuthClient()
	if err != nil {
		return nil, trace.Wrap(err)
	}

	stateToken, err := utils.CryptoRandomHex(TokenLenBytes)
	if err != nil {
		return nil, trace.Wrap(err)
	}

	req.StateToken = stateToken

	// online indicates that this login should only work online
	req.RedirectURL = oauthClient.AuthCodeURL(req.StateToken, teleport.OIDCAccessTypeOnline, connector.GetPrompt())

	// if the connector has an Authentication Context Class Reference (ACR) value set,
	// update redirect url and add it as a query value.
	acrValue := connector.GetACR()
	if acrValue != "" {
		u, err := url.Parse(req.RedirectURL)
		if err != nil {
			return nil, trace.Wrap(err)
		}
		q := u.Query()
		q.Set("acr_values", acrValue)
		u.RawQuery = q.Encode()
		req.RedirectURL = u.String()
	}

	log.Debugf("OIDC redirect URL: %v.", req.RedirectURL)

	err = s.Identity.CreateOIDCAuthRequest(req, defaults.OIDCAuthRequestTTL)
	if err != nil {
		return nil, trace.Wrap(err)
	}
	return &req, nil
}

// ValidateOIDCAuthCallback is called by the proxy to check OIDC query parameters
// returned by OIDC Provider, if everything checks out, auth server
// will respond with OIDCAuthResponse, otherwise it will return error
func (a *AuthServer) ValidateOIDCAuthCallback(q url.Values) (*OIDCAuthResponse, error) {
	re, err := a.validateOIDCAuthCallback(q)
	if err != nil {
		fields := events.EventFields{
			events.LoginMethod:        events.LoginMethodOIDC,
			events.AuthAttemptSuccess: false,
			// log the original internal error in audit log
			events.AuthAttemptErr: trace.Unwrap(err).Error(),
		}
		if re != nil && re.claims != nil {
			fields[events.IdentityAttributes] = re.claims
		}
		a.EmitAuditEvent(events.UserSSOLoginFailure, fields)
		return nil, trace.Wrap(err)
	}
	fields := events.EventFields{
		events.EventUser:          re.auth.Username,
		events.AuthAttemptSuccess: true,
		events.LoginMethod:        events.LoginMethodOIDC,
	}
	if re.claims != nil {
		fields[events.IdentityAttributes] = re.claims
	}
	a.EmitAuditEvent(events.UserSSOLogin, fields)
	return &re.auth, nil
}

type oidcAuthResponse struct {
	auth   OIDCAuthResponse
	claims jose.Claims
}

func (a *AuthServer) validateOIDCAuthCallback(q url.Values) (*oidcAuthResponse, error) {
	if error := q.Get("error"); error != "" {
		return nil, trace.OAuth2(oauth2.ErrorInvalidRequest, error, q)
	}

	code := q.Get("code")
	if code == "" {
		return nil, trace.OAuth2(
			oauth2.ErrorInvalidRequest, "code query param must be set", q)
	}

	stateToken := q.Get("state")
	if stateToken == "" {
		return nil, trace.OAuth2(
			oauth2.ErrorInvalidRequest, "missing state query param", q)
	}

	clusterName, err := a.GetClusterName()
	if err != nil {
		return nil, trace.Wrap(err)
	}

	req, err := a.Identity.GetOIDCAuthRequest(stateToken)
	if err != nil {
		return nil, trace.Wrap(err)
	}

	connector, err := a.Identity.GetOIDCConnector(req.ConnectorID, true)
	if err != nil {
		return nil, trace.Wrap(err)
	}

	oidcClient, err := a.getOrCreateOIDCClient(connector)
	if err != nil {
		return nil, trace.Wrap(err)
	}

	// extract claims from both the id token and the userinfo endpoint and merge them
	claims, err := a.getClaims(oidcClient, connector, code)
	if err != nil {
		return nil, trace.WrapWithMessage(
			// preserve the original error message, to avoid leaking
			// server errors to the user in the UI, but override
			// user message to the high level instruction to check audit log for details
			trace.OAuth2(
				oauth2.ErrorUnsupportedResponseType, err.Error(), q),
			"unable to construct claims, check audit log for details",
		)
	}
	re := &oidcAuthResponse{
		claims: claims,
	}

	log.Debugf("OIDC claims: %v.", re.claims)

	// if we are sending acr values, make sure we also validate them
	acrValue := connector.GetACR()
	if acrValue != "" {
		err := a.validateACRValues(acrValue, connector.GetProvider(), claims)
		if err != nil {
			return re, trace.Wrap(err)
		}
		log.Debugf("OIDC ACR values %q successfully validated.", acrValue)
	}

	ident, err := oidc.IdentityFromClaims(claims)
	if err != nil {
		return re, trace.OAuth2(
			oauth2.ErrorUnsupportedResponseType, "unable to convert claims to identity", q)
	}
	log.Debugf("OIDC user %q expires at: %v.", ident.Email, ident.ExpiresAt)

	if len(connector.GetClaimsToRoles()) == 0 {
		return re, trace.BadParameter("no claims to roles mapping, check connector documentation")
	}
	log.Debugf("Applying %v OIDC claims to roles mappings.", len(connector.GetClaimsToRoles()))

	// Calculate (figure out name, roles, traits, session TTL) of user and
	// create the user in the backend.
	params, err := a.calculateOIDCUser(connector, claims, ident, req)
	if err != nil {
		return re, trace.Wrap(err)
	}
	user, err := a.createOIDCUser(params)
	if err != nil {
		return re, trace.Wrap(err)
	}

	// Auth was successful, return session, certificate, etc. to caller.
	re.auth = OIDCAuthResponse{
		Req: *req,
		Identity: services.ExternalIdentity{
			ConnectorID: params.connectorName,
			Username:    params.username,
		},
		Username: user.GetName(),
	}

	if !req.CheckUser {
		return re, nil
	}

	// If the request is coming from a browser, create a web session.
	if req.CreateWebSession {
		session, err := a.createWebSession(user, params.sessionTTL)
		if err != nil {
			return nil, trace.Wrap(err)
		}
		re.auth.Session = session
	}

	// If a public key was provided, sign it and return a certificate.
	if len(req.PublicKey) != 0 {
		sshCert, tlsCert, err := a.createSessionCert(user, params.sessionTTL, req.PublicKey, req.Compatibility)
		if err != nil {
			return nil, trace.Wrap(err)
		}

		re.auth.Cert = sshCert
		re.auth.TLSCert = tlsCert

		// Return the host CA for this cluster only.
		authority, err := a.GetCertAuthority(services.CertAuthID{
			Type:       services.HostCA,
			DomainName: clusterName.GetClusterName(),
		}, false)
		if err != nil {
			return nil, trace.Wrap(err)
		}
		re.auth.HostSigners = append(re.auth.HostSigners, authority)
	}

	return re, nil
}

// OIDCAuthResponse is returned when auth server validated callback parameters
// returned from OIDC provider
type OIDCAuthResponse struct {
	// Username is authenticated teleport username
	Username string `json:"username"`
	// Identity contains validated OIDC identity
	Identity services.ExternalIdentity `json:"identity"`
	// Web session will be generated by auth server if requested in OIDCAuthRequest
	Session services.WebSession `json:"session,omitempty"`
	// Cert will be generated by certificate authority
	Cert []byte `json:"cert,omitempty"`
	// TLSCert is PEM encoded TLS certificate
	TLSCert []byte `json:"tls_cert,omitempty"`
	// Req is original oidc auth request
	Req services.OIDCAuthRequest `json:"req"`
	// HostSigners is a list of signing host public keys
	// trusted by proxy, used in console login
	HostSigners []services.CertAuthority `json:"host_signers"`
}

// buildOIDCRoles takes a connector and claims and returns a slice of roles.
func (a *AuthServer) buildOIDCRoles(connector services.OIDCConnector, claims jose.Claims) ([]string, error) {
	roles := connector.MapClaims(claims)
	if len(roles) == 0 {
		return nil, trace.AccessDenied("unable to map claims to role for connector: %v", connector.GetName())
	}

	return roles, nil
}

// claimsToTraitMap extracts all string claims and creates a map of traits
// that can be used to populate role variables.
func claimsToTraitMap(claims jose.Claims) map[string][]string {
	traits := make(map[string][]string)

	for claimName := range claims {
		claimValue, ok, _ := claims.StringClaim(claimName)
		if ok {
			traits[claimName] = []string{claimValue}
		}
		claimValues, ok, _ := claims.StringsClaim(claimName)
		if ok {
			traits[claimName] = claimValues
		}
	}

	return traits
}

func (a *AuthServer) calculateOIDCUser(connector services.OIDCConnector, claims jose.Claims, ident *oidc.Identity, request *services.OIDCAuthRequest) (*createUserParams, error) {
	var err error

	p := createUserParams{
		connectorName: connector.GetName(),
		username:      ident.Email,
	}

	p.roles, err = a.buildOIDCRoles(connector, claims)
	if err != nil {
		return nil, trace.Wrap(err)
	}
	p.traits = claimsToTraitMap(claims)

	// Pick smaller for role: session TTL from role or requested TTL.
	roles, err := services.FetchRoles(p.roles, a.Access, p.traits)
	if err != nil {
		return nil, trace.Wrap(err)
	}
	roleTTL := roles.AdjustSessionTTL(defaults.MaxCertDuration)
	p.sessionTTL = utils.MinTTL(roleTTL, request.CertTTL)

	return &p, nil
}

func (a *AuthServer) createOIDCUser(p *createUserParams) (services.User, error) {
	expires := a.GetClock().Now().UTC().Add(p.sessionTTL)

	log.Debugf("Generating dynamic OIDC identity %v/%v with roles: %v.", p.connectorName, p.username, p.roles)
	user, err := services.GetUserMarshaler().GenerateUser(&services.UserV2{
		Kind:    services.KindUser,
		Version: services.V2,
		Metadata: services.Metadata{
			Name:      p.username,
			Namespace: defaults.Namespace,
			Expires:   &expires,
		},
		Spec: services.UserSpecV2{
			Roles:  p.roles,
			Traits: p.traits,
			OIDCIdentities: []services.ExternalIdentity{
				services.ExternalIdentity{
					ConnectorID: p.connectorName,
					Username:    p.username,
				},
			},
			CreatedBy: services.CreatedBy{
				User: services.UserRef{Name: "system"},
				Time: a.clock.Now().UTC(),
				Connector: &services.ConnectorRef{
					Type:     teleport.ConnectorOIDC,
					ID:       p.connectorName,
					Identity: p.username,
				},
			},
		},
	})
	if err != nil {
		return nil, trace.Wrap(err)
	}

	// Get the user to check if it already exists or not.
	existingUser, err := a.Identity.GetUser(p.username, false)
	if err != nil {
		if !trace.IsNotFound(err) {
			return nil, trace.Wrap(err)
		}
	}

	// Overwrite exisiting user if it was created from an external identity provider.
	if existingUser != nil {
		connectorRef := existingUser.GetCreatedBy().Connector

		// If the exisiting user is a local user, fail and advise how to fix the problem.
		if connectorRef == nil {
			return nil, trace.AlreadyExists("local user with name '%v' already exists. Either change "+
				"email in OIDC identity or remove local user and try again.", existingUser.GetName())
		}

		log.Debugf("Overwriting exisiting user '%v' created with %v connector %v.",
			existingUser.GetName(), connectorRef.Type, connectorRef.ID)
	}

	// Upsert the new user creating or updating whatever is in the database.
	err = a.UpsertUser(user)
	if err != nil {
		return nil, trace.Wrap(err)
	}

	return user, nil
}

// claimsFromIDToken extracts claims from the ID token.
func claimsFromIDToken(oidcClient *oidc.Client, idToken string) (jose.Claims, error) {
	jwt, err := jose.ParseJWT(idToken)
	if err != nil {
		return nil, trace.Wrap(err)
	}

	err = oidcClient.VerifyJWT(jwt)
	if err != nil {
		return nil, trace.Wrap(err)
	}

	log.Debugf("Extracting OIDC claims from ID token.")

	claims, err := jwt.Claims()
	if err != nil {
		return nil, trace.Wrap(err)
	}

	return claims, nil
}

// claimsFromUserInfo finds the UserInfo endpoint from the provider config and then extracts claims from it.
//
// Note: We don't request signed JWT responses for UserInfo, instead we force the provider config and
// the issuer to be HTTPS and leave integrity and confidentiality to TLS. Authenticity is taken care of
// during the token exchange.
func claimsFromUserInfo(oidcClient *oidc.Client, issuerURL string, accessToken string) (jose.Claims, error) {
	err := isHTTPS(issuerURL)
	if err != nil {
		return nil, trace.Wrap(err)
	}

	oac, err := oidcClient.OAuthClient()
	if err != nil {
		return nil, trace.Wrap(err)
	}
	hc := oac.HttpClient()

	// go get the provider config so we can find out where the UserInfo endpoint
	// is. if the provider doesn't offer a UserInfo endpoint return not found.
	pc, err := oidc.FetchProviderConfig(oac.HttpClient(), issuerURL)
	if err != nil {
		return nil, trace.Wrap(err)
	}
	if pc.UserInfoEndpoint == nil {
		return nil, trace.NotFound("UserInfo endpoint not found")
	}

	endpoint := pc.UserInfoEndpoint.String()
	err = isHTTPS(endpoint)
	if err != nil {
		return nil, trace.Wrap(err)
	}
	log.Debugf("Fetching OIDC claims from UserInfo endpoint: %q.", endpoint)

	req, err := http.NewRequest("GET", endpoint, nil)
	if err != nil {
		return nil, trace.Wrap(err)
	}
	req.Header.Set("Authorization", fmt.Sprintf("Bearer %s", accessToken))

	resp, err := hc.Do(req)
	if err != nil {
		return nil, trace.Wrap(err)
	}
	defer resp.Body.Close()

	if resp.StatusCode < 200 || resp.StatusCode > 299 {
		return nil, trace.AccessDenied("bad status code: %v", resp.StatusCode)
	}

	var claims jose.Claims
	err = json.NewDecoder(resp.Body).Decode(&claims)
	if err != nil {
		return nil, trace.Wrap(err)
	}

	return claims, nil
}

func (a *AuthServer) claimsFromGSuite(config *jwt.Config, issuerURL string, userEmail string, domain string) (jose.Claims, error) {
	client, err := a.newGsuiteClient(config, issuerURL, userEmail, domain)
	if err != nil {
		return nil, trace.Wrap(err)
	}
	return client.fetchGroups()
}

func (a *AuthServer) newGsuiteClient(config *jwt.Config, issuerURL string, userEmail string, domain string) (*gsuiteClient, error) {
	err := isHTTPS(issuerURL)
	if err != nil {
		return nil, trace.Wrap(err)
	}

	u, err := url.Parse(teleport.GSuiteGroupsEndpoint)
	if err != nil {
		return nil, trace.Wrap(err)
	}

	return &gsuiteClient{
		domain:    domain,
		client:    config.Client(context.TODO()),
		url:       *u,
		userEmail: userEmail,
		config:    config,
		auditLog:  a,
	}, nil
}

type gsuiteClient struct {
	client    *http.Client
	url       url.URL
	userEmail string
	domain    string
	config    *jwt.Config
	auditLog  events.IAuditLog
}

// fetchGroups fetches GSuite groups a user belongs to and returns
// "groups" claim with
func (g *gsuiteClient) fetchGroups() (jose.Claims, error) {
	count := 0
	var groups []string
	var nextPageToken string
collect:
	for {
		if count > MaxPages {
			warningMessage := "Truncating list of teams used to populate claims: " +
				"hit maximum number pages that can be fetched from GSuite."

			// Print warning to Teleport logs as well as the Audit Log.
			log.Warnf(warningMessage)
			g.auditLog.EmitAuditEvent(events.UserSSOLoginFailure, events.EventFields{
				events.LoginMethod:        events.LoginMethodOIDC,
				events.AuthAttemptMessage: warningMessage,
			})
			break collect
		}
		response, err := g.fetchGroupsPage(nextPageToken)
		if err != nil {
			return nil, trace.Wrap(err)
		}
		groups = append(groups, response.groups()...)
		if response.NextPageToken == "" {
			break collect
		}
		count++
		nextPageToken = response.NextPageToken
	}
	return jose.Claims{"groups": groups}, nil
}

func (g *gsuiteClient) fetchGroupsPage(pageToken string) (*gsuiteGroups, error) {
	// copy URL to avoid modifying the same url
	// with query parameters
	u := g.url
	q := u.Query()
	q.Set("userKey", g.userEmail)
	q.Set("domain", g.domain)
	if pageToken != "" {
		q.Set("pageToken", pageToken)
	}
	u.RawQuery = q.Encode()
	endpoint := u.String()

	log.Debugf("Fetching OIDC claims from GSuite groups endpoint: %q.", endpoint)

	req, err := http.NewRequest("GET", endpoint, nil)
	if err != nil {
		return nil, trace.Wrap(err)
	}

	resp, err := g.client.Do(req)
	if err != nil {
		return nil, trace.Wrap(err)
	}
	defer resp.Body.Close()

	bytes, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return nil, trace.Wrap(err)
	}
	if resp.StatusCode < 200 || resp.StatusCode > 299 {
		return nil, trace.AccessDenied("bad status code: %v %v", resp.StatusCode, string(bytes))
	}
	var response gsuiteGroups
	if err := json.Unmarshal(bytes, &response); err != nil {
		return nil, trace.BadParameter("failed to parse response: %v", err)
	}
	return &response, nil
}

type gsuiteGroups struct {
	NextPageToken string        `json:"nextPageToken"`
	Groups        []gsuiteGroup `json:"groups"`
}

func (g gsuiteGroups) groups() []string {
	groups := make([]string, len(g.Groups))
	for i, group := range g.Groups {
		groups[i] = group.Email
	}
	return groups
}

type gsuiteGroup struct {
	Email string `json:"email"`
}

// mergeClaims merges b into a.
func mergeClaims(a jose.Claims, b jose.Claims) (jose.Claims, error) {
	for k, v := range b {
		_, ok := a[k]
		if !ok {
			a[k] = v
		}
	}

	return a, nil
}

// getClaims gets claims from ID token and UserInfo and returns UserInfo claims merged into ID token claims.
func (a *AuthServer) getClaims(oidcClient *oidc.Client, connector services.OIDCConnector, code string) (jose.Claims, error) {
	var err error

	oac, err := oidcClient.OAuthClient()
	if err != nil {
		return nil, trace.Wrap(err)
	}

	t, err := oac.RequestToken(oauth2.GrantTypeAuthCode, code)
	if err != nil {
		return nil, trace.Wrap(err)
	}

	idTokenClaims, err := claimsFromIDToken(oidcClient, t.IDToken)
	if err != nil {
		log.Debugf("Unable to fetch OIDC ID token claims: %v.", err)
		return nil, trace.Wrap(err)
	}
	log.Debugf("OIDC ID Token claims: %v.", idTokenClaims)

	userInfoClaims, err := claimsFromUserInfo(oidcClient, connector.GetIssuerURL(), t.AccessToken)
	if err != nil {
		if trace.IsNotFound(err) {
			log.Debugf("OIDC provider doesn't offer UserInfo endpoint. Returning token claims: %v.", idTokenClaims)
			return idTokenClaims, nil
		}
		log.Debugf("Unable to fetch UserInfo claims: %v.", err)
		return nil, trace.Wrap(err)
	}
	log.Debugf("UserInfo claims: %v.", userInfoClaims)

	// make sure that the subject in the userinfo claim matches the subject in
	// the id token otherwise there is the possibility of a token substitution attack.
	// see section 16.11 of the oidc spec for more details.
	var idsub string
	var uisub string
	var exists bool
	if idsub, exists, err = idTokenClaims.StringClaim("sub"); err != nil || !exists {
		log.Debugf("Unable to extract OIDC sub claim from ID token.")
		return nil, trace.Wrap(err)
	}
	if uisub, exists, err = userInfoClaims.StringClaim("sub"); err != nil || !exists {
		log.Debugf("Unable to extract OIDC sub claim from UserInfo.")
		return nil, trace.Wrap(err)
	}
	if idsub != uisub {
		log.Debugf("OIDC claim subjects don't match '%v' != '%v'.", idsub, uisub)
		return nil, trace.BadParameter("invalid subject in UserInfo")
	}

	claims, err := mergeClaims(idTokenClaims, userInfoClaims)
	if err != nil {
		log.Debugf("Unable to merge OIDC claims: %v.", err)
		return nil, trace.Wrap(err)
	}

	// for GSuite users, fetch extra data from the proprietary google API
	// only if scope includes admin groups readonly scope
	if connector.GetIssuerURL() == teleport.GSuiteIssuerURL {
		email, _, err := claims.StringClaim("email")
		if err != nil {
			return nil, trace.Wrap(err)
		}

		serviceAccountURI := connector.GetGoogleServiceAccountURI()
		if serviceAccountURI == "" {
			return nil, trace.NotFound(
				"the gsuite connector requires google_service_account_uri parameter to be specified and pointing to a valid google service account file with credentials, read this article for more details https://developers.google.com/admin-sdk/directory/v1/guides/delegation")
		}

		uri, err := utils.ParseSessionsURI(serviceAccountURI)
		if err != nil {
			return nil, trace.BadParameter("failed to parse google_service_account_uri: %v", err)
		}

		impersonateAdmin := connector.GetGoogleAdminEmail()
		if impersonateAdmin == "" {
			return nil, trace.NotFound(
				"the gsuite connector requires google_admin_email user to impersonate, as service accounts can not be used directly https://developers.google.com/identity/protocols/OAuth2ServiceAccount#delegatingauthority")
		}

		jsonCredentials, err := ioutil.ReadFile(uri.Path)
		if err != nil {
			return nil, trace.Wrap(err)
		}

		config, err := google.JWTConfigFromJSON(jsonCredentials, teleport.GSuiteGroupsScope)
		if err != nil {
			return nil, trace.BadParameter("unable to parse client secret file to config: %v", err)
		}
		// User should impersonate admin user, otherwise it won't work:
		//
		// https://developers.google.com/admin-sdk/directory/v1/guides/delegation
		//
		// "Note: Only users with access to the Admin APIs can access the Admin SDK Directory API, therefore your service account needs to impersonate one of those users to access the Admin SDK Directory API. Additionally, the user must have logged in at least once and accepted the G Suite Terms of Service."
		//
		domain, exists, err := userInfoClaims.StringClaim(teleport.GSuiteDomainClaim)
		if err != nil || !exists {
			return nil, trace.BadParameter("hd is the required claim for GSuite")
		}
		config.Subject = impersonateAdmin

		gsuiteClaims, err := a.claimsFromGSuite(config, connector.GetIssuerURL(), email, domain)
		if err != nil {
			if !trace.IsNotFound(err) {
				return nil, trace.Wrap(err)
			}
			log.Debugf("Found no GSuite claims.")
		} else {
			if gsuiteClaims != nil {
				log.Debugf("Got GSuiteclaims: %v.", gsuiteClaims)
			}
			claims, err = mergeClaims(claims, gsuiteClaims)
			if err != nil {
				return nil, trace.Wrap(err)
			}
		}
	}

	return claims, nil
}

// validateACRValues validates that we get an appropriate response for acr values. By default
// we expect the same value we send, but this function also handles Identity Provider specific
// forms of validation.
func (a *AuthServer) validateACRValues(acrValue string, identityProvider string, claims jose.Claims) error {
	switch identityProvider {
	case teleport.NetIQ:
		log.Debugf("Validating OIDC ACR values with '%v' rules.", identityProvider)

		tokenAcr, ok := claims["acr"]
		if !ok {
			return trace.BadParameter("acr not found in claims")
		}
		tokenAcrMap, ok := tokenAcr.(map[string]interface{})
		if !ok {
			return trace.BadParameter("acr unexpected type: %T", tokenAcr)
		}
		tokenAcrValues, ok := tokenAcrMap["values"]
		if !ok {
			return trace.BadParameter("acr.values not found in claims")
		}
		tokenAcrValuesSlice, ok := tokenAcrValues.([]interface{})
		if !ok {
			return trace.BadParameter("acr.values unexpected type: %T", tokenAcr)
		}

		acrValueMatched := false
		for _, v := range tokenAcrValuesSlice {
			vv, ok := v.(string)
			if !ok {
				continue
			}
			if acrValue == vv {
				acrValueMatched = true
				break
			}
		}
		if !acrValueMatched {
			log.Debugf("No OIDC ACR match found for '%v' in '%v'.", acrValue, tokenAcrValues)
			return trace.BadParameter("acr claim does not match")
		}
	default:
		log.Debugf("Validating OIDC ACR values with default rules.")

		claimValue, exists, err := claims.StringClaim("acr")
		if !exists {
			return trace.BadParameter("acr claim does not exist")
		}
		if err != nil {
			return trace.Wrap(err)
		}
		if claimValue != acrValue {
			log.Debugf("No OIDC ACR match found '%v' != '%v'.", acrValue, claimValue)
			return trace.BadParameter("acr claim does not match")
		}
	}

	return nil
}