[client] Verify the SSO login came back for the hinted account

login_hint is a suggestion the IdP may ignore: with a silent flow configured
(DisablePromptLogin or max_age=0) and a live IdP session for another account,
the login completes with that account's token. On a registered peer the
management server rejects it as a user mismatch, but on a fresh profile the
peer silently registers under the wrong account and the profile is then bound
to it — every later login follows the stored hint straight back.

After the token exchange, compare the ID token's email against the hint the
flow was sent with. On a mismatch, do not log in to management with the token;
run one more round asking the IdP to re-decide the account (prompt=login, via
ForceAccountPrompt — DisablePromptLogin still wins there). If the prompted
round also comes back different, proceed with a warning: the address may
legitimately have changed, and refusing forever would lock the user out of the
profile while the management server still rejects a token that does not own
the peer. A token or profile with no email to compare is not judged.

The retry differs per platform because of who opens the browser:

- CLI (netbird login foreground) and Android run the whole flow in one
  process, so the mismatch retries automatically: the browser reopens with
  the account prompt within the same login attempt.
- On desktop the login is split between the daemon and the GUI: Login hands
  the authorize URL to the GUI, WaitSSOLogin blocks for the token, and only
  the GUI can open a browser. A new URL cannot be handed out from inside
  WaitSSOLogin (its response has no field for one, kept that way to avoid a
  proto change), so the daemon arms forceAccountPrompt, fails the round with
  "connect again to choose the account", and builds the next Login's flow
  with the prompt — the user's next connect is the retry.

The flag and the flow annotations live in daemon memory only; SwitchProfile
drops them so the previous profile's hint cannot judge the next profile's
token. The device code flow has no prompt parameter (RFC 8628), so a prompted
round there runs as-is and a repeated mismatch is let through with the
warning rather than looping.

client/server/login_account_test.go

@@ -0,0 +1,125 @@
+package server
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/netbirdio/netbird/client/internal"
+	"github.com/netbirdio/netbird/client/internal/auth"
+	"github.com/netbirdio/netbird/client/proto"
+)
+
+type stubOAuthFlow struct {
+	token auth.TokenInfo
+}
+
+func (f *stubOAuthFlow) RequestAuthInfo(context.Context) (auth.AuthFlowInfo, error) {
+	return auth.AuthFlowInfo{}, nil
+}
+
+func (f *stubOAuthFlow) WaitToken(context.Context, auth.AuthFlowInfo) (auth.TokenInfo, error) {
+	return f.token, nil
+}
+
+func (f *stubOAuthFlow) GetClientID(context.Context) string {
+	return "stub-client"
+}
+
+func TestWaitSSOLogin_WrongAccountArmsPromptAndFails(t *testing.T) {
+	s := newSSOTestServer(t, "user@example.com", false, "other@example.com")
+	attempts := 0
+	s.loginAttemptFn = func(context.Context, string, string) (internal.StatusType, error) {
+		attempts++
+		return "", nil
+	}
+
+	resp, err := s.WaitSSOLogin(context.Background(), &proto.WaitSSOLoginRequest{UserCode: "code"})
+	require.Error(t, err)
+	require.Nil(t, resp)
+	require.Equal(t, 0, attempts, "the wrong account's token reached the management login")
+	require.True(t, s.forceAccountPrompt, "the next login was not armed to ask for the account")
+	require.Nil(t, s.oauthAuthFlow.flow, "the mismatched flow stayed cached for reuse")
+
+	status, stateErr := internal.CtxGetState(s.rootCtx).Status()
+	require.NoError(t, stateErr)
+	require.Equal(t, internal.StatusNeedsLogin, status, "the mismatch must stay retryable")
+}
+
+func TestWaitSSOLogin_WrongAccountAfterPromptProceeds(t *testing.T) {
+	s := newSSOTestServer(t, "user@example.com", true, "other@example.com")
+	attempts := 0
+	s.loginAttemptFn = func(context.Context, string, string) (internal.StatusType, error) {
+		attempts++
+		return "", nil
+	}
+
+	resp, err := s.WaitSSOLogin(context.Background(), &proto.WaitSSOLoginRequest{UserCode: "code"})
+	require.NoError(t, err, "a prompted round must not error again on a mismatch")
+	require.NotNil(t, resp)
+	require.Equal(t, "other@example.com", resp.Email)
+	require.Equal(t, 1, attempts)
+	require.False(t, s.forceAccountPrompt)
+}
+
+func TestWaitSSOLogin_MatchingAccountProceeds(t *testing.T) {
+	s := newSSOTestServer(t, "user@example.com", false, "User@Example.com")
+	attempts := 0
+	s.loginAttemptFn = func(context.Context, string, string) (internal.StatusType, error) {
+		attempts++
+		return "", nil
+	}
+
+	resp, err := s.WaitSSOLogin(context.Background(), &proto.WaitSSOLoginRequest{UserCode: "code"})
+	require.NoError(t, err)
+	require.NotNil(t, resp)
+	require.Equal(t, 1, attempts)
+	require.False(t, s.forceAccountPrompt)
+}
+
+func TestWaitSSOLogin_NoHintIsNotJudged(t *testing.T) {
+	s := newSSOTestServer(t, "", false, "whoever@example.com")
+	attempts := 0
+	s.loginAttemptFn = func(context.Context, string, string) (internal.StatusType, error) {
+		attempts++
+		return "", nil
+	}
+
+	_, err := s.WaitSSOLogin(context.Background(), &proto.WaitSSOLoginRequest{UserCode: "code"})
+	require.NoError(t, err)
+	require.Equal(t, 1, attempts)
+	require.False(t, s.forceAccountPrompt)
+}
+
+func TestSwitchProfile_DropsAccountPromptAndPendingFlow(t *testing.T) {
+	s, ctx, _, _, _ := setupServerWithProfile(t)
+	s.forceAccountPrompt = true
+	cancelled := false
+	s.oauthAuthFlow = oauthAuthFlow{
+		flow:       &stubOAuthFlow{},
+		hint:       "user@example.com",
+		waitCancel: func() { cancelled = true },
+	}
+
+	_, err := s.SwitchProfile(ctx, nil)
+	require.NoError(t, err)
+	require.False(t, s.forceAccountPrompt, "the prompt flag leaked across a profile switch")
+	require.Nil(t, s.oauthAuthFlow.flow, "the previous profile's flow leaked across a profile switch")
+	require.Empty(t, s.oauthAuthFlow.hint)
+	require.True(t, cancelled, "the pending wait was not cancelled")
+}
+
+func newSSOTestServer(t *testing.T, hint string, accountPrompted bool, tokenEmail string) *Server {
+	t.Helper()
+	s := New(internal.CtxInitState(context.Background()), "console", "", false, false, false, false)
+	s.oauthAuthFlow = oauthAuthFlow{
+		flow:            &stubOAuthFlow{token: auth.TokenInfo{Email: tokenEmail}},
+		info:            auth.AuthFlowInfo{UserCode: "code"},
+		expiresAt:       time.Now().Add(time.Minute),
+		hint:            hint,
+		accountPrompted: accountPrompted,
+	}
+	return s
+}

client/server/server.go

@@ -82,6 +82,12 @@ type Server struct {
 	uiLogPath string
 
 	oauthAuthFlow oauthAuthFlow
+	// forceAccountPrompt makes the next startSSOLogin build its flow with a
+	// forced account prompt. Armed when a login came back for an account other
+	// than the hinted one: that flow's browser is gone, so the correction has to
+	// ride on the user's next connect. Guarded by mutex; deliberately not
+	// persisted — a lost flag only costs one more mismatch round.
+	forceAccountPrompt bool
 	// extendAuthSessionFlow holds the pending PKCE flow created by
 	// RequestExtendAuthSession until WaitExtendAuthSession resolves it.
 	// Kept separate from oauthAuthFlow (which is reserved for the SSH
@@ -153,6 +159,14 @@ type oauthAuthFlow struct {
 	flow       auth.OAuthFlow
 	info       auth.AuthFlowInfo
 	waitCancel context.CancelFunc
+	// hint is the account the flow was asked to sign in (login_hint). The token
+	// that comes back is compared against it; empty means nothing to compare.
+	hint string
+	// accountPrompted records that this flow already asked the IdP to re-decide
+	// the account (or could not ask — the device flow has no way to). A token
+	// for the wrong account is then let through with a warning instead of
+	// erroring again, so the flow cannot loop.
+	accountPrompted bool
 }
 
 // New server instance constructor.
@@ -709,6 +723,16 @@ func (s *Server) startSSOLogin(ctx context.Context, msg *proto.LoginRequest, con
 		return nil, err
 	}
 
+	s.mutex.Lock()
+	promptForAccount := s.forceAccountPrompt
+	s.forceAccountPrompt = false
+	s.mutex.Unlock()
+	if promptForAccount && auth.RetryFlowForAccount(oAuthFlow) == nil {
+		// The device flow cannot ask; run it as-is. accountPrompted still goes
+		// true below so a second mismatch is let through instead of looping.
+		log.Warnf("the previous login returned a different account, but this flow cannot ask the IdP to choose one")
+	}
+
 	if resp := s.reuseOAuthFlow(ctx, oAuthFlow, state); resp != nil {
 		return resp, nil
 	}
@@ -723,6 +747,8 @@ func (s *Server) startSSOLogin(ctx context.Context, msg *proto.LoginRequest, con
 	s.oauthAuthFlow.flow = oAuthFlow
 	s.oauthAuthFlow.info = authInfo
 	s.oauthAuthFlow.expiresAt = time.Now().Add(time.Duration(authInfo.ExpiresIn) * time.Second)
+	s.oauthAuthFlow.hint = hint
+	s.oauthAuthFlow.accountPrompted = promptForAccount
 	s.mutex.Unlock()
 
 	state.Set(internal.StatusNeedsLogin)
@@ -923,8 +949,33 @@ func (s *Server) WaitSSOLogin(callerCtx context.Context, msg *proto.WaitSSOLogin
 
 	s.mutex.Lock()
 	s.oauthAuthFlow.expiresAt = time.Now()
+	hint := s.oauthAuthFlow.hint
+	accountPrompted := s.oauthAuthFlow.accountPrompted
 	s.mutex.Unlock()
 
+	if !tokenInfo.MatchesAccount(hint) {
+		if !accountPrompted {
+			// The IdP answered from a session belonging to another account. The
+			// browser for this flow is gone, so a new URL cannot be handed out
+			// here — arm the prompt for the user's next connect and fail this
+			// round. Never log in with the token: on a registered peer the
+			// server would reject it, and on a fresh one it would silently
+			// register the peer under the wrong account.
+			log.Warnf("login returned an account other than the one this profile is bound to; the next connect will ask the IdP to choose")
+			s.mutex.Lock()
+			s.oauthAuthFlow = oauthAuthFlow{}
+			s.forceAccountPrompt = true
+			s.mutex.Unlock()
+			state.Set(internal.StatusNeedsLogin)
+			return nil, gstatus.Errorf(codes.FailedPrecondition, "the login used a different account than this profile; connect again to choose the account")
+		}
+		// Already asked once; the account may legitimately differ (a changed
+		// email address). Refusing again would lock the user out of the profile,
+		// and the management server still rejects a token that does not own the
+		// peer.
+		log.Warnf("login still returned a different account after the prompt, continuing with it")
+	}
+
 	if loginStatus, err := s.attemptLogin(ctx, "", tokenInfo.GetTokenToUse()); err != nil {
 		state.Set(loginStatus)
 		return nil, err
@@ -1225,6 +1276,16 @@ func (s *Server) SwitchProfile(callerCtx context.Context, msg *proto.SwitchProfi
 
 	s.config = config
 
+	// A pending login flow and the account-prompt flag describe the previous
+	// profile's login; carried across a switch they would judge the new
+	// profile's token against the old profile's account. CancelFunc is
+	// non-blocking, so calling it under the mutex is safe.
+	if cancel := s.oauthAuthFlow.waitCancel; cancel != nil {
+		cancel()
+	}
+	s.oauthAuthFlow = oauthAuthFlow{}
+	s.forceAccountPrompt = false
+
 	if msg != nil && msg.ProfileName != nil {
 		s.publishProfileListChanged(*msg.ProfileName)
 	}