types: omit secret fields from JSON marshalling

Add json:"-" to PostgresConfig.Pass, OIDCConfig.ClientSecret, and
CLIConfig.APIKey so they are excluded from json.Marshal output
(e.g. the /debug/config endpoint).

hscontrol/types/config.go

@@ -148,7 +148,7 @@ type PostgresConfig struct {
 	Port                int
 	Name                string
 	User                string
-	Pass                string
+	Pass                string `json:"-"` // never serialise the database password
 	Ssl                 string
 	MaxOpenConnections  int
 	MaxIdleConnections  int
@@ -198,7 +198,7 @@ type OIDCConfig struct {
 	OnlyStartIfOIDCIsAvailable bool
 	Issuer                     string
 	ClientID                   string
-	ClientSecret               string
+	ClientSecret               string `json:"-"` // never serialise the OIDC client secret
 	Scope                      []string
 	ExtraParams                map[string]string
 	AllowedDomains             []string
@@ -237,7 +237,7 @@ type TaildropConfig struct {
 
 type CLIConfig struct {
 	Address  string
-	APIKey   string
+	APIKey   string `json:"-"` // never serialise the headscale admin API key
 	Timeout  time.Duration
 	Insecure bool
 }

hscontrol/types/config_test.go

@@ -1,6 +1,7 @@
 package types
 
 import (
+	"encoding/json"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -472,3 +473,40 @@ func TestSafeServerURL(t *testing.T) {
 		})
 	}
 }
+
+// TestConfigJSONOmitsSecrets verifies that marshalling a Config to JSON
+// (as /debug/config does via state.DebugConfig) does not leak the
+// Postgres password, the OIDC client secret, or the headscale admin
+// API key. Operators who widen metrics_listen_addr to 0.0.0.0 should
+// not be able to read these back via debug endpoints reachable over
+// CGNAT/loopback.
+func TestConfigJSONOmitsSecrets(t *testing.T) {
+	const (
+		secretPostgresPass = "p0stgres-secret-marker"
+		secretClientSecret = "oidc-client-secret-marker"    //nolint:gosec // test marker, not a real credential
+		secretAPIKey       = "headscale-cli-api-key-marker" //nolint:gosec // test marker, not a real credential
+	)
+
+	cfg := &Config{
+		Database: DatabaseConfig{
+			Postgres: PostgresConfig{
+				Pass: secretPostgresPass,
+			},
+		},
+		OIDC: OIDCConfig{
+			ClientSecret: secretClientSecret,
+		},
+		CLI: CLIConfig{
+			APIKey: secretAPIKey,
+		},
+	}
+
+	out, err := json.Marshal(cfg)
+	require.NoError(t, err)
+
+	body := string(out)
+	for _, secret := range []string{secretPostgresPass, secretClientSecret, secretAPIKey} {
+		assert.NotContains(t, body, secret,
+			"marshalled Config must not contain secret %q", secret)
+	}
+}