feat(api): warn at startup when the debug route has no authentication

pkg/modules/api/api.go

@@ -383,12 +383,37 @@ func (a *Api) Provision(ctx *gotenberg.Context) error {
 	// Logger.
 	a.logger = gotenberg.Logger(a)
 
+	a.warnInsecureDebugRoute()
+
 	// File system.
 	a.fs = gotenberg.NewFileSystem(new(gotenberg.OsMkdirAll))
 
 	return nil
 }
 
+// warnInsecureDebugRoute logs a warning when the debug route is reachable
+// without authentication.
+//
+// The route reports the resolved configuration of every module, which is
+// useful to an operator and equally useful to anyone else who can reach it.
+// This warns rather than refuses: an operator may sit behind a gateway that
+// authenticates on Gotenberg's behalf, and failing startup would break them.
+func (a *Api) warnInsecureDebugRoute() {
+	if !a.enableDebugRoute || a.basicAuthUsername != "" || a.oidcEnabled {
+		return
+	}
+	if a.logger == nil {
+		return
+	}
+
+	a.logger.WarnContext(
+		context.Background(),
+		"--api-enable-debug-route (API_ENABLE_DEBUG_ROUTE) is enabled but no authentication is configured, so anyone who can reach Gotenberg can read its configuration. Set --api-enable-basic-auth (API_ENABLE_BASIC_AUTH) with GOTENBERG_API_BASIC_AUTH_USERNAME and GOTENBERG_API_BASIC_AUTH_PASSWORD, set --api-enable-oidc-auth (API_ENABLE_OIDC_AUTH), or disable the route.",
+		slog.String("flag", "--api-enable-debug-route"),
+		slog.String("env", "API_ENABLE_DEBUG_ROUTE"),
+	)
+}
+
 // Validate validates the module properties.
 func (a *Api) Validate() error {
 	var err error

pkg/modules/api/api_test.go

@@ -1,6 +1,8 @@
 package api
 
 import (
+	"bytes"
+	"log/slog"
 	"strings"
 	"testing"
 )
@@ -65,3 +67,68 @@ func TestApi_Validate_Auth(t *testing.T) {
 		})
 	}
 }
+
+func TestApi_warnInsecureDebugRoute(t *testing.T) {
+	for _, tc := range []struct {
+		scenario     string
+		api          Api
+		expectWarn   bool
+		expectFields []string
+	}{
+		{
+			scenario:     "debug route on with no auth warns",
+			api:          Api{enableDebugRoute: true},
+			expectWarn:   true,
+			expectFields: []string{"--api-enable-debug-route", "API_ENABLE_DEBUG_ROUTE", "--api-enable-basic-auth", "API_ENABLE_BASIC_AUTH", "--api-enable-oidc-auth", "API_ENABLE_OIDC_AUTH"},
+		},
+		{
+			scenario:   "debug route off is silent",
+			api:        Api{enableDebugRoute: false},
+			expectWarn: false,
+		},
+		{
+			scenario:   "basic auth silences it",
+			api:        Api{enableDebugRoute: true, basicAuthUsername: "foo"},
+			expectWarn: false,
+		},
+		{
+			scenario:   "oidc silences it",
+			api:        Api{enableDebugRoute: true, oidcEnabled: true},
+			expectWarn: false,
+		},
+	} {
+		t.Run(tc.scenario, func(t *testing.T) {
+			buf := new(bytes.Buffer)
+			tc.api.logger = slog.New(slog.NewJSONHandler(buf, &slog.HandlerOptions{Level: slog.LevelWarn}))
+
+			tc.api.warnInsecureDebugRoute()
+
+			logged := buf.String()
+			if !tc.expectWarn {
+				if logged != "" {
+					t.Fatalf("expected no warning, got: %s", logged)
+				}
+				return
+			}
+
+			if logged == "" {
+				t.Fatal("expected a warning, got none")
+			}
+			// Every flag named must carry its environment variable.
+			for _, want := range tc.expectFields {
+				if !strings.Contains(logged, want) {
+					t.Fatalf("warning does not mention %q: %s", want, logged)
+				}
+			}
+			if strings.Contains(logged, "—") {
+				t.Fatalf("warning must not contain an em dash: %s", logged)
+			}
+		})
+	}
+}
+
+// The warning reads a.logger, which is nil until Provision assigns it.
+func TestApi_warnInsecureDebugRoute_NilLoggerDoesNotPanic(t *testing.T) {
+	api := Api{enableDebugRoute: true}
+	api.warnInsecureDebugRoute()
+}