fix(workspace-server): default-bind to 127.0.0.1 in dev-mode fail-open

In dev mode (`MOLECULE_ENV=dev|development`, `ADMIN_TOKEN` unset) the
AdminAuth chain fails open by design so canvas at :3000 can call
workspace-server at :8080 without a bearer token. Combined with the
existing wildcard bind on `:8080`, that exposed unauthenticated
`POST /workspaces` to any same-LAN peer (S-8 in the audit RFC v1).

Couple the bind narrowness to the same signal that drives the auth
fail-open: when `middleware.IsDevModeFailOpen()` returns true, default
the listener to `127.0.0.1`. Production (`ADMIN_TOKEN` set) keeps
binding to all interfaces — its auth chain is doing the work. Operators
who need LAN exposure set `BIND_ADDR=<host>` explicitly.

* `cmd/server/main.go` — `resolveBindHost()` precedence: BIND_ADDR
  explicit > IsDevModeFailOpen() loopback > "" (all interfaces).
  Startup log line now includes the resolved bind + dev-mode-fail-open
  state for post-deploy auditing.
* `cmd/server/bind_test.go` — 8 t.Setenv table cases covering
  precedence, explicit overrides, dev/prod env words. Mutation-tested:
  removing the `IsDevModeFailOpen()` branch makes the dev-mode cases
  fail with "" vs "127.0.0.1".

Refs: molecule-core#7

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

workspace-server/cmd/server/bind_test.go

@@ -0,0 +1,89 @@
+package main
+
+import "testing"
+
+// TestResolveBindHost pins the precedence: BIND_ADDR explicit > dev-mode
+// fail-open default of 127.0.0.1 > production-shape empty (all interfaces).
+//
+// Mutation-test invariant: removing the IsDevModeFailOpen() branch makes
+// "no_bindaddr_devmode_unset_admin" fail (returns "" instead of "127.0.0.1").
+// Removing the BIND_ADDR branch makes "explicit_bindaddr_*" cases fail.
+func TestResolveBindHost(t *testing.T) {
+	cases := []struct {
+		name       string
+		bindAddr   string
+		adminToken string
+		molEnv     string
+		want       string
+	}{
+		{
+			name:       "no_bindaddr_devmode_unset_admin",
+			bindAddr:   "",
+			adminToken: "",
+			molEnv:     "dev",
+			want:       "127.0.0.1",
+		},
+		{
+			name:       "no_bindaddr_devmode_unset_admin_full_word",
+			bindAddr:   "",
+			adminToken: "",
+			molEnv:     "development",
+			want:       "127.0.0.1",
+		},
+		{
+			name:       "no_bindaddr_admin_set_in_dev_env",
+			bindAddr:   "",
+			adminToken: "secret",
+			molEnv:     "dev",
+			want:       "", // ADMIN_TOKEN flips IsDevModeFailOpen to false → all interfaces
+		},
+		{
+			name:       "no_bindaddr_production_env",
+			bindAddr:   "",
+			adminToken: "",
+			molEnv:     "production",
+			want:       "", // production is not a dev value → all interfaces
+		},
+		{
+			name:       "no_bindaddr_unset_env",
+			bindAddr:   "",
+			adminToken: "",
+			molEnv:     "",
+			want:       "", // unset MOLECULE_ENV → not dev → all interfaces
+		},
+		{
+			name:       "explicit_bindaddr_loopback_overrides_devmode",
+			bindAddr:   "127.0.0.1",
+			adminToken: "",
+			molEnv:     "dev",
+			want:       "127.0.0.1",
+		},
+		{
+			name:       "explicit_bindaddr_wildcard_overrides_devmode_default",
+			bindAddr:   "0.0.0.0",
+			adminToken: "",
+			molEnv:     "dev",
+			want:       "0.0.0.0",
+		},
+		{
+			name:       "explicit_bindaddr_in_production",
+			bindAddr:   "10.0.5.7",
+			adminToken: "secret",
+			molEnv:     "production",
+			want:       "10.0.5.7",
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Setenv("BIND_ADDR", tc.bindAddr)
+			t.Setenv("ADMIN_TOKEN", tc.adminToken)
+			t.Setenv("MOLECULE_ENV", tc.molEnv)
+			got := resolveBindHost()
+			if got != tc.want {
+				t.Errorf("resolveBindHost() = %q, want %q (BIND_ADDR=%q ADMIN_TOKEN=%q MOLECULE_ENV=%q)",
+					got, tc.want, tc.bindAddr, tc.adminToken, tc.molEnv)
+			}
+		})
+	}
+}

workspace-server/cmd/server/main.go

@@ -19,6 +19,7 @@ import (
 	"github.com/Molecule-AI/molecule-monorepo/platform/internal/handlers"
 	"github.com/Molecule-AI/molecule-monorepo/platform/internal/imagewatch"
 	memwiring "github.com/Molecule-AI/molecule-monorepo/platform/internal/memory/wiring"
+	"github.com/Molecule-AI/molecule-monorepo/platform/internal/middleware"
 	"github.com/Molecule-AI/molecule-monorepo/platform/internal/pendinguploads"
 	"github.com/Molecule-AI/molecule-monorepo/platform/internal/provisioner"
 	"github.com/Molecule-AI/molecule-monorepo/platform/internal/registry"
@@ -337,15 +338,23 @@ func main() {
 	// Router
 	r := router.Setup(hub, broadcaster, prov, platformURL, configsDir, wh, channelMgr, memBundle)
 
-	// HTTP server with graceful shutdown
+	// HTTP server with graceful shutdown.
+	//
+	// Bind host: in dev-mode (no ADMIN_TOKEN, MOLECULE_ENV=dev|development)
+	// the AdminAuth chain fails open by design; pairing that with a wildcard
+	// bind would expose unauth /workspaces to any same-LAN peer. Default to
+	// loopback when fail-open is active. Operators who need LAN exposure set
+	// BIND_ADDR=0.0.0.0 explicitly. Production (ADMIN_TOKEN set) is unchanged.
+	// See molecule-core#7.
+	bindHost := resolveBindHost()
 	srv := &http.Server{
-		Addr:    fmt.Sprintf(":%s", port),
+		Addr:    fmt.Sprintf("%s:%s", bindHost, port),
 		Handler: r,
 	}
 
 	// Start server in goroutine
 	go func() {
-		log.Printf("Platform starting on :%s", port)
+		log.Printf("Platform starting on %s:%s (dev-mode-fail-open=%v)", bindHost, port, middleware.IsDevModeFailOpen())
 		if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
 			log.Fatalf("Server failed: %v", err)
 		}
@@ -380,6 +389,29 @@ func envOr(key, fallback string) string {
 	return fallback
 }
 
+// resolveBindHost picks the listener interface for the HTTP server.
+//
+// Precedence:
+//  1. BIND_ADDR — explicit operator override (any value, including "0.0.0.0").
+//  2. dev-mode fail-open active → "127.0.0.1" (loopback only).
+//  3. otherwise → "" (Go binds every interface; existing prod/self-host shape).
+//
+// Coupling the loopback default to middleware.IsDevModeFailOpen() means the
+// two safety levers — bind narrowness and auth strength — move together. A
+// production deploy (ADMIN_TOKEN set) keeps binding to all interfaces because
+// the auth chain is doing its job; a dev Mac (no ADMIN_TOKEN, MOLECULE_ENV=dev)
+// is reachable only via loopback because the auth chain is fail-open. See
+// molecule-core#7 for the original LAN exposure finding.
+func resolveBindHost() string {
+	if v := os.Getenv("BIND_ADDR"); v != "" {
+		return v
+	}
+	if middleware.IsDevModeFailOpen() {
+		return "127.0.0.1"
+	}
+	return ""
+}
+
 func findConfigsDir() string {
 	candidates := []string{
 		"workspace-configs-templates",