diff --git a/plans/web-ui-management-plan.md b/plans/web-ui-management-plan.md
index bd264bc..91026d0 100644
--- a/plans/web-ui-management-plan.md
+++ b/plans/web-ui-management-plan.md
@@ -23,7 +23,7 @@ Stato passi (spunta man mano):
 - [x] **W2** — Layer `operations/cache.py` + `operations/host.py`: stato cache cargo, purge, disco, runner, backup template. Refactor `retention.py` (`monitor.py` lasciato invariato — refactor opzionale; `disk_status`/`runner_status` verranno consumati dalla web app in W4).
 - [x] **W3** — `operations/audit.py`: append-only JSONL + lettura. Config `[web]` in `config.py`.
 - [x] **W4** — App web core: `web/app.py` factory + `commands/web.py` (`ci web serve`). Dashboard **read-only** (VM, cache, disco, runner). Ancora **senza auth**, bind localhost.
-- [ ] **W5** — Auth: Gitea OIDC (Authlib) + sessione firmata + allowlist utenti. Tutte le route protette.
+- [x] **W5** — Auth: Gitea OIDC (Authlib) + sessione firmata + allowlist utenti (fail-closed) + secret solo da keyring. Tutte le route dati protette (`/healthz`,`/static`,`/login`,`/auth/callback` aperte). Hardening: `ci web serve` rifiuta bind non-loopback quando `[web.oidc]` assente (open mode solo su localhost).
 - [ ] **W6** — Azioni mutanti + safety: remove VM, cleanup orfane, purge cache, backup. Confirm gate + CSRF + audit + dry-run. HTMX partials.
 - [ ] **W7** — UI dark Gitea-style (asset vendorizzati) + installer ripetibile (`setup-web-linux.sh`, unit systemd, `docs/WEB-UI.md`) + validazione finale (test, coverage, mypy, ruff, smoke manuale).
 
diff --git a/src/ci_orchestrator/commands/web.py b/src/ci_orchestrator/commands/web.py
index f8fad74..583e7b6 100644
--- a/src/ci_orchestrator/commands/web.py
+++ b/src/ci_orchestrator/commands/web.py
@@ -10,8 +10,49 @@ the optional ``[web]`` extra is not installed.
 
 from __future__ import annotations
 
+import ipaddress
+from typing import TYPE_CHECKING
+
 import click
 
+if TYPE_CHECKING:  # pragma: no cover
+    from ci_orchestrator.config import Config
+
+_LOOPBACK_HOSTNAMES = {"localhost", ""}
+
+
+def _is_loopback(host: str) -> bool:
+    """True if ``host`` is a loopback address / ``localhost``.
+
+    Unresolvable hostnames are treated as NON-loopback (conservative — we
+    would rather refuse an ambiguous bind than expose the dashboard).
+    """
+    if host in _LOOPBACK_HOSTNAMES:
+        return True
+    try:
+        return ipaddress.ip_address(host).is_loopback
+    except ValueError:
+        return False
+
+
+def _bind_guard_error(host: str, config: Config) -> str | None:
+    """Refuse an unauthenticated bind on a non-loopback address.
+
+    When ``[web.oidc]`` is not configured the dashboard runs in OPEN mode
+    (no login — see ``web/auth.py``). That is only safe on loopback. Binding
+    a LAN/any address in open mode would expose host-management actions
+    unauthenticated, so it is refused here. Returns an error message to abort
+    with, or ``None`` when the bind is allowed.
+    """
+    if config.web.oidc is None and not _is_loopback(host):
+        return (
+            f"refusing to bind {host!r} without authentication: [web.oidc] is "
+            "not configured, so the dashboard would be served unauthenticated "
+            "on a non-loopback address. Configure [web.oidc] (Gitea OIDC) or "
+            "bind 127.0.0.1."
+        )
+    return None
+
 
 @click.group("web")
 def web() -> None:
@@ -49,6 +90,10 @@ def serve(host: str | None, port: int | None) -> None:
     bind_host = host if host is not None else config.web.bind_host
     bind_port = port if port is not None else config.web.bind_port
 
+    guard_error = _bind_guard_error(bind_host, config)
+    if guard_error is not None:
+        raise click.ClickException(guard_error)
+
     app = create_app(config)
     uvicorn.run(app, host=bind_host, port=bind_port)
 
diff --git a/tests/python/test_commands_web.py b/tests/python/test_commands_web.py
index 44e304e..43e5a25 100644
--- a/tests/python/test_commands_web.py
+++ b/tests/python/test_commands_web.py
@@ -148,3 +148,71 @@ def test_web_group_registered_on_root_cli() -> None:
 def test_config_import_does_not_require_web_extra() -> None:
     """``Config``/``load_config`` (imported lazily inside serve) is core, not [web]."""
     assert Config is not None
+
+
+# ── W5 security guard: no unauthenticated bind on a non-loopback address ──────
+
+
+def _cfg(*, oidc: bool, tmp: Path) -> Config:
+    from ci_orchestrator.config import (
+        BackendConfig,
+        Paths,
+        WebConfig,
+        WebOidcConfig,
+    )
+
+    web_oidc = (
+        WebOidcConfig(issuer="https://gitea.lan", client_id="cid", redirect_url="http://x/cb")
+        if oidc
+        else None
+    )
+    return Config(
+        paths=Paths(
+            root=tmp,
+            templates=tmp / "templates",
+            build_vms=tmp / "build-vms",
+            artifacts=tmp / "artifacts",
+            keys=tmp / "keys",
+        ),
+        backend=BackendConfig(),
+        web=WebConfig(oidc=web_oidc),
+    )
+
+
+@pytest.mark.parametrize("host", ["127.0.0.1", "::1", "localhost", ""])
+def test_is_loopback_true(host: str) -> None:
+    assert web_module._is_loopback(host) is True
+
+
+@pytest.mark.parametrize("host", ["192.168.1.50", "10.0.0.5", "0.0.0.0", "gitea.lan"])
+def test_is_loopback_false(host: str) -> None:
+    assert web_module._is_loopback(host) is False
+
+
+def test_bind_guard_blocks_lan_bind_without_oidc(tmp_path: Path) -> None:
+    err = web_module._bind_guard_error("192.168.1.50", _cfg(oidc=False, tmp=tmp_path))
+    assert err is not None
+    assert "refusing to bind" in err
+
+
+def test_bind_guard_allows_lan_bind_with_oidc(tmp_path: Path) -> None:
+    assert web_module._bind_guard_error("192.168.1.50", _cfg(oidc=True, tmp=tmp_path)) is None
+
+
+def test_bind_guard_allows_loopback_without_oidc(tmp_path: Path) -> None:
+    assert web_module._bind_guard_error("127.0.0.1", _cfg(oidc=False, tmp=tmp_path)) is None
+
+
+def test_web_serve_refuses_lan_bind_without_auth(monkeypatch: pytest.MonkeyPatch) -> None:
+    """End-to-end: ``serve`` aborts (and never starts uvicorn) on an unsafe bind.
+
+    The autouse fixture points config at an empty tmp dir → ``web.oidc`` is
+    ``None`` (open mode), so a non-loopback ``--host`` must be refused.
+    """
+    calls = _install_fake_uvicorn(monkeypatch)
+
+    result = CliRunner().invoke(cli, ["web", "serve", "--host", "10.0.0.5", "--port", "8800"])
+
+    assert result.exit_code != 0
+    assert "refusing to bind" in result.output
+    assert calls == [], "uvicorn.run must never be called on a refused bind"