diff --git a/decnet/env.py b/decnet/env.py
index 6e892bf2..8473a570 100644
--- a/decnet/env.py
+++ b/decnet/env.py
@@ -46,13 +46,19 @@ def _require_env(name: str) -> str:
             f"Environment variable '{name}' is set to an insecure default ('{value}'). "
             f"Choose a strong, unique value before starting DECNET."
         )
+    _developer = os.environ.get("DECNET_DEVELOPER", "False").lower() == "true"
     if name == "DECNET_JWT_SECRET" and len(value) < 32:
-        _developer = os.environ.get("DECNET_DEVELOPER", "False").lower() == "true"
         if not _developer:
             raise ValueError(
                 f"DECNET_JWT_SECRET is too short ({len(value)} bytes). "
                 f"Use at least 32 characters to satisfy HS256 requirements (RFC 7518 §3.2)."
             )
+    if name == "DECNET_ADMIN_PASSWORD" and len(value) < 12:
+        if not _developer:
+            raise ValueError(
+                f"DECNET_ADMIN_PASSWORD is too short ({len(value)} chars). "
+                f"Use at least 12 characters, or set DECNET_DEVELOPER=true for local runs."
+            )
     return value
 
 
@@ -132,7 +138,9 @@ DECNET_BATCH_MAX_WAIT_MS: int = int(os.environ.get("DECNET_BATCH_MAX_WAIT_MS", "
 DECNET_WEB_HOST: str = os.environ.get("DECNET_WEB_HOST", "127.0.0.1")
 DECNET_WEB_PORT: int = _port("DECNET_WEB_PORT", 8080)
 DECNET_ADMIN_USER: str = os.environ.get("DECNET_ADMIN_USER", "admin")
-DECNET_ADMIN_PASSWORD: str = os.environ.get("DECNET_ADMIN_PASSWORD", "admin")
+# DECNET_ADMIN_PASSWORD is resolved lazily via __getattr__ (like DECNET_JWT_SECRET)
+# so it is validated only on the master processes that seed the admin user, and
+# never silently defaults to "admin". See _require_env + __getattr__ below.
 DECNET_DEVELOPER: bool = os.environ.get("DECNET_DEVELOPER", "False").lower() == "true"
 
 # Host role — seeded by /etc/decnet/decnet.ini or exported directly.
@@ -277,6 +285,6 @@ def validate_public_binding() -> None:
 
 def __getattr__(name: str) -> str:
     """Lazy resolution for secrets only the master web/api process needs."""
-    if name == "DECNET_JWT_SECRET":
-        return _require_env("DECNET_JWT_SECRET")
+    if name in ("DECNET_JWT_SECRET", "DECNET_ADMIN_PASSWORD"):
+        return _require_env(name)
     raise AttributeError(f"module 'decnet.env' has no attribute {name!r}")
diff --git a/tests/core/test_env_secrets.py b/tests/core/test_env_secrets.py
new file mode 100644
index 00000000..6de921f9
--- /dev/null
+++ b/tests/core/test_env_secrets.py
@@ -0,0 +1,66 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+"""Fail-closed validation of security-sensitive env secrets (env._require_env).
+
+DECNET_ADMIN_PASSWORD must never silently default to "admin": it is resolved
+lazily and validated like DECNET_JWT_SECRET. These tests drive _require_env
+against a controlled environ so the production raise paths (which are bypassed
+under live pytest) are actually exercised.
+"""
+from __future__ import annotations
+
+import pytest
+
+import decnet.env as envmod
+
+
+def _require(monkeypatch: pytest.MonkeyPatch, environ: dict[str, str]) -> str:
+    # Replace the whole environ for the call so the PYTEST_* short-circuit in
+    # _require_env doesn't fire — we want the real production behaviour.
+    monkeypatch.setattr(envmod.os, "environ", dict(environ))
+    return envmod._require_env("DECNET_ADMIN_PASSWORD")
+
+
+def test_admin_password_unset_raises(monkeypatch: pytest.MonkeyPatch) -> None:
+    with pytest.raises(ValueError, match="not set"):
+        _require(monkeypatch, {})
+
+
+def test_admin_password_known_bad_default_raises(monkeypatch: pytest.MonkeyPatch) -> None:
+    with pytest.raises(ValueError, match="insecure default"):
+        _require(monkeypatch, {"DECNET_ADMIN_PASSWORD": "admin"})
+
+
+@pytest.mark.parametrize("bad", ["secret", "password", "changeme", "ADMIN"])
+def test_admin_password_other_known_bad_raises(monkeypatch: pytest.MonkeyPatch, bad: str) -> None:
+    with pytest.raises(ValueError, match="insecure default"):
+        _require(monkeypatch, {"DECNET_ADMIN_PASSWORD": bad})
+
+
+def test_admin_password_too_short_raises_in_production(monkeypatch: pytest.MonkeyPatch) -> None:
+    with pytest.raises(ValueError, match="too short"):
+        _require(monkeypatch, {"DECNET_ADMIN_PASSWORD": "short1"})
+
+
+def test_admin_password_short_allowed_in_developer_mode(monkeypatch: pytest.MonkeyPatch) -> None:
+    val = _require(monkeypatch, {"DECNET_ADMIN_PASSWORD": "short1", "DECNET_DEVELOPER": "true"})
+    assert val == "short1"
+
+
+def test_admin_password_strong_value_passes(monkeypatch: pytest.MonkeyPatch) -> None:
+    val = _require(monkeypatch, {"DECNET_ADMIN_PASSWORD": "a-strong-unique-password"})
+    assert val == "a-strong-unique-password"
+
+
+def test_pytest_short_circuit_returns_value_unchecked() -> None:
+    # Under live pytest (PYTEST_* present), _require_env returns the configured
+    # value without the production checks — documents why the dev loop is safe.
+    # conftest sets a strong value, so this also proves lazy resolution works.
+    assert envmod._require_env("DECNET_ADMIN_PASSWORD") == "test-password-123"
+
+
+def test_lazy_getattr_resolves_admin_password() -> None:
+    # Accessing the attribute (not a module global anymore) routes through
+    # __getattr__ -> _require_env.
+    assert envmod.DECNET_ADMIN_PASSWORD == "test-password-123"
+    with pytest.raises(AttributeError):
+        envmod.NOT_A_REAL_SECRET  # noqa: B018