diff --git a/src/memos/api/middleware/auth.py b/src/memos/api/middleware/auth.py
index 15b217651..125d17278 100644
--- a/src/memos/api/middleware/auth.py
+++ b/src/memos/api/middleware/auth.py
@@ -6,6 +6,7 @@
 """
 
 import hashlib
+import hmac
 import os
 import time
 
@@ -25,6 +26,7 @@
 # Environment configuration
 AUTH_ENABLED = os.getenv("AUTH_ENABLED", "false").lower() == "true"
 MASTER_KEY_HASH = os.getenv("MASTER_KEY_HASH")  # SHA-256 hash of master key
+INTERNAL_SERVICE_SECRET = os.getenv("INTERNAL_SERVICE_SECRET")  # shared secret for X-Internal-Service header
 INTERNAL_SERVICE_IPS = {"127.0.0.1", "::1", "memos-mcp", "moltbot", "clawdbot"}
 
 # Connection pool for auth queries (lazy init)
@@ -142,16 +144,35 @@ async def lookup_api_key(key_hash: str) -> dict[str, Any] | None:
 
 
 def is_internal_request(request: Request) -> bool:
-    """Check if request is from internal service."""
+    """Check if request is from internal service.
+
+    Two authorised paths:
+
+    1. The client host address is a well-known trusted host in
+       ``INTERNAL_SERVICE_IPS``.
+    2. The request carries an ``X-Internal-Service`` header whose value matches
+       the operator-configured ``INTERNAL_SERVICE_SECRET`` env var. Comparison
+       is constant-time (``hmac.compare_digest``) to avoid timing side channels.
+
+    The header branch is **disabled** whenever either the environment secret or
+    the request header is missing or empty. This closes GHSA-9pw6-vmgx-qgwx /
+    issue #2259, where an unset secret combined with an absent header caused
+    ``None == None`` to be treated as a match and grant the ``internal``
+    principal to unauthenticated remote callers.
+    """
     client_host = request.client.host if request.client else None
 
     # Check internal IPs
     if client_host in INTERNAL_SERVICE_IPS:
         return True
 
-    # Check internal header (for container-to-container)
+    # Check internal header (for container-to-container). Treat an unset /
+    # empty secret or missing / empty header as "disabled" and fail closed.
+    secret = INTERNAL_SERVICE_SECRET
     internal_header = request.headers.get("X-Internal-Service")
-    return internal_header == os.getenv("INTERNAL_SERVICE_SECRET")
+    if not secret or not internal_header:
+        return False
+    return hmac.compare_digest(internal_header, secret)
 
 
 async def verify_api_key(
@@ -180,7 +201,8 @@ async def verify_api_key(
 
     # Allow internal services
     if is_internal_request(request):
-        logger.debug(f"Internal request from {request.client.host}")
+        client_host = request.client.host if request.client else "<unknown>"
+        logger.debug(f"Internal request from {client_host}")
         return {
             "user_name": "internal",
             "scopes": ["all"],
diff --git a/tests/api/test_auth_internal_request.py b/tests/api/test_auth_internal_request.py
new file mode 100644
index 000000000..9a38b3820
--- /dev/null
+++ b/tests/api/test_auth_internal_request.py
@@ -0,0 +1,195 @@
+"""Tests for `memos.api.middleware.auth.is_internal_request` fail-open regression.
+
+Regression test suite for GHSA-9pw6-vmgx-qgwx / issue #2259.
+
+The bug: when `INTERNAL_SERVICE_SECRET` is unset (its default across every shipped
+Dockerfile / compose / Helm config) and an external client does not send the
+`X-Internal-Service` header, `is_internal_request()` compared
+``request.headers.get("X-Internal-Service")`` to ``os.getenv("INTERNAL_SERVICE_SECRET")``
+directly, so ``None == None`` returned True and the request was granted the
+`internal` principal with ``scopes: ["all"]``.
+
+These tests lock in the fixed behaviour: unset secret OR missing header → not internal;
+matching non-empty secret + header → internal (constant-time compare).
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+import pytest
+
+from fastapi import HTTPException
+from starlette.requests import Request
+
+from memos.api.middleware import auth as auth_module
+from memos.api.middleware.auth import is_internal_request, verify_api_key
+
+
+def _make_request(
+    *,
+    client: tuple[str, int] | None = ("203.0.113.9", 53124),
+    headers: dict[str, str] | None = None,
+) -> Request:
+    """Build a bare-bones Starlette Request for the dependency under test."""
+    scope: dict[str, Any] = {
+        "type": "http",
+        "method": "GET",
+        "path": "/admin/keys",
+        "headers": [(k.lower().encode(), v.encode()) for k, v in (headers or {}).items()],
+    }
+    if client is not None:
+        scope["client"] = client
+    return Request(scope)
+
+
+class TestIsInternalRequest:
+    def test_returns_false_when_secret_unset_and_header_missing(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """Regression: None == None must not authenticate."""
+        monkeypatch.setattr(auth_module, "INTERNAL_SERVICE_SECRET", None)
+        request = _make_request(headers={})
+
+        assert is_internal_request(request) is False
+
+    def test_returns_false_when_secret_unset_and_header_present(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """Header path is disabled when secret is not configured."""
+        monkeypatch.setattr(auth_module, "INTERNAL_SERVICE_SECRET", None)
+        request = _make_request(headers={"X-Internal-Service": "guessed-value"})
+
+        assert is_internal_request(request) is False
+
+    def test_returns_false_when_secret_empty_string(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        """Empty string secret is treated as unset (defence in depth)."""
+        monkeypatch.setattr(auth_module, "INTERNAL_SERVICE_SECRET", "")
+        request = _make_request(headers={"X-Internal-Service": ""})
+
+        assert is_internal_request(request) is False
+
+    def test_returns_false_when_secret_set_but_header_missing(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        monkeypatch.setattr(auth_module, "INTERNAL_SERVICE_SECRET", "super-secret-value")
+        request = _make_request(headers={})
+
+        assert is_internal_request(request) is False
+
+    def test_returns_false_when_secret_set_but_header_wrong(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        monkeypatch.setattr(auth_module, "INTERNAL_SERVICE_SECRET", "super-secret-value")
+        request = _make_request(headers={"X-Internal-Service": "wrong-value"})
+
+        assert is_internal_request(request) is False
+
+    def test_returns_true_when_secret_matches_header(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        monkeypatch.setattr(auth_module, "INTERNAL_SERVICE_SECRET", "super-secret-value")
+        request = _make_request(headers={"X-Internal-Service": "super-secret-value"})
+
+        assert is_internal_request(request) is True
+
+    def test_returns_true_when_client_ip_is_internal(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        """The internal-IP branch remains untouched by the fix."""
+        monkeypatch.setattr(auth_module, "INTERNAL_SERVICE_SECRET", None)
+        request = _make_request(client=("127.0.0.1", 12345), headers={})
+
+        assert is_internal_request(request) is True
+
+    def test_uses_constant_time_compare(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        """The fix must call `hmac.compare_digest` (not plain ==) to avoid timing leaks."""
+        monkeypatch.setattr(auth_module, "INTERNAL_SERVICE_SECRET", "super-secret-value")
+        calls: list[tuple[str, str]] = []
+
+        real_compare = auth_module.hmac.compare_digest
+
+        def spy(a: str, b: str) -> bool:
+            calls.append((a, b))
+            return real_compare(a, b)
+
+        monkeypatch.setattr(auth_module.hmac, "compare_digest", spy)
+        request = _make_request(headers={"X-Internal-Service": "super-secret-value"})
+
+        assert is_internal_request(request) is True
+        assert calls, "expected hmac.compare_digest to be invoked for header comparison"
+
+    def test_no_client_and_no_header_and_no_secret(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        """Missing request.client + no header + no secret must NOT authenticate."""
+        monkeypatch.setattr(auth_module, "INTERNAL_SERVICE_SECRET", None)
+        request = _make_request(client=None, headers={})
+
+        assert is_internal_request(request) is False
+
+
+class TestVerifyApiKeyEndToEnd:
+    """End-to-end reproduction of the advisory PoC against `verify_api_key`."""
+
+    @pytest.mark.asyncio
+    async def test_external_request_no_key_is_rejected_when_secret_unset(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """The exact PoC from GHSA-9pw6-vmgx-qgwx must now be rejected with 401."""
+        monkeypatch.setattr(auth_module, "AUTH_ENABLED", True)
+        monkeypatch.setattr(auth_module, "INTERNAL_SERVICE_SECRET", None)
+        request = _make_request(headers={})
+
+        with pytest.raises(HTTPException) as exc_info:
+            await verify_api_key(request, api_key=None)
+
+        assert exc_info.value.status_code == 401
+        assert "Missing API key" in exc_info.value.detail
+
+    @pytest.mark.asyncio
+    async def test_external_request_with_wrong_header_still_rejected(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        monkeypatch.setattr(auth_module, "AUTH_ENABLED", True)
+        monkeypatch.setattr(auth_module, "INTERNAL_SERVICE_SECRET", None)
+        request = _make_request(headers={"X-Internal-Service": "guess"})
+
+        with pytest.raises(HTTPException) as exc_info:
+            await verify_api_key(request, api_key=None)
+
+        assert exc_info.value.status_code == 401
+
+    @pytest.mark.asyncio
+    async def test_internal_request_with_matching_secret_is_authorised(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        monkeypatch.setattr(auth_module, "AUTH_ENABLED", True)
+        monkeypatch.setattr(auth_module, "INTERNAL_SERVICE_SECRET", "shared-secret")
+        request = _make_request(
+            headers={"X-Internal-Service": "shared-secret"},
+            client=("10.0.0.42", 40000),
+        )
+
+        result = await verify_api_key(request, api_key=None)
+        assert result == {
+            "user_name": "internal",
+            "scopes": ["all"],
+            "is_master_key": False,
+            "is_internal": True,
+        }
+
+    @pytest.mark.asyncio
+    async def test_internal_request_via_header_with_no_client_does_not_crash(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """Regression for finding #1: ``request.client`` may be None on the header path.
+
+        `verify_api_key` must not raise ``AttributeError`` when a request without a
+        ``client`` tuple is granted the internal principal via a matching
+        ``X-Internal-Service`` header.
+        """
+        monkeypatch.setattr(auth_module, "AUTH_ENABLED", True)
+        monkeypatch.setattr(auth_module, "INTERNAL_SERVICE_SECRET", "shared-secret")
+        request = _make_request(
+            client=None,
+            headers={"X-Internal-Service": "shared-secret"},
+        )
+
+        result = await verify_api_key(request, api_key=None)
+        assert result["is_internal"] is True
+        assert result["user_name"] == "internal"