diff --git a/muxplex/auth.py b/muxplex/auth.py
index 4d5a7ba..a7535e5 100644
--- a/muxplex/auth.py
+++ b/muxplex/auth.py
@@ -136,6 +136,9 @@ def authenticate_pam(username: str, password: str) -> bool:
 # Paths that bypass auth (login page itself, static assets it needs)
 _AUTH_EXEMPT_PATHS = {"/login", "/auth/mode", "/auth/logout"}
 
+# Socket-level localhost addresses — cannot be forged via HTTP headers
+_LOCALHOST_ADDRS = {"127.0.0.1", "::1"}
+
 
 class AuthMiddleware(BaseHTTPMiddleware):
     """FastAPI middleware that enforces authentication on non-localhost requests."""
@@ -155,13 +158,10 @@ class AuthMiddleware(BaseHTTPMiddleware):
         self.password = password
 
     async def dispatch(self, request: Request, call_next) -> Response:
-        # 1. Localhost bypass — check both client IP and server hostname.
-        # In production, client.host is the connecting IP (127.0.0.1 for local).
-        # In tests (TestClient), base_url affects request.url.hostname instead.
+        # 1. Localhost bypass — client.host is the socket-level IP and cannot
+        # be forged by the client (unlike the HTTP Host header).
         client_host = request.client.host if request.client else ""
-        server_host = request.url.hostname or ""
-        _localhost_addrs = ("127.0.0.1", "::1", "localhost")
-        if client_host in _localhost_addrs or server_host in _localhost_addrs:
+        if client_host in _LOCALHOST_ADDRS:
             return await call_next(request)
 
         # 2. Exempt paths (login page, auth endpoints)
@@ -177,6 +177,7 @@ class AuthMiddleware(BaseHTTPMiddleware):
         auth_header = request.headers.get("authorization", "")
         if auth_header.lower().startswith("basic "):
             try:
+                # Strip "Basic " prefix (6 chars) before base64-decoding
                 decoded = base64.b64decode(auth_header[6:]).decode()
                 username, _, pw = decoded.partition(":")
                 if self._check_credentials(username, pw):
diff --git a/muxplex/tests/test_auth.py b/muxplex/tests/test_auth.py
index 895d983..8c6ceec 100644
--- a/muxplex/tests/test_auth.py
+++ b/muxplex/tests/test_auth.py
@@ -247,6 +247,25 @@ def test_authenticate_pam_wrong_user_rejected(monkeypatch):
 # ---------------------------------------------------------------------------
 
 
+class _InjectClientMiddleware:
+    """Thin ASGI wrapper that injects a fake client address into the scope.
+
+    Starlette's TestClient sets request.client.host to "testclient" regardless
+    of base_url.  Wrapping the app with this middleware lets tests supply a
+    specific socket-level IP so the AuthMiddleware localhost check is exercised
+    with real values rather than relying on the (user-controlled) Host header.
+    """
+
+    def __init__(self, app, client_host: str, client_port: int = 50000):
+        self.app = app
+        self._client = (client_host, client_port)
+
+    async def __call__(self, scope, receive, send):
+        if scope.get("type") == "http":
+            scope = {**scope, "client": self._client}
+        await self.app(scope, receive, send)
+
+
 def _make_test_app(auth_mode: str = "password", password: str = "test-pw") -> FastAPI:
     """Create a minimal FastAPI app with AuthMiddleware for testing."""
     test_app = FastAPI()
@@ -269,7 +288,11 @@ def _make_test_app(auth_mode: str = "password", password: str = "test-pw") -> Fa
 def test_middleware_localhost_bypasses_auth():
     """Requests from 127.0.0.1 pass through without auth."""
     app = _make_test_app()
-    client = TestClient(app, base_url="http://127.0.0.1")
+    # TestClient always sets request.client.host to "testclient".  Wrap the app
+    # with _InjectClientMiddleware to set the socket-level client IP to 127.0.0.1
+    # so the middleware's localhost check is exercised with a real address.
+    app_with_client = _InjectClientMiddleware(app, "127.0.0.1")
+    client = TestClient(app_with_client)
     response = client.get("/protected")
     assert response.status_code == 200
     assert response.text == "OK"
@@ -280,7 +303,8 @@ def test_middleware_valid_session_cookie_passes():
     app = _make_test_app()
     cookie = create_session_cookie("test-secret", ttl_seconds=3600)
     client = TestClient(app, base_url="http://192.168.1.1")
-    response = client.get("/protected", cookies={"muxplex_session": cookie})
+    client.cookies.set("muxplex_session", cookie)
+    response = client.get("/protected")
     assert response.status_code == 200
     assert response.text == "OK"
 
@@ -289,7 +313,8 @@ def test_middleware_tampered_cookie_redirects():
     """Non-localhost request with a tampered cookie redirects to /login."""
     app = _make_test_app()
     client = TestClient(app, base_url="http://192.168.1.1", follow_redirects=False)
-    response = client.get("/protected", cookies={"muxplex_session": "bad.cookie.value"})
+    client.cookies.set("muxplex_session", "bad.cookie.value")
+    response = client.get("/protected")
     assert response.status_code == 307
     assert "/login" in response.headers["location"]