diff --git a/src/lh_harness/webapi/server.py b/src/lh_harness/webapi/server.py
index ed3a0e84..fc97b641 100644
--- a/src/lh_harness/webapi/server.py
+++ b/src/lh_harness/webapi/server.py
@@ -43,6 +43,7 @@
 }
 
 _MAX_ARTIFACT_BYTES = 8 * 1024 * 1024
+_MAX_CONTROL_BODY_BYTES = 1 * 1024 * 1024
 
 # Agent-produced artifacts are untrusted.  Only a small, explicit raster
 # allow-list is rendered in the dashboard origin; everything else is a
@@ -104,6 +105,37 @@ def _is_loopback_host(host: str) -> bool:
         return False
 
 
+def _request_hostname(host_header: str) -> str:
+    """Return the hostname from a Host header, ignoring an optional port."""
+
+    value = str(host_header or "").strip()
+    if not value:
+        return ""
+    if value.startswith("["):
+        end = value.find("]")
+        if end == -1:
+            return ""
+        return value[1:end]
+    if value.count(":") == 1:
+        return value.rsplit(":", 1)[0]
+    return value
+
+
+def _host_header_allowed(host_header: str, bind_host: str) -> bool:
+    hostname = _request_hostname(host_header)
+    if not hostname:
+        return False
+    if _is_loopback_host(hostname):
+        return True
+    configured = str(bind_host or "").strip().lower().strip("[]")
+    return bool(configured) and hostname.lower() == configured
+
+
+def _is_json_content_type(value: str | None) -> bool:
+    media = (value or "").split(";", 1)[0].strip().lower()
+    return media == "application/json"
+
+
 def _configured_token(explicit: str | None) -> str | None:
     value = explicit if explicit is not None else os.environ.get("LH_HARNESS_WEB_TOKEN")
     value = str(value or "").strip()
@@ -550,6 +582,7 @@ def create_app(
     supervisor: RunSupervisor | None = None,
     auth_token: str | None = None,
     allowed_origins: set[str] | list[str] | tuple[str, ...] | None = None,
+    bind_host: str = "127.0.0.1",
 ) -> FastAPI:
     """Create an API app over a live shared state or a historical runs root."""
 
@@ -570,6 +603,7 @@ def create_app(
     app.state.registry = registry
     app.state.auth_token = token
     app.state.allowed_origins = origins
+    app.state.bind_host = bind_host
     if supervisor is not None:
         async def _shutdown_owned_workers() -> None:
             await asyncio.to_thread(supervisor.shutdown)
@@ -582,6 +616,29 @@ async def _security_headers_and_auth(request: Request, call_next):
         # shell, but every API route (including legacy compatibility routes and
         # artifact reads) shares one authentication boundary when a token is
         # configured.
+        if _is_loopback_host(bind_host) and not _host_header_allowed(
+            request.headers.get("host", ""), bind_host
+        ):
+            return JSONResponse({"detail": "host is not allowed"}, status_code=403)
+        if request.method in {"POST", "PUT", "PATCH"} and request.url.path.startswith("/api/"):
+            content_type = request.headers.get("content-type")
+            content_length = request.headers.get("content-length")
+            if content_length is not None:
+                try:
+                    declared = int(content_length)
+                except ValueError:
+                    return JSONResponse({"detail": "invalid content-length"}, status_code=400)
+                if declared > _MAX_CONTROL_BODY_BYTES:
+                    return JSONResponse({"detail": "request body is too large"}, status_code=413)
+            body = await request.body()
+            if len(body) > _MAX_CONTROL_BODY_BYTES:
+                return JSONResponse({"detail": "request body is too large"}, status_code=413)
+            if body or content_type:
+                if not _is_json_content_type(content_type):
+                    return JSONResponse(
+                        {"detail": "request must be application/json"},
+                        status_code=415,
+                    )
         authenticated = _bearer_matches(request.headers.get("authorization"), token)
         if token and request.url.path.startswith("/api/") and not authenticated:
             return JSONResponse(
@@ -731,6 +788,9 @@ async def stream(
             return
         origin = websocket.headers.get("origin")
         host = websocket.headers.get("host", "")
+        if _is_loopback_host(bind_host) and not _host_header_allowed(host, bind_host):
+            await websocket.close(code=4403, reason="host is not allowed")
+            return
         authenticated, selected_subprotocol = _websocket_auth(websocket, token)
         if not authenticated:
             await websocket.close(code=4401, reason="invalid or missing bearer token")
@@ -1081,6 +1141,7 @@ def run_web_server(
         supervisor=supervisor,
         auth_token=token,
         allowed_origins=allowed_origins,
+        bind_host=host,
     )
     uvicorn.run(app, host=host, port=port, log_level="info")
     return 0
@@ -1167,6 +1228,7 @@ def start_web_server(
         supervisor=supervisor,
         auth_token=token,
         allowed_origins=allowed_origins,
+        bind_host=host,
     )
     config = uvicorn.Config(app, host=host, port=port, log_level="warning", access_log=False)
     server = uvicorn.Server(config)
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 00000000..209b6d31
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,37 @@
+"""Default TestClient Host to loopback so Host-header hardening stays on."""
+
+from __future__ import annotations
+
+from urllib.parse import urljoin
+
+import fastapi.testclient as fastapi_testclient
+import starlette.testclient as starlette_testclient
+
+_Orig = starlette_testclient.TestClient
+_Upgrade = starlette_testclient._Upgrade
+
+
+class LoopbackTestClient(_Orig):
+    def __init__(self, app, *args, **kwargs):
+        kwargs.setdefault("base_url", "http://127.0.0.1")
+        super().__init__(app, *args, **kwargs)
+
+    def websocket_connect(self, url, subprotocols=None, **kwargs):
+        # Starlette hardcodes ws://testserver; rewrite so Host stays loopback.
+        url = urljoin("ws://127.0.0.1", url)
+        headers = kwargs.get("headers", {})
+        headers.setdefault("connection", "upgrade")
+        headers.setdefault("sec-websocket-key", "testserver==")
+        headers.setdefault("sec-websocket-version", "13")
+        if subprotocols is not None:
+            headers.setdefault("sec-websocket-protocol", ", ".join(subprotocols))
+        kwargs["headers"] = headers
+        try:
+            super().request("GET", url, **kwargs)
+        except _Upgrade as exc:
+            return exc.session
+        raise RuntimeError("Expected WebSocket upgrade")
+
+
+starlette_testclient.TestClient = LoopbackTestClient
+fastapi_testclient.TestClient = LoopbackTestClient
diff --git a/tests/webapi/test_hardening.py b/tests/webapi/test_hardening.py
index 8b78b0d7..8e24ff20 100644
--- a/tests/webapi/test_hardening.py
+++ b/tests/webapi/test_hardening.py
@@ -15,7 +15,7 @@
 import lh_harness.dashboard.state as dashboard_state
 from lh_harness.dashboard.state import DashboardState
 from lh_harness.webapi.events import EventTailer
-from lh_harness.webapi.server import create_app
+from lh_harness.webapi.server import _MAX_CONTROL_BODY_BYTES, create_app
 
 
 def _run(tmp_path: Path) -> tuple[Path, DashboardState]:
@@ -614,3 +614,79 @@ def test_attached_api_rejects_foreign_run_paths(tmp_path: Path) -> None:
 
     assert client.get("/api/runs/run-2/snapshot").status_code == 404
     assert client.post("/api/runs/run-2/stop").status_code == 404
+
+
+def test_loopback_host_header_rejects_dns_rebind(tmp_path: Path) -> None:
+    root, state = _run(tmp_path)
+    client = TestClient(create_app(state=state, runs_root=root, run_id="run-1"))
+
+    assert client.get("/api/meta").status_code == 200
+    assert client.get("/api/meta", headers={"Host": "127.0.0.1:8799"}).status_code == 200
+    assert client.get("/api/meta", headers={"Host": "localhost"}).status_code == 200
+    assert client.get("/api/meta", headers={"Host": "[::1]:8799"}).status_code == 200
+    assert client.get("/api/meta", headers={"Host": "evil.example"}).status_code == 403
+    assert client.get("/api/meta", headers={"Host": "evil.example:8799"}).status_code == 403
+    assert client.post(
+        "/api/runs/run-1/instructions",
+        headers={"Host": "evil.example"},
+        json={"instructions": "injected"},
+    ).status_code == 403
+
+
+def test_control_posts_require_json_and_bound_body(tmp_path: Path) -> None:
+    root, state = _run(tmp_path)
+    client = TestClient(create_app(state=state, runs_root=root, run_id="run-1"))
+
+    assert client.post(
+        "/api/runs/run-1/instructions",
+        content=b'{"instructions":"keep going"}',
+        headers={"Content-Type": "text/plain"},
+    ).status_code == 415
+    assert client.post(
+        "/api/runs/run-1/instructions",
+        data={"instructions": "keep going"},
+    ).status_code == 415
+    assert client.post(
+        "/api/runs/run-1/instructions",
+        content=b'{"instructions":"keep going"}',
+        headers={"Content-Type": "application/json; charset=utf-8"},
+    ).status_code == 200
+    oversized = client.post(
+        "/api/runs/run-1/instructions",
+        content=b"x" * (_MAX_CONTROL_BODY_BYTES + 1),
+        headers={
+            "Content-Type": "application/json",
+            "Content-Length": str(_MAX_CONTROL_BODY_BYTES + 1),
+        },
+    )
+    assert oversized.status_code == 413
+
+
+def test_bearer_and_non_loopback_host_behavior(tmp_path: Path) -> None:
+    root, state = _run(tmp_path)
+    loopback = TestClient(
+        create_app(state=state, runs_root=root, run_id="run-1", auth_token="secret")
+    )
+    assert loopback.get("/api/meta").status_code == 401
+    assert loopback.get(
+        "/api/meta", headers={"Authorization": "Bearer secret"}
+    ).status_code == 200
+    assert loopback.get(
+        "/api/meta",
+        headers={"Authorization": "Bearer secret", "Host": "evil.example"},
+    ).status_code == 403
+
+    exposed = TestClient(
+        create_app(
+            state=state,
+            runs_root=root,
+            run_id="run-1",
+            auth_token="secret",
+            bind_host="0.0.0.0",
+        )
+    )
+    assert exposed.get("/api/meta", headers={"Host": "evil.example"}).status_code == 401
+    assert exposed.get(
+        "/api/meta",
+        headers={"Authorization": "Bearer secret", "Host": "lan.example:8799"},
+    ).status_code == 200