diff --git a/pyproject.toml b/pyproject.toml
index 44b4693..13e4ba2 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "uipath-dev"
-version = "0.0.82"
+version = "0.0.83"
 description = "UiPath Developer Console"
 readme = { file = "README.md", content-type = "text/markdown" }
 requires-python = ">=3.11"
diff --git a/src/uipath/dev/server/app.py b/src/uipath/dev/server/app.py
index 6c7b0d6..085f47b 100644
--- a/src/uipath/dev/server/app.py
+++ b/src/uipath/dev/server/app.py
@@ -12,6 +12,10 @@
 from fastapi.responses import HTMLResponse
 
 from uipath.dev.server import UiPathDeveloperServer
+from uipath.dev.server.security import (
+    LoopbackGuardMiddleware,
+    loopback_origin_regex,
+)
 
 logger = logging.getLogger(__name__)
 
@@ -103,12 +107,15 @@ def create_app(server: UiPathDeveloperServer) -> FastAPI:
 
     app.add_middleware(
         CORSMiddleware,
-        allow_origins=["*"],
+        allow_origin_regex=loopback_origin_regex(),
         allow_credentials=True,
         allow_methods=["*"],
         allow_headers=["*"],
     )
 
+    # Added last so it wraps CORS and runs first on every request and WebSocket.
+    app.add_middleware(LoopbackGuardMiddleware)
+
     # Store server reference on app state for route access
     app.state.server = server
 
diff --git a/src/uipath/dev/server/routes/files.py b/src/uipath/dev/server/routes/files.py
index d69b667..96a3a40 100644
--- a/src/uipath/dev/server/routes/files.py
+++ b/src/uipath/dev/server/routes/files.py
@@ -84,6 +84,20 @@ def _resolve_safe(path: str) -> Path:
     return resolved
 
 
+def _reject_if_secret(target: Path) -> None:
+    """Block reads/writes of credential files inside the project root."""
+    rel_parts = target.relative_to(ROOT).parts if target.is_relative_to(ROOT) else ()
+    if ".uipath" in rel_parts:
+        raise HTTPException(
+            status_code=403, detail="Access to this file is not allowed"
+        )
+    name = target.name
+    if name == ".env" or name.startswith(".env."):
+        raise HTTPException(
+            status_code=403, detail="Access to this file is not allowed"
+        )
+
+
 def _detect_language(filepath: Path) -> str | None:
     name = filepath.name.lower()
     if name == "dockerfile":
@@ -139,6 +153,7 @@ async def list_directory(path: str = "") -> list[dict[str, Any]]:
 async def read_file(path: str) -> dict[str, Any]:
     """Read file content. Returns content, binary flag, size, and language."""
     target = _resolve_safe(path)
+    _reject_if_secret(target)
     if not target.is_file():
         raise HTTPException(status_code=404, detail="File not found")
 
@@ -189,6 +204,7 @@ class SaveFileBody(BaseModel):
 async def save_file(path: str, body: SaveFileBody) -> dict[str, str]:
     """Save file content. Creates parent dirs if needed."""
     target = _resolve_safe(path)
+    _reject_if_secret(target)
     target.parent.mkdir(parents=True, exist_ok=True)
     # Write bytes directly to avoid Python text-mode converting \n → \r\n on
     # Windows, which would add extra carriage returns on every save since
diff --git a/src/uipath/dev/server/security.py b/src/uipath/dev/server/security.py
new file mode 100644
index 0000000..9597f1f
--- /dev/null
+++ b/src/uipath/dev/server/security.py
@@ -0,0 +1,80 @@
+"""Loopback protection for the unauthenticated developer server."""
+
+from __future__ import annotations
+
+from typing import Any
+
+from starlette.types import ASGIApp, Receive, Scope, Send
+
+_LOOPBACK_HOSTS = {"localhost", "127.0.0.1", "::1"}
+
+
+def _hostname(header_value: str) -> str:
+    """Extract the hostname from a Host/Origin header, dropping scheme and port."""
+    value = header_value.strip()
+    if "://" in value:
+        value = value.split("://", 1)[1]
+    if value.startswith("["):
+        return value[1 : value.index("]")] if "]" in value else value[1:]
+    return value.rsplit(":", 1)[0] if ":" in value else value
+
+
+class LoopbackGuardMiddleware:
+    """Reject requests whose Host or Origin is not loopback."""
+
+    def __init__(self, app: ASGIApp) -> None:
+        """Wrap the downstream ASGI app."""
+        self.app = app
+
+    def _allowed_hosts(self, scope: Scope) -> set[str]:
+        allowed = set(_LOOPBACK_HOSTS)
+        server = getattr(scope.get("app"), "state", None)
+        configured = getattr(getattr(server, "server", None), "host", None)
+        if configured:
+            allowed.add(configured)
+        return allowed
+
+    def _is_allowed(self, scope: Scope) -> bool:
+        headers = {k.lower(): v for k, v in scope.get("headers", [])}
+        allowed = self._allowed_hosts(scope)
+
+        host = headers.get(b"host")
+        if host is not None and _hostname(host.decode("latin-1")) not in allowed:
+            return False
+
+        origin = headers.get(b"origin")
+        if origin is not None and _hostname(origin.decode("latin-1")) not in allowed:
+            return False
+
+        return True
+
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        """Guard HTTP and WebSocket scopes, pass everything else through."""
+        if scope["type"] not in ("http", "websocket") or self._is_allowed(scope):
+            await self.app(scope, receive, send)
+            return
+
+        if scope["type"] == "websocket":
+            await send({"type": "websocket.close", "code": 1008})
+            return
+
+        await self._forbidden(send)
+
+    async def _forbidden(self, send: Send) -> None:
+        body = b"Forbidden: request rejected by loopback guard"
+        await send(
+            {
+                "type": "http.response.start",
+                "status": 403,
+                "headers": [
+                    (b"content-type", b"text/plain; charset=utf-8"),
+                    (b"content-length", str(len(body)).encode("latin-1")),
+                ],
+            }
+        )
+        await send({"type": "http.response.body", "body": body})
+
+
+def loopback_origin_regex(*args: Any) -> str:
+    """Regex matching loopback origins for CORS ``allow_origin_regex``."""
+    return r"^https?://(localhost|127\.0\.0\.1|\[::1\])(:\d+)?$"
diff --git a/tests/test_security.py b/tests/test_security.py
new file mode 100644
index 0000000..4cbc835
--- /dev/null
+++ b/tests/test_security.py
@@ -0,0 +1,116 @@
+"""Security tests for the developer server loopback protections.
+
+Covers VULN-45558: the unauthenticated file/run APIs must reject cross-site
+(``Origin``) and DNS-rebinding (``Host``) requests, and must never serve the
+UiPath Cloud access token stored in ``.env`` / ``.uipath``.
+"""
+
+import pytest
+from fastapi import FastAPI, WebSocket
+from starlette.testclient import TestClient
+
+from uipath.dev.server.security import (
+    LoopbackGuardMiddleware,
+    _hostname,
+    loopback_origin_regex,
+)
+
+
+def _app() -> FastAPI:
+    app = FastAPI()
+    app.add_middleware(LoopbackGuardMiddleware)
+
+    @app.get("/api/ping")
+    async def ping() -> dict[str, str]:
+        return {"status": "ok"}
+
+    @app.websocket("/ws")
+    async def ws(websocket: WebSocket) -> None:
+        await websocket.accept()
+        await websocket.send_text("ok")
+        await websocket.close()
+
+    return app
+
+
+def _client() -> TestClient:
+    return TestClient(_app(), base_url="http://localhost")
+
+
+@pytest.mark.parametrize("host", ["localhost", "127.0.0.1", "localhost:8080"])
+def test_loopback_host_allowed(host: str) -> None:
+    resp = _client().get("/api/ping", headers={"Host": host})
+    assert resp.status_code == 200
+
+
+@pytest.mark.parametrize("host", ["evil.com", "attacker.example:8080", "169.254.1.1"])
+def test_non_loopback_host_rejected(host: str) -> None:
+    resp = _client().get("/api/ping", headers={"Host": host})
+    assert resp.status_code == 403
+
+
+def test_cross_site_origin_rejected() -> None:
+    resp = _client().get("/api/ping", headers={"Origin": "https://evil.example"})
+    assert resp.status_code == 403
+
+
+def test_same_origin_allowed() -> None:
+    resp = _client().get("/api/ping", headers={"Origin": "http://localhost:8080"})
+    assert resp.status_code == 200
+
+
+def test_no_origin_allowed() -> None:
+    assert _client().get("/api/ping").status_code == 200
+
+
+def test_websocket_rejects_cross_site_origin() -> None:
+    from starlette.websockets import WebSocketDisconnect
+
+    with pytest.raises(WebSocketDisconnect):
+        with _client().websocket_connect(
+            "/ws", headers={"Host": "localhost", "Origin": "https://evil.example"}
+        ) as ws:
+            ws.receive_text()
+
+
+def test_websocket_allows_loopback() -> None:
+    with _client().websocket_connect("/ws", headers={"Host": "localhost"}) as ws:
+        assert ws.receive_text() == "ok"
+
+
+@pytest.mark.parametrize(
+    "value,expected",
+    [
+        ("http://evil.example:8080", "evil.example"),
+        ("localhost:8080", "localhost"),
+        ("[::1]:8080", "::1"),
+        ("127.0.0.1", "127.0.0.1"),
+    ],
+)
+def test_hostname_parsing(value: str, expected: str) -> None:
+    assert _hostname(value) == expected
+
+
+def test_cors_regex_matches_loopback_only() -> None:
+    import re
+
+    pattern = re.compile(loopback_origin_regex())
+    assert pattern.match("http://localhost:8080")
+    assert pattern.match("http://127.0.0.1:8080")
+    assert not pattern.match("https://evil.example")
+
+
+def test_secret_files_are_blocked(monkeypatch) -> None:
+    from fastapi import HTTPException
+
+    from uipath.dev.server.routes import files
+
+    for name in (".env", ".env.local"):
+        with pytest.raises(HTTPException) as exc:
+            files._reject_if_secret(files.ROOT / name)
+        assert exc.value.status_code == 403
+
+    with pytest.raises(HTTPException):
+        files._reject_if_secret(files.ROOT / ".uipath" / ".auth.json")
+
+    files._reject_if_secret(files.ROOT / "main.py")
diff --git a/uv.lock b/uv.lock
index dbe6d4a..c386558 100644
--- a/uv.lock
+++ b/uv.lock
@@ -2592,7 +2592,7 @@ wheels = [
 
 [[package]]
 name = "uipath-dev"
-version = "0.0.82"
+version = "0.0.83"
 source = { editable = "." }
 dependencies = [
     { name = "aiosqlite" },