feat(proxy): support LITELLM_ENABLE_NO_STORE in SecurityHeadersMiddleware

Add opt-in Cache-Control: no-store header via LITELLM_ENABLE_NO_STORE env var to prevent browsers and proxies from caching sensitive responses. Uses setdefault so route-specific Cache-Control headers are preserved.

Closes #46078

litellm/proxy/middleware/security_headers_middleware.py

@@ -8,6 +8,10 @@ X-Content-Type-Options: nosniff stops MIME sniffing.
 Strict-Transport-Security is opt-in via LITELLM_ENABLE_HSTS because it only
 makes sense over HTTPS and would lock browsers out of plain-http deployments.
 
+Cache-Control: no-store is opt-in via LITELLM_ENABLE_NO_STORE to prevent
+browsers and intermediaries from caching sensitive responses (e.g. key lists,
+spend logs, user credentials).
+
 Headers are set with setdefault so a route that intentionally sets its own
 value is never overridden.
 """
@@ -24,12 +28,17 @@ STATIC_SECURITY_HEADERS: Final = (
     ("X-Content-Type-Options", "nosniff"),
 )
 HSTS_HEADER: Final = ("Strict-Transport-Security", "max-age=31536000; includeSubDomains")
+NO_STORE_HEADER: Final = ("Cache-Control", "no-store")
 
 
 def _hsts_enabled() -> bool:
     return os.getenv("LITELLM_ENABLE_HSTS", "false").strip().lower() == "true"
 
 
+def _no_store_enabled() -> bool:
+    return os.getenv("LITELLM_ENABLE_NO_STORE", "false").strip().lower() == "true"
+
+
 class SecurityHeadersMiddleware:
     def __init__(self, app: ASGIApp) -> None:
         self.app = app
@@ -42,7 +51,11 @@ class SecurityHeadersMiddleware:
         async def send_with_security_headers(message: Message) -> None:
             if message["type"] == "http.response.start":
                 headers: Final = MutableHeaders(scope=message)
-                applied: Final = (*STATIC_SECURITY_HEADERS, HSTS_HEADER) if _hsts_enabled() else STATIC_SECURITY_HEADERS
+                applied: list[tuple[str, str]] = list(STATIC_SECURITY_HEADERS)
+                if _hsts_enabled():
+                    applied.append(HSTS_HEADER)
+                if _no_store_enabled():
+                    applied.append(NO_STORE_HEADER)
                 for name, value in applied:
                     headers.setdefault(name, value)
             await send(message)

tests/unit/proxy/middleware/test_security_headers_middleware.py

@@ -2,7 +2,8 @@
 Tests for SecurityHeadersMiddleware.
 
 Verifies anti-framing / content-type headers are present on every response and
-that HSTS is opt-in via LITELLM_ENABLE_HSTS.
+that HSTS is opt-in via LITELLM_ENABLE_HSTS and Cache-Control: no-store is
+opt-in via LITELLM_ENABLE_NO_STORE.
 """
 
 from starlette.applications import Starlette
@@ -69,3 +70,45 @@ def test_does_not_override_existing_header(monkeypatch):
     assert resp.headers["x-frame-options"] == "SAMEORIGIN"
     # other headers still applied
     assert resp.headers["x-content-type-options"] == "nosniff"
+
+
+def test_no_store_absent_by_default(monkeypatch):
+    monkeypatch.delenv("LITELLM_ENABLE_NO_STORE", raising=False)
+    resp = _make_client(_ok).get("/")
+    assert "cache-control" not in resp.headers
+
+
+def test_no_store_present_when_enabled(monkeypatch):
+    monkeypatch.setenv("LITELLM_ENABLE_NO_STORE", "true")
+    resp = _make_client(_ok).get("/")
+    assert resp.headers["cache-control"] == "no-store"
+
+
+def test_no_store_not_enabled_by_arbitrary_value(monkeypatch):
+    monkeypatch.setenv("LITELLM_ENABLE_NO_STORE", "1")
+    resp = _make_client(_ok).get("/")
+    assert "cache-control" not in resp.headers
+
+
+def test_no_store_does_not_override_existing_header(monkeypatch):
+    """A route that sets its own Cache-Control must win."""
+
+    async def custom(request):
+        return Response("hi", headers={"Cache-Control": "private, max-age=60"})
+
+    monkeypatch.setenv("LITELLM_ENABLE_NO_STORE", "true")
+    resp = _make_client(custom).get("/")
+    assert resp.headers["cache-control"] == "private, max-age=60"
+    assert resp.headers["x-content-type-options"] == "nosniff"
+
+
+def test_both_hsts_and_no_store_enabled(monkeypatch):
+    monkeypatch.setenv("LITELLM_ENABLE_HSTS", "true")
+    monkeypatch.setenv("LITELLM_ENABLE_NO_STORE", "true")
+    resp = _make_client(_ok).get("/")
+    assert resp.headers["strict-transport-security"] == (
+        "max-age=31536000; includeSubDomains"
+    )
+    assert resp.headers["cache-control"] == "no-store"
+    assert resp.headers["x-frame-options"] == "DENY"
+