harden(dataflows): bound the Reddit feed read before parsing

- ElementTree does not resolve external entities, so the reported XXE flag
  doesn't apply; the real residual is an unbounded read of untrusted network XML
- cap both the RSS and JSON reads at 5 MiB; overflow degrades to empty / RSS
  fallback through the existing failure paths #1206 #1276

tests/test_reddit_fallback.py

@@ -36,8 +36,9 @@ def _resp(read_fn):
         def __exit__(self_inner, *a):
             return False
 
-        def read(self_inner):
-            return read_fn()
+        def read(self_inner, size=-1):
+            data = read_fn()
+            return data if size is None or size < 0 else data[:size]
     return _Resp()
 
 
@@ -183,6 +184,14 @@ class TestChunkedTransferErrorsHandled:
             reddit._fetch_subreddit_json("NVDA", "stocks", 5, 5.0)
         rss.assert_called_once()
 
+    def test_oversized_rss_feed_is_refused_not_parsed(self):
+        # A hostile/misbehaving endpoint streaming an unbounded body must not be
+        # read into memory before parsing; overflow degrades to an empty feed.
+        big = _resp(lambda: b"x" * 100)
+        with patch.object(reddit, "_MAX_FEED_BYTES", 10), \
+             patch.object(reddit, "urlopen", return_value=big):
+            assert reddit._fetch_subreddit_rss("NVDA", "stocks", 5, 5.0) == []
+
 
 @pytest.mark.unit
 class TestFormatterHandlesRssPosts:

tradingagents/dataflows/reddit.py

@@ -126,6 +126,23 @@ def _retry_after_seconds(exc: HTTPError) -> float | None:
         return None
 
 
+# Reddit search feeds are small (a page of results); cap the read so a
+# compromised or misbehaving endpoint can't stream an unbounded body into
+# memory before we parse it. Overflow raises http.client.HTTPException, which
+# both fetch paths already treat as a failed fetch (degrade to empty / RSS).
+_MAX_FEED_BYTES = 5 * 1024 * 1024
+
+
+def _read_capped(resp) -> bytes:
+    """Read a response body bounded to ``_MAX_FEED_BYTES``, raising on overflow."""
+    data = resp.read(_MAX_FEED_BYTES + 1)
+    if len(data) > _MAX_FEED_BYTES:
+        raise http.client.HTTPException(
+            f"Reddit feed exceeded {_MAX_FEED_BYTES} bytes; refusing to parse"
+        )
+    return data
+
+
 def _fetch_subreddit_rss(
     ticker: str,
     sub: str,
@@ -144,7 +161,7 @@ def _fetch_subreddit_rss(
     req = Request(url, headers={"User-Agent": _UA})
     try:
         with urlopen(req, timeout=timeout) as resp:
-            root = ET.fromstring(resp.read())
+            root = ET.fromstring(_read_capped(resp))
     except HTTPError as exc:
         if exc.code == 429 and _retry:
             # Honour a server-supplied Retry-After exactly (including 0); jitter
@@ -201,7 +218,7 @@ def _fetch_subreddit_json(
     req = Request(url, headers={"User-Agent": _UA, "Accept": "application/json"})
     try:
         with urlopen(req, timeout=timeout) as resp:
-            payload = json.loads(resp.read())
+            payload = json.loads(_read_capped(resp))
         children = (payload.get("data") or {}).get("children") or []
         return [c.get("data", {}) for c in children if isinstance(c, dict)]
     except (OSError, http.client.HTTPException, json.JSONDecodeError) as exc: