diff --git a/src/broker/overseas.py b/src/broker/overseas.py
index 13f3903..69504a6 100644
--- a/src/broker/overseas.py
+++ b/src/broker/overseas.py
@@ -121,6 +121,7 @@ class OverseasBroker:
             tr_id = self._broker._settings.OVERSEAS_RANKING_VOLUME_TR_ID
             path = self._broker._settings.OVERSEAS_RANKING_VOLUME_PATH
             params: dict[str, str] = {
+                "KEYB": "",  # NEXT KEY BUFF — Required, 공백
                 "AUTH": "",
                 "EXCD": ranking_excd,
                 "MIXN": "0",
@@ -130,10 +131,11 @@ class OverseasBroker:
             tr_id = self._broker._settings.OVERSEAS_RANKING_FLUCT_TR_ID
             path = self._broker._settings.OVERSEAS_RANKING_FLUCT_PATH
             params = {
+                "KEYB": "",  # NEXT KEY BUFF — Required, 공백
                 "AUTH": "",
                 "EXCD": ranking_excd,
                 "NDAY": "0",
-                "GUBN": "0",  # 0=전체(상승+하락), 1=상승만 — 변동성 스캐너는 전체 필요
+                "GUBN": "1",  # 0=하락율, 1=상승율 — 변동성 스캐너는 급등 종목 우선
                 "VOL_RANG": "0",
             }
 
diff --git a/tests/test_overseas_broker.py b/tests/test_overseas_broker.py
index 3f346f6..0f47bec 100644
--- a/tests/test_overseas_broker.py
+++ b/tests/test_overseas_broker.py
@@ -122,9 +122,10 @@ class TestFetchOverseasRankings:
         params = call_args[1]["params"]
 
         assert "/uapi/overseas-stock/v1/ranking/updown-rate" in url
+        assert params["KEYB"] == ""  # Required by KIS API spec
         assert params["EXCD"] == "NAS"
         assert params["NDAY"] == "0"
-        assert params["GUBN"] == "0"  # 0=전체(상승+하락), 변동성 스캐너에 필요
+        assert params["GUBN"] == "1"  # 1=상승율 — 변동성 스캐너는 급등 종목 우선
         assert params["VOL_RANG"] == "0"
 
         overseas_broker._broker._auth_headers.assert_called_with("HHDFS76290000")
@@ -157,6 +158,7 @@ class TestFetchOverseasRankings:
         params = call_args[1]["params"]
 
         assert "/uapi/overseas-stock/v1/ranking/volume-surge" in url
+        assert params["KEYB"] == ""  # Required by KIS API spec
         assert params["EXCD"] == "NYS"
         assert params["MIXN"] == "0"
         assert params["VOL_RANG"] == "0"