diff --git a/cmd/server/routes.go b/cmd/server/routes.go
index 3346e430..24301052 100644
--- a/cmd/server/routes.go
+++ b/cmd/server/routes.go
@@ -501,6 +501,13 @@ func (s *Server) handlePutConfigGeoFilter(w http.ResponseWriter, r *http.Request
 		}
 	}
 
+	// bufferKm must be finite, non-negative, and ≤ 20000 km (half Earth circumference).
+	if math.IsNaN(body.BufferKm) || math.IsInf(body.BufferKm, 0) ||
+		body.BufferKm < 0 || body.BufferKm > 20000 {
+		writeError(w, http.StatusBadRequest, "bufferKm must be a finite number in [0, 20000]")
+		return
+	}
+
 	var gf *GeoFilterConfig
 	if len(body.Polygon) >= 3 {
 		gf = &GeoFilterConfig{Polygon: body.Polygon, BufferKm: body.BufferKm}
diff --git a/cmd/server/routes_test.go b/cmd/server/routes_test.go
index f87e3673..a45d74e1 100644
--- a/cmd/server/routes_test.go
+++ b/cmd/server/routes_test.go
@@ -4149,6 +4149,32 @@ func TestPutConfigGeoFilter(t *testing.T) {
 			t.Fatalf("expected 401, got %d", w.Code)
 		}
 	})
+
+	t.Run("rejects negative bufferKm", func(t *testing.T) {
+		_, router, _ := setupGeoFilterServer(t, apiKey)
+		body := `{"polygon":[[51.0,4.0],[51.0,5.0],[50.5,4.0]],"bufferKm":-1}`
+		req := httptest.NewRequest("PUT", "/api/config/geo-filter", strings.NewReader(body))
+		req.Header.Set("X-API-Key", apiKey)
+		req.Header.Set("Content-Type", "application/json")
+		w := httptest.NewRecorder()
+		router.ServeHTTP(w, req)
+		if w.Code != http.StatusBadRequest {
+			t.Fatalf("expected 400 for negative bufferKm, got %d: %s", w.Code, w.Body.String())
+		}
+	})
+
+	t.Run("rejects excessive bufferKm", func(t *testing.T) {
+		_, router, _ := setupGeoFilterServer(t, apiKey)
+		body := `{"polygon":[[51.0,4.0],[51.0,5.0],[50.5,4.0]],"bufferKm":99999999}`
+		req := httptest.NewRequest("PUT", "/api/config/geo-filter", strings.NewReader(body))
+		req.Header.Set("X-API-Key", apiKey)
+		req.Header.Set("Content-Type", "application/json")
+		w := httptest.NewRecorder()
+		router.ServeHTTP(w, req)
+		if w.Code != http.StatusBadRequest {
+			t.Fatalf("expected 400 for excessive bufferKm, got %d: %s", w.Code, w.Body.String())
+		}
+	})
 }
 
 func TestSaveGeoFilter(t *testing.T) {