# UNDF: UNDF-2026-000000929
--- a/BTCPayServer/Services/Apps/AppService.cs
+++ b/BTCPayServer/Services/Apps/AppService.cs
@@ -113,9 +113,12 @@

             // fill up the gaps
+            // CWE-407: series.All(e => e.Date != date) inside foreach is O(D*S) where
+            // D = numberOfDays and S = series.Count. Build a HashSet for O(1) lookup.
+            var existingDates = new HashSet<DateTime>(series.Select(e => e.Date));
             foreach (var i in Enumerable.Range(0, numberOfDays))
             {
                 var date = (DateTimeOffset.UtcNow - TimeSpan.FromDays(i)).Date;
-                if (series.All(e => e.Date != date))
+                if (!existingDates.Contains(date))
                 {
                     series.Add(new AppSalesStatsItem
                     {
                         Date = date,
                         Label = date.ToString("MMM dd", CultureInfo.InvariantCulture)
                     });
+                    existingDates.Add(date);
                 }
             }