diff --git a/devdocs/modernization/NET10-PORT.md b/devdocs/modernization/NET10-PORT.md
index b13d46c3..6030a825 100644
--- a/devdocs/modernization/NET10-PORT.md
+++ b/devdocs/modernization/NET10-PORT.md
@@ -181,7 +181,7 @@ John's "next release" cases for AyaNova (exported 2026-10-24). Each is its own s
 | 4711 | JWT secret setting removed: the server generates its signing key once into `jwt-signing.key` in the data folder (4698 folded in) | F1. **Done** (placeholder-forged token 401; key survives restart; auth tests pass). Hosted setup script's secret step now does nothing; tidy later in `sockeye-client` |
 | 4712 | Customer contact route allowlist + ownership checks | F13, F14, F15, F16, F17 |
 | 4713 | Contact users limited to customer roles | F28 |
-| 4715 | Download file names checked (one helper) | F21 |
+| 4715 | Download file names checked (one helper) | F21. **Done** (`FileUtil.FullPathInFolderOrNull` in temp/backup exists checks and both log routes; test `DownloadNames`) |
 | 4716 | Pick-list search text as SQL parameters | F18 |
 | 4717 | KPI interval allowlist | F26 |
 | 4718 | Report and translation import need create rights | F19, F24. **Done** (contact and Service 403, BizAdmin 202; test `ImportRights`) |
diff --git a/devdocs/modernization/SECURITY-REVIEW.md b/devdocs/modernization/SECURITY-REVIEW.md
index b71d51c9..563ff699 100644
--- a/devdocs/modernization/SECURITY-REVIEW.md
+++ b/devdocs/modernization/SECURITY-REVIEW.md
@@ -194,7 +194,7 @@ take the next free number; a fixed or accepted finding keeps its ID and changes
 | F5 | **Critical** | **fixed in case 4710** | 2FA login step accepts an empty or missing temp token: the password is skipped for users with 2FA on |
 | F1 | Critical (installs that kept it) | **fixed in case 4711** | Placeholder JWT secret in the LAN/IIS and Linux server config templates |
 | F19 | Critical | **fixed in case 4718** | Any logged-in user, customer contacts included, can import report templates, whose JavaScript runs in the server's browser |
-| F21 | Critical | candidate | Download file names from the URL are not checked: `report/download` and `export/download` may serve files outside the temp folder on Windows |
+| F21 | Critical | **fixed in case 4715** | Download file names from the URL are not checked: `report/download` and `export/download` may serve files outside the temp folder on Windows |
 | F18 | Critical | confirmed (code) | Pick-list search text is put into SQL without escaping; any logged-in user can reach it |
 | F26 | Critical | confirmed (code) | KPI `interval` criterion is put into SQL unchecked; reachable by staff users down to TechRestricted |
 | F13 | High | **confirmed** | Customer contacts can open any customer's work order (customer view, customer report, attachments) by id |
diff --git a/server/AyaNova/Controllers/LogFilesController.cs b/server/AyaNova/Controllers/LogFilesController.cs
index fbe9fae6..9236f548 100644
--- a/server/AyaNova/Controllers/LogFilesController.cs
+++ b/server/AyaNova/Controllers/LogFilesController.cs
@@ -69,9 +69,10 @@ namespace AyaNova.Api.Controllers
             //stream the file contents into a json object and return
 
             //build the full path from the log file name and defined path
-            var logFilePath = System.IO.Path.Combine(ServerBootConfig.AYANOVA_LOG_PATH, logname);
+            //case 4715: only a bare file name inside the log folder
+            var logFilePath = FileUtil.FullPathInFolderOrNull(ServerBootConfig.AYANOVA_LOG_PATH, logname);
             //does file exist?
-            if (!System.IO.File.Exists(logFilePath))
+            if (logFilePath == null || !System.IO.File.Exists(logFilePath))
             {
                 return NotFound(new ApiErrorResponse(ApiErrorCode.NOT_FOUND));
             }
@@ -149,8 +150,9 @@ namespace AyaNova.Api.Controllers
             if (!Authorized.HasReadFullRole(user.Roles, AyaType.LogFile))
                 return StatusCode(403, new ApiNotAuthorizedResponse());
 
-            var logFilePath = System.IO.Path.Combine(ServerBootConfig.AYANOVA_LOG_PATH, logname);
-            if (!System.IO.File.Exists(logFilePath))
+            //case 4715: only a bare file name inside the log folder
+            var logFilePath = FileUtil.FullPathInFolderOrNull(ServerBootConfig.AYANOVA_LOG_PATH, logname);
+            if (logFilePath == null || !System.IO.File.Exists(logFilePath))
                 return NotFound(new ApiErrorResponse(ApiErrorCode.NOT_FOUND));
                 
             //Note: this works with new Nlog file locking for write so it must be opening it as read and shareable 
diff --git a/server/AyaNova/util/FileUtil.cs b/server/AyaNova/util/FileUtil.cs
index c276f4be..f88f3c03 100644
--- a/server/AyaNova/util/FileUtil.cs
+++ b/server/AyaNova/util/FileUtil.cs
@@ -116,12 +116,27 @@ namespace AyaNova.Util
         /// <returns>duh!</returns>
         internal static bool TemporaryFileExists(string fileName)
         {
-            if (string.IsNullOrWhiteSpace(fileName))
-                return false;
-
-            var FilePath = GetFullPathForTemporaryFile(fileName);
-            return File.Exists(FilePath);
+            //case 4715: only a bare file name inside the temporary files folder
+            var FilePath = FullPathInFolderOrNull(TemporaryFilesFolder, fileName);
+            return FilePath != null && File.Exists(FilePath);
+        }
 
+        /// <summary>
+        /// case 4715: full path of a file name that came from a request, or null unless it is a bare file name
+        /// (no folder part, no "..") that resolves to a file directly inside the folder
+        /// </summary>
+        internal static string FullPathInFolderOrNull(string folder, string fileName)
+        {
+            if (string.IsNullOrWhiteSpace(folder) || string.IsNullOrWhiteSpace(fileName))
+                return null;
+            if (fileName != Path.GetFileName(fileName) || fileName.Contains("..") || fileName.IndexOfAny(Path.GetInvalidFileNameChars()) >= 0)
+                return null;
+            var folderFullPath = Path.GetFullPath(folder);
+            var fileFullPath = Path.GetFullPath(Path.Combine(folderFullPath, fileName));
+            var comparison = OperatingSystem.IsWindows() ? StringComparison.OrdinalIgnoreCase : StringComparison.Ordinal;
+            if (!string.Equals(Path.GetDirectoryName(fileFullPath), folderFullPath.TrimEnd(Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar), comparison))
+                return null;
+            return fileFullPath;
         }
 
         /// <summary>
@@ -275,12 +290,9 @@ namespace AyaNova.Util
         /// <returns>duh!</returns>
         internal static bool BackupFileExists(string fileName)
         {
-            if (string.IsNullOrWhiteSpace(fileName))
-                return false;
-
-            var utilityFilePath = GetFullPathForBackupFile(fileName);
-            return File.Exists(utilityFilePath);
-
+            //case 4715: only a bare file name inside the backup files folder
+            var utilityFilePath = FullPathInFolderOrNull(ServerBootConfig.AYANOVA_BACKUP_FILES_PATH, fileName);
+            return utilityFilePath != null && File.Exists(utilityFilePath);
         }
 
         // /// <summary>