Fix directory traversal in the HlsSegmentController in a fairly rudimentary but working way.

GHSL-2021-050: Issue 1,2,3 Arbitrary file read and directory traversal.
The segment id's can probably just be verified to be an actual ID or to not contain any forward or backward slashes

Jellyfin.Api/Controllers/HlsSegmentController.cs

@@ -62,6 +62,13 @@ namespace Jellyfin.Api.Controllers
             // TODO: Deprecate with new iOS app
             var file = segmentId + Path.GetExtension(Request.Path);
             file = Path.Combine(_serverConfigurationManager.GetTranscodePath(), file);
+            var transcodePath = _serverConfigurationManager.GetTranscodePath();
+            file = Path.GetFullPath(Path.Combine(transcodePath, file));
+            var fileDir = Path.GetDirectoryName(file);
+            if (string.IsNullOrEmpty(fileDir) || !fileDir.StartsWith(transcodePath))
+            {
+                return BadRequest("Invalid segment.");
+            }
 
             return FileStreamResponseHelpers.GetStaticFileResult(file, MimeTypes.GetMimeType(file)!, false, HttpContext);
         }
@@ -82,6 +89,13 @@ namespace Jellyfin.Api.Controllers
         {
             var file = playlistId + Path.GetExtension(Request.Path);
             file = Path.Combine(_serverConfigurationManager.GetTranscodePath(), file);
+            var transcodePath = _serverConfigurationManager.GetTranscodePath();
+            file = Path.GetFullPath(Path.Combine(transcodePath, file));
+            var fileDir = Path.GetDirectoryName(file);
+            if (string.IsNullOrEmpty(fileDir) || !fileDir.StartsWith(transcodePath) || Path.GetExtension(file) != ".m3u8")
+            {
+                return BadRequest("Invalid segment.");
+            }
 
             return GetFileResult(file, file);
         }
@@ -131,6 +145,12 @@ namespace Jellyfin.Api.Controllers
             var transcodeFolderPath = _serverConfigurationManager.GetTranscodePath();
 
             file = Path.Combine(transcodeFolderPath, file);
+            file = Path.GetFullPath(Path.Combine(transcodeFolderPath, file));
+            var fileDir = Path.GetDirectoryName(file);
+            if (string.IsNullOrEmpty(fileDir) || !fileDir.StartsWith(transcodeFolderPath))
+            {
+                return BadRequest("Invalid segment.");
+            }
 
             var normalizedPlaylistId = playlistId;