Fix path transversal exposure in Plugins

The request path is not validated to a valid path and could allow escaping the transcode path and downloading of any arbitrary file in GetHlsPlaylistLegacy .

GetHlsAudioSegmentLegacy and GetHlsVideoSegmentLegacy have the same issue, and are NOT behind an Authorize so they are publicly exploitable.

Added a ValidateTranscodePath that verifies that requested file paths start with the transcode path setting. Also ensure that all filename comparisons are OrdinalIgnoreCase because we might be running on a filesystem where filename-casing doesn't have to match. Switched from InvariantCulture because the underlying OS filename comparisons are always byte-wise (with case insensitivity here).

Fixed a similar issue in GetPluginImage

Jellyfin.Api/Controllers/PluginsController.cs

@@ -226,10 +226,11 @@ public class PluginsController : BaseJellyfinApiController
             return NotFound();
         }
 
-        if (!string.IsNullOrEmpty(plugin.Manifest.ImagePath))
+        string? imagePath = plugin.Manifest.ImagePath;
+        if (!string.IsNullOrWhiteSpace(imagePath))
         {
-            var imagePath = Path.Combine(plugin.Path, plugin.Manifest.ImagePath);
-            if (!System.IO.File.Exists(imagePath))
+            imagePath = Path.GetFullPath(imagePath, plugin.Path);
+            if (imagePath.StartsWith(plugin.Path, StringComparison.OrdinalIgnoreCase) is false || System.IO.File.Exists(imagePath) is false)
             {
                 return NotFound();
             }