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Upload Issue After OS Upgrade (Debian 11 → 13) and Serv-U Upgrade (15.5.2 → 15.5.4)

File upload failures were observed after upgrading the server OS from Debian 11 to Debian 13 and Serv-U from version 15.5.2 to 15.5.4. The issue occurs when attempting to upload files to restricted system directories.

First published date

3/30/2026 6:56 AM

Last published date

3/30/2026 3:30 PM

Overview

Following an upgrade, file uploads with attachments are no longer successful. Investigation confirms that the behavior is reproducible and is not caused by the OS upgrade itself, but by a security enhancement introduced in Serv-U version 15.5.3.

Product section

Serv-U Managed File Transfer & Serv-U FTP Server

Cause

This issue is due to a security improvement introduced starting in Serv-U 15.5.3. On Linux systems, Serv-U now restricts write (upload) access to critical system directories to prevent unintended or unsafe modifications.

The following directories are protected and cannot be modified:

  • /
  • /bin
  • /sbin
  • /usr/bin
  • /usr/sbin
  • /usr/local/bin
  • /usr/local/sbin
  • /etc
  • /dev
  • /boot
  • /lib
  • /lib64
  • /opt

Additionally:

  • All subdirectories within these paths (except /) are also restricted from uploads, even if directory access rules allow write permissions.
  • Downloads from these directories are still permitted if allowed by directory access rules.
  • All other directories behave as in previous Serv-U versions.

 

Resolution

Upload files to directories outside the restricted list.