copyparty FTP: upload outside defined volumes (limited path traversal)

The FTP and FTPS server -- if enabled -- would allow uploading to any folder that the copyparty process had permission to write to.

The FTP-server is default-disabled, and must be explicitly enabled in the config with global-option ftp or ftps to be affected. Additionally, SFTP was not affected at all; only FTP and FTPS.

This was not a full read/write path-traversal; it had the following limitations:

• Only applied to uploads; it was not possible to read/download existing files in this manner.
• The output filename was not fully controllable; a randomized suffix would always be appended to the filename.
• Existing files could not be overwritten or appended to; only new files could be created.
• Filesystem-permissions would prevent writing to system folders, as copyparty surely does not run as root in most deployments.
• Copyparty itself will not execute arbitrary files as code by default, so this vulnerability could not be used to execute arbitrary code through copyparty.

The main concern is the potential effect on other unrelated software which is running on the same system.

The severity is assuming the worst case (SC:H/SI:H), that the same machine is running some unrelated software that will execute new arbitrarily-named files as code, and that the copyparty process has sufficient privileges to write files into the config-folder of that unrelated software.