Copyparty ftp/sftp: Sharing a single file did not fully restrict source-folder access

Copyparty is a portable file server. Prior to 1.20.12, there was a missing permission-check in the shares feature (the shr global-option). This vulnerability only applies when the shares feature is used for the specific purpose of creating a share of just a single file inside a folder or either the FTP or SFTP server is enabled, and also made publicly accessible. Given these conditions, when a user is browsing a share through either FTP or SFTP (not http or https), they can gain read-access to the remaining files inside the shared folder by guessing/bruteforcing the filenames. It was not possible to descend into subdirectories in this manner; only the sibling files were accessible. This vulnerability is similar to CVE-2025-58753 which was previously fixed for HTTP and HTTPS, but not for FTP. The FTPS server did not yet exist at that time. This vulnerability is fixed in 1.20.12.