ruby -- Command injection vulnerability in Net::FTP

Etienne Stalmans from the Heroku product security team reports:

There is a command injection vulnerability in Net::FTP bundled with Ruby.
Net::FTP#get, getbinaryfile, gettextfile, put, putbinaryfile, and puttextfile use Kernel#open to open a local file. If the localfile argument starts with the pipe character “|”, the command following the pipe character is executed. The default value of localfile is File.basename(remotefile), so malicious FTP servers could cause arbitrary command execution.