FTP sites function as online file caches and are accessible remotely - usually via Web browsers.
Users who have the required login credentials can upload and download files from them, but other users can also retrieve certain files hosted on such a server if given a specific link that leads to the file (and without needing to provide login credentials).
It is this latter capacity that makes login credentials to FTP servers a prized haul for cyber scammers, as they upload malware and malicious links to the server, then embed direct links to them in spam emails delivered to potential victims.
Access to a FTP server can also be occasionally leveraged by the attackers to compromise connected web services.
"The victim companies hosting exploited FTP sites are spread across the spectrum – from small companies and individual accounts with ISPs to major multi-national corporations," noted the researchers.
"Hackers planted PHP scripts armed with backdoors (shells) and viruses in multiple directories hoping that these directories map to web servers of the victim companies to gain control of the web services. They also uploaded HTML files with seamless re-directs to malicious sites."
Alex Holden, the company's chief information security officer, has shared with Jeremy Kirk that among the compromised file transfer servers are also some that belong to The New York Times and UNICEF. Affected organizations have been notified of the problem and some have already moved to fix it.
It is unknown who stole the FTP credentials, and who is using them, but judging by the complexity of some of the passwords, it's natural to assume that they haven't been guessed, but stolen via information-stealing malware. Also, some sites have default or publicized login credentials, so exploitation of them is easy.