Researchers from AVAST are warning users to beware of modified and malicious versions of the popular open source FileZilla FTP client that captures login details and sends them to a server hosted in Germany.
The malicious FTP program is fully functional and maintains the look of the official version, and has no evidence of suspicious behavior such as entries in the system registry, communication or changes in application look and feel, AVAST cautioned.
According to the security firm, the interface of the malicious installer is nearly identical to the official version, with just a slight difference in the version of NullSoft installer, which the malware uses 2.46.3-Unicode, while the official installer uses v2.45-Unicode.
“The whole operation is very quick and quiet,” AVAST noted in a blog post. “Log in details are sent to attackers from the ongoing FTP connection only once. Malware doesn’t search bookmarks or send any other files or saved connections.”
Stolen login credentials are encoded using a custom base64 algorithm and then sent to the IP 220.127.116.11, a server hosted at “Server Block” in Germany.
AVAST found 3 domains linked to the IP, all registered through Naunet.ru, a domain registrar associated with malicious activities and masks registrant contact information and ignores requests to suspend domains, acccording to AVAST.
One sign that users may be using the malicious version of FileZilla is if the software update feature is not functioning. “Any attempt to update the application fails, which is most likely a protection to prevent overwriting of malware binaries,” AVAST said.
One version of the malware (version 3.5.3) was compiled in September 2012, indicating that these attackers have been running this attack campaign for some time.
The first suspicious signs are "bogus download URLs", AVAST said, noting that the malcious FileZilla installers are typically hosted on hacked websites with fake content.
“We assume that the stolen FTP accounts are further abused for upload and spread of malware. Attackers also can download whole webpage source code containing database log in, payment system, customer private information etc.,” AVAST said.
As of publishing time, SecurityWeek found that detection rates for four separate malicious FileZilla installers was very low, with between 2 and 4 anti-virus vendors identifying the files as malicious, based on data provided by VirusTotal.
[Update: Shortly after publishing time, the number of anti-virus vendors detecting the malware has increased for one sample to 9 of 50. Expect this to jump even further over the next few days.]
More details on the malware are available on AVAST’s blog.