Cybercrooks have embraced the open-source model in the development of banking Trojans following the release of source code for the infamous ZeuS cybercrime toolkit last year.

Multiple variants of a Zeus Trojan called Citadel have emerged over recent weeks as VXers have embraced a new development lifecycle - dubbed open-source malware.

Net security firm Seculert reports that the main developers of the software have also created forums where customers of Citadel (other cybercriminals) can suggest new features and modules for the malware, report bugs and other errors, or enter into discussion with their fellow e-banking fraudsters.

The approach is fuelling development of extremely well-developed malware, Seculert reports.

"Because of the new development lifecycle, this malware evolves quickly, and since late December we have seen over five new versions released by the Citadel authors, with new modules and features added to each version," Aviv Raff, CTO at Seculert, told El Reg.

New modules and features include support for AES Encryption of malware configuration files, support for security vendor websites blacklisting and technology designed to keep off the radar of Zeus tracking websites.

Cybercriminals have embraced open-source malware with the creation of 20 unique botnets using the new Citadel malware, which is starting to become the FreeBSD of e-banking fraudsters. Several of those botnets have infected more than 100,000 machines, Raff reports.

Cybercrooks have embraced trends from the legitimate business world for some years. For example, the Citadel authors provide their customers with a user manual, release notes and a licence agreement.

"We suspect that the open-source model may be the next growing trend," Seculert concludes. "The cybercrime world is characterised by rapid development, cutting-edge technology, and hackers’ constant cravings for recognition. By looking at the developments in the software world, the open-source model may be well accepted in the cybercrime ecosystem as well."

A blog post from Seculert, showing admin panels for the malware and a pie-chart illustrating Citadel infections per country, can be found here. ®