Customers of the crimeware-as-a-service syndicate behind the "Gameover" variant of the venerable Zeus banking Trojan have been customizing the malware to steal data from a number of new banking sites.

Denmark-based CSIS Security Group sounded that warning Wednesday, reporting that the number of unique targets for Zeus web injection capabilities has recently increased by 38% -- from 1,097 at the beginning of 2014, to 1,515 at the end of March.

Using web injection allows Zeus -- like many other types of malware -- to hook into Windows processes and gain raw access to HTTP data traveling over the network. Zeus then modifies that HTTP data, for example to surreptitiously make bank transfers in the background while showing users a normal-looking banking website page.

According to a report issued last year by Poland's Computer Emergency Response Team (CERT Polska), Zeus can compare a visited site's URL with a preset list, and then execute predefined actions, which are either contained in a configuration file or retrieved from an external server by using a peer-to-peer (P2P) proxy to disguise related communications. These actions can include recording data or making screenshots, for example to capture banking website login codes for sites that require customers to employ onscreen keyboards.

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The abuse.ch Zeus Tracker reports that there are currently about 450 Zeus command-and-control servers online, and the average anti-malware detection rate for Zeus binaries is only about 40%. According to CSIS, various versions of Zeus are thought to infect and control a few million PCs around the world. The ongoing use of Zeus has been driven in part by the malware source code getting leaked in 2011.

Unlike many Zeus operators, however, the gang behind Gameover doesn't sell their version of the code directly, but instead allows customers to lease the botnet for their attacks. In the past, the group has also tapped some third-party malicious infrastructure, including using the Cutwail spamming botnet to distribute related malware.

Historically, Zeus-using online criminals have tended to focus their efforts on better-known banks, especially in the United States, Canada, and Western Europe. But according to Peter Kruse, a security specialist at CSIS Security Group, Gameover's user base has recently been launching attacks against customers of financial services firms and other organizations for the first time. "Amongst the 'new countries' being significantly targeted by this malware family we are seeing: South Africa, Nigeria, India, Singapore, Turkey, UAE, Saudi Arabia, Australia, Croatia, Greece, but in general we are seeing brands from all over the world being attacked," he said in a blog post.

Gameover's geographic expansion has been facilitated by its developers giving their crimeware-as-a-service (CaaS) customers the ability to customize and tweak related configuration profiles to their liking. "The malware also includes a basic ZeuS webinject template, but each customer in the ZeuS P2P CaaS can modify and add new advanced webinjects and increase the number of targets," Kruse said.

Enabling Gameover to be used in new markets -- even if they offer less bang for the buck than existing, higher-net-worth banking customer targets -- makes economic sense. For starters, the customizations allow criminals to use Gameover to attack local banks and generate new sources of revenue. Likewise, every new customer that leases the malware helps increase the profits for the Gameover gang, at least some of which they'll plow back into research and development to help attract future cybercrime customers.