And says in-browser security specialist Trusteer, this will almost certainly result in the number and variety of trojans increasing in the months ahead.
Amit Klein, Trusteer's chief technology officer, claims that, since his research team confirmed in early February that the Zeus source code was being hawked around for $100,000, it seems that market forces have taken over, with the code's exclusivity and price taking a tumble.
"We've observed before that that the old adage of there being no honour amongst thieves applies equally to the cybercriminal world, and now it seems that this even truer when it comes to electronic crime", he said.
"We said at the start of February that our research teams were seeing multiple variants of Zeus appearing on users' machines, and now our colleagues over at Trend Micro are reporting that the source code being offered for sale on multiple forums from different people", he added.
Citing a Trend Micro blog, Klein went on to say that elements of the source code have been available for a couple of weeks, but adding it now appears that the situation has become more serious after the code is being file-shared amongst potentially hundreds of users.
The only piece of good news, he notes, is that the RAR files are reported to be password protected, but there are also reports, he adds, that some groups of hackers are attempting to brute force hack the file password.
This means it is only a matter of time before the source code for Zeus is released in the wild at little or no cost, a step that potentially means that thousands of cybercriminals can then develop toolkits to maximise their revenues from the malware, he explained.
"As we said in early February, the extensible nature of Zeus, and its flexible ability to be recoded, means that the malware is likely to continue to be problem for financial users of the internet, and their organisations, for some time to come", he said.
"What we didn't foresee was how rapidly this prediction would turn out to come true, and on a vastly larger scale than anyone could have foreseen. We may yet see even more variants of Zeus appearing on a larger scale - and shorter timeframe - than anyone could have predicted", he added.