Coming on the heels of recent, similar attacks on Flash files and the Real player, the latest of these campaigns also targets computers running Microsoft's Windows operating system, allowing hackers to steal passwords, system information and reports on Internet surfing preferences, along with online bank account information, login names and more.
Estimates vary on the size of the attack, which could have compromised up to 10,000 compromised computer systems.
"It's safe to say that there are thousands of these out there," Yuval Ben-Itzhak, security firm Finjan's CTO, said.
Researchers haven't uncovered all of the new attacks secrets, which they've been monitoring since December, but say that the poisoned websites rely on similar server and administration software.
"We know some of the methods," Ben-Itzhak said. "They are trying to exploit known vulnerabilities in open source content management software that the sites are using."
Many adult websites are driven by content management systems (CMS), which could face similar vulnerabilities to the compromised platforms.
The malicious code hides itself by generating random character names for each unique visitor and by remembering repeat visitors, which are not attacked a second time.
According to Simon Heron, managing director for the security firm Network Box, the attack finds vulnerabilities in common browsing software, and other applications such as instant messaging and multimedia programs, which it can exploit by installing a Trojan that will remain undetected as it waits for sensitive data such as online banking logins to be used.
"It looks like the root kit type technique that we have been worried about for the last two or three years," Heron said. "It's very clever."
Many anti-virus programs fail to detect the presence of the Trojan.