An exploit for an unpatched vulnerability in the Microsoft XML Core Services (MSXML) has been incorporated into Blackhole, one of the most widely used Web attack toolkits, according to security researchers from antivirus firm Sophos.
The security flaw is identified as CVE-2012-1889 and is what security researchers call a zero-day vulnerability -- an actively exploited vulnerability for which an official patch doesn't yet exist.
CVE-2012-1889 allows remote attackers to execute malicious code on a user's computer if they convince the user to visit a specially crafted Web page in Internet Explorer.
On June 12, Microsoft published a security advisory regarding this security flaw and provided users with several work-arounds for it, including a semi-automated "Fix it" solution.
Since then, exploit code for the vulnerability was added to the open-source Metasploit penetration testing framework, making it available for anyone to copy and potentially misuse.
"Within a week, CVE-2012-1889 exploiting code very similar to that published to Metasploit was seen within the landing page of a Blackhole exploit kit site," said Fraser Howard, a principal virus researcher at Sophos, in a blog post on Friday.
Blackhole is one of the most popular exploit toolkits -- malicious Web applications designed to exploit vulnerabilities in browser plug-ins and other components in order to distribute malware.
Exploit kits are used in the majority of Web-based attacks launched from compromised websites and are one of the primary methods of infected computers with malware.