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.
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.
The integration of a CVE-2012-1889 exploit into Blackhole could suggest that the number of attacks targeting this flaw will increase significantly. "As soon as we see exploit kits targeting new vulnerabilities we can expect to see a lot more users getting infected - especially if the vulnerabilities are zero-days," Howard said.
However, this hasn't happened yet and the use of such an exploit together with Blackhole was limited to one incident detected on 21 June. Howard speculated that this might be because the exploit's integration with Blackhole is still unreliable, or because this particular exploit is only available for new, more expensive, versions of the toolkit.
Regardless of the reason, the exploit's lack of adoption among Blackhole attackers might only be temporary. Therefore, users should deploy Microsoft's "Fix it" solution as soon as possible and keep their antivirus programs up-to-date in order to block attacks targeting this vulnerability.