America Online is working on a patch for what security researchers are calling a "major vulnerability" in the company's highly popular Instant Messenger application.

Researchers at Core Security Technologies Wednesday disclosed a bug that they say could severely impact the millions of registered users of AOL's instant-messaging service, AIM. The flaw, according to Core Security, would enable a series of attacks -- enabling a remote hacker to execute malicious code, exploit Internet Explorer bugs, and inject scripting code in the IE browser.

"This vulnerability poses a significant security risk to millions of AIM users," said Ivan Arce, CTO at Core Security, in a written statement. "Core Security has alerted AOL to this threat and has provided full technical details about the vulnerability so that they can address it in their products. Since we notified AOL, this vulnerability has emerged on several public bug-tracking Web sites. Therefore, we believe it is necessary to bring precise details about this issue to light immediately, so that AIM users and organizations using AIM can be made aware of the threat, assess their risk, and take the appropriate measures to ensure that they are protected."

In a response e-mailed to InformationWeek, an AOL spokesman said its technicians are working on the problem.

"The safety and security of AIM users is of utmost importance to us," she wrote. "To that end, we quickly take the necessary steps to block malicious content from reaching our users. We have addressed the issues that Core Security has brought to us on the server side. We are comfortable with the server side fixes we have in place, but we are also working on a client fix."

According to Core Security, the vulnerability affects AIM V6.1, as well as the V6.2 beta, which is the latest version of AOL's instant messaging application. It also affects AIM Pro, the instant messaging version for corporate users, and AIM Lite, a simplified version of the client application.

The security company is suggesting that AIM users running vulnerable client software switch to AIM V5.9, the latest version of the AIM client 6.5, which is still in beta, or the web-based AIM Express.

The researchers did not give any specific details about the bug, but did note that all of the vulnerable AIM clients include support for enhanced message types that enable AIM users to take advantage of HTML to customize text messages with different fonts and colors.

The bug reportedly allows remote arbitrary code execution, direct exploitation of IE bugs without user interaction, injection of scripting code in IE, and cross-site request forgery.