Security researchers are warning of a security hole in Yahoo 's Messenger that could allow attackers to run their own code on computers using the instant messaging program.

The buffer overrun vulnerability was discovered by researcher Tri Huynh in a file named "yauto.dll," which is an ActiveX component of Yahoo! Messenger software versions up to 5.6.0.1347, according to a security alert released Wednesday by Secunia of Copenhagen, Denmark.

Yahoo did not immediately respond to requests for comment.

The company was notified via e-mail about the hole one month ago, but did not respond, Secunia said.

Yahoo! Messenger allows users to instantaneously communicate with each other over the Internet using text messages. It also lets users send files or links to Web pages. Instant messaging applications such as Yahoo! Messenger, Microsoft Corp.'s MSN Messenger and America Online's AOL Instant Messenger are increasingly used at companies to let workers communicate with each other over corporate LANs. ActiveX is a Microsoft technology that allows software developers to create small, reusable bits of code, called "controls" that enable programs to share information over computer networks and the Internet.

Attackers could trigger a buffer overrun on machines running Yahoo! Messenger by sending a long stream of data in the form of a Web page URL (uniform resource locator) to a vulnerable function within yauto.dll, crashing the application or allowing the attacker to place his or her own malicious code on the machine, according to Secunia.

To launch an attack, hackers could set up a Web page, then lure Yahoo! Messenger users into visiting the site and clicking on a link that triggers the buffer overrun and runs the attack code, Secunia said.

In buffer overflow attacks, hackers use flaws in a software program's underlying code to overwrite areas of the computer's memory, replacing legitimate computer instructions with bad data or other instructions.

Secunia rated the Messenger vulnerability "highly critical," saying that researchers tested the hole and successfully exploited it by downloading and running a Trojan horse program.

Yahoo! Messenger users running vulnerable versions of the program were advised to remove yauto.dll from their computer hard drive. Users should also consider modifying their Web browser configuration to prevent ActiveX controls and Active Scripting from running, except on approved Web sites, Secunia said.