Yahoo! Messenger users have been alerted to a number of newly discovered holes in the instant messaging system, leaving their PCs open to malicious code and denial of service attacks.
CERT, a computer security response organisation, issued the warning on Wednesday, referring to two flaws discovered in late May. The flaws, a buffer overrun and a URL validation vulnerability, affect Yahoo! Messenger versions 5,0,0,1064 and earlier. Users are advised to upgrade to version 5,0,0,1065, released on Yahoo!'s site on 22 May, which patches the holes.
The first bug is a buffer overflow affecting Messenger's handler for Uniform Resource Indicators (URIs), software installed at the system level that is used by applications like Web browsers in processing Internet addresses. A URI sent in a Messenger message, embedded in a Web site or sent in an HTML email message can trigger the overflow, allowing hackers to execute code with the security privileges of the system's user, or shut down the system.
Yahoo! warned of this bug late last month.
The second bug affects Messenger's "addview" function, allowing an attacker to send malicious script or HTML in a message, which is then rendered in a Web browser.
CERT noted that a problem with Yahoo! servers after 22 May resulted in some users downloading the vulnerable Messenger version 5,0,0,1036 instead of the new version. The problem has since been fixed. Users can check which version they have by selecting the "About Yahoo! Messenger..." option from the Help menu.
Robert Mead, coordination centre manager for AusCERT, the organisation's Australian arm, said there is a danger that "people are pretty much executing (malicious) code on users' machines... Instant messaging is very widely used, at least in non-business environments."
However, businesses may also be at risk because many workers run the software on their office PCs. CERT noted that it had not yet detected hackers actively scanning for the vulnerabilities.
According Jupiter Media Metrix, 16 percent of workers with access to the Internet will be using Instant Messaging (IM) by the end of the year, with that figure expected to reach 46 percent by the year 2005.
ZDNet Australia's Rachel Lebihan contributed to this report.