MSN Messenger users are being forced to upgrade to Windows Live Messenger in response to a vulnerability in the older program.
According to a blog post by a Microsoft security program manager who identified himself only as 'Anand', the instant messaging service requires users to update to the safe Live Messenger 8.1.
"When a user using an older version of Messenger tries to log in, the client will help the user with a mandatory upgrade to Messenger 8.1," Anand said.
The update, he added, will be rolled out over several days, so users running MSN Messenger 6.2, 7.0 and 7.5, as well as Live Messenger 8.0, may not see the notification immediately. The dialogue reads: "A newer version is available. You must install the newer version in order to continue. Would you like to do this now?"
Windows Live Messenger 8.1 has been offered to users since February, but until now, the update has been optional. "Some of you might feel this inconvenient, but in order to protect you and protect the health of the network, we have chosen to take this step," Anand added.
The move isn't unprecedented. In early 2005, Microsoft made patched versions of MSN Messenger mandatory when security researchers posted attack code that targeted flaws that had been disclosed only hours before.
The vulnerability that prompted the compulsory upgrade was described by Microsoft in Tuesday's MS07-054 security bulletin, which only recommended that users upgrade. A bug in Messenger's webcam and video chat features was reported late last month on a Chinese-language security mailing list, and exploit code quickly followed. Users who accepted malicious webcam or video chat invitations risked losing control of their PC to hijacking attackers.
The enterprise-grade version of Microsoft's instant messaging client - Office Communicator - does not contain the buggy component, and is not vulnerable. But businesses whose users run MSN Messenger or Live Messenger 8.0 that rely on Windows Server Update Services to patch PCs, have a hoop or two to jump through, according to messages on the WSUS support newsgroup.
The revamped MSN Messenger will carry version number 7.0.0820.