Microsoft will force users of its aged MSN Messenger instant messaging software to upgrade to Windows Live Messenger 8.1 in response to a vulnerability in the older program that was made public Tuesday.

According to a blog post by a Microsoft security program manager who identified himself only as "Anand," the instant messaging service will require users to update to the safe Live Messenger 8.1.

"We will soon configure the service such that any user on Windows XP or later has to use Windows Live Messenger 8.1," Anand said. "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."

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 will read: "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.

"Why isn't MS07-054 showing up in WSUS?" asked a user tagged as Henry Johnston. "The security bulletin says the update is being distributed via MSN Messenger itself, but that's no use -- it still leaves us having to log into every computer individually, one by one, in order to install it."

Others who responded to Johnston said that the MSN Messenger and Live Messenger updates weren't deployable via WSUS. "Since the product [MSN/Windows Live Messenger] is considered an out-of-band product, it doesn't really fit in with the normal enterprise deployment methods that we have," wrote an unidentified Microsoft support representative.

Consumers weren't happy with the situation, either. The mandatory upgrade to Live Messenger got a thumbs down from many users posting to Microsoft's IM support newsgroups. "Well, I tried it and HATE it and can't revert it," complained a user with the nickname bodeelifts on the microsoft.public.msn.messenger forum. "I am absolutely livid. It is hard on the eyes, not easy to use, is filled with too many bells and whistles that I have no need for, and I am sick and tired of being forced into things I don't want."

Some users of the now-obsolete MSN Messenger said they were ditching Microsoft's IM client, and had switched to alternatives, such as Cerulean Studio's free Trillian or the open-source Pidgen, formerly known as Gaim.

However, one edition of MSN Messenger will continue to work, Microsoft said. "Because Windows 2000 isn't supported by Windows Live Messenger 8.1, we will provide an updated version of MSN Messenger 7.0," said Anand on the Messenger blog. "We will upgrade Windows 2000 users to the updated version of MSN Messenger 7.0 after the Windows Live Messenger upgrades."

The revamped MSN Messenger will carry version number 7.0.0820.