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The flaw exploits a feature in MSN Instant Messenger, which notifies users when they have received new e-mail in their Hotmail accounts.
Though Microsoft is treating the flaw as low risk, it will release a new version of its Messenger products early next week, a spokeswoman said. Users will be notified that a new version is available and will be prompted to download it.
In the meantime, Microsoft is advising concerned users to go to the MSN Messenger support Web site ( messenger.msn.com/support/status.asp ) for information about the issue and steps they can take to protect themselves.