Two instant messaging worms that affect MSN Messenger and Windows Messenger are in the wild.
They have been detected by various anti-virus software companies and are classed as "medium risk" by the industry-backed instant messaging body IMLogic.
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W32/Kelvir and W32/Sumom-A have both been spreading since the beginning of the week, said IMLogic, which is backed by the likes of Microsoft, AOL, Symantec, Sophos and Yahoo.
Users infect themselves and others by opening a .pif file via a link in a malicious message that contains the worm.
The increasing number of hackers targeting instant messaging programmes from the likes of Microsoft, AOL and Yahoo, is adding to pressure on organisations to block instant messaging programs on their networks altogether.
Microsoft issued a patch last month against a different vulnerability in its MSN Messenger and Windows Messenger systems.
Just one third of British companies give staff access to instant messaging, according to a recently published study for Telewest Business by the Manchester Business School.
Security concerns were cited as a major factor for the low take-up.
More details of the latest worms are available here: http://www.imlogic.com/