Social networking site Twitter has been hit by phishing and spam attacks.
Twitter members are warned to be on guard against an evolving attack that threatens to steal personal information, said security firm Sophos.
Thousands of Twitter users have reported receiving messages from friends inviting them to click on a link to funny pictures or blog articles about the recipients.
Celebrity Stephen Fry is among those who unwittingly clicked on the link without realising it was a phishing attempt.
"Lawks. Hope I haven't been phished for all my details. Clicked on scam URL last night before I knew what it was," he said in a recent posting known as a Tweet.
Fry's account is not believed to have been compromised, but Sophos said the links take users to a bogus Twitter page that steals users' login names and passwords.
Having hacked into Twitter accounts with information from the phishing attack, cybercriminals are using the compromised Twitter identities to send spam to other users.
These messages claim that recipients could win an Apple iPhone by clicking on a link.
Twitter users who may have lost control of their accounts need to change their passwords before more harm is done, said Graham Cluley, senior technology consultant at Sophos.
"Compromised social networking accounts are valuable for hackers as they can use them as a springboard for spam campaigns, identity theft and other crimes," he said.
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Twitter users are being targeted by a phishing campaign designed to steal passwords and use hijacked accounts to spread money-making spam campaigns.