Skype is being used by hackers to spread a malicious 'worm' that infects Windows PCs and can lock users out of their machines.
Skype users are warned not to click on any messages or links that they do not recognise.
The security threat comes in the form of a seemingly innocent instant message on Skype saying "lol is this your new profile pic?" When users click on the message they unwittingly download a file containing a Trojan horse malware file.
This Trojan horse opens a backdoor, allowing a remote hacker to take control of the infected PCs.
"Before you know it, your computer has been recruited into a botnet [security threats that spread via Facebook, Twitter and instant messengers] and could fall victim to a ransomware attack," explains a spokesperson for internet security specialist, Sophos.
Skype has not disclosed how many users are affected by the threat. It said in a statement: "Skype takes the user experience very seriously, particularly when it comes to security. We are aware of this malicious activity and are working quickly to mitigate its impact.
"Additionally, following links - even when from your contacts - that look strange or are unexpected is not advisable."
Sophos warns that Skype users may be less in the habit of being suspicious about links sent to them than, say, Facebook users.
"Always remember to be suspicious of unsolicited out-of-character messages sent to you by your online friends," said a spokesperson.
"You don't know that it was a friend who sent you the message, all you know is that it was their account which posted it to you... and who knows if it was compromised or not?"