Skype this week has prompted Mac users to install an update that kills a zero-day vulnerability.
Pure Hacking researcher Gordon Maddern stumbled on the flaw last month.
The bug could have enabled an attacker to take control of a victim's computer simply by sending a malicious message.
"About a month ago I was chatting on Skype to a colleague about a payload for one of our clients," Maddern wrote in a blog post.
"Completely by accident, my payload executed in my colleague's Skype client...The long and the short of it is that an attacker needs only to send a victim a message and they can gain remote control of the victim's Mac. It is extremely wormable and dangerous."
Maddern said he used the open-source Metasploit framework to craft a proof-of-concept, then notified Skype about the vulnerability. But it never was fixed.
The flaw was remediated 14 April when Skype issued a hotfix for version 22.214.171.1242 for Mac, however, Skype did not prompt its users to install the update because there were no reports of active exploits.
Skype chief security officer Adrian Asher downplayed the risk of the bug. He said the worm would have had difficulty spreading because Skype is set up in a such a way that it does not permit users to receive messages from people with whom they have not approved.
Users are now being asked to manually install the update that began circulating Monday, 126.96.36.1995, Asher said. Clients for Windows and Linux are not affected by the vulnerability.