Businesses using Skype are offering hackers a way into the corporate network, according to security firm Network Box.
The company said Skype is an easy target for hackers and could be undermined by a malevolent insider working to open hidden back doors.
It's more bad news for Skype, following a week in which its users have struggled with crippling service outages.
In the short report, 'Skype - Friend of Foe? ', Network Box suggests how the program could be compromised. Because Skype uses a proprietary protocol to evade detection, it could not only stymie blocking systems, but if hijacked would be a perfect system to compromise the security of any communications made using it. Any built-in hack would be invisible to corporate security systems until it was too late.
"The security of the Skype system depends entirely on the good will of Skype's programmers and the organisation running Skype's back-end servers. It is possible that there are back doors in the system, allowing the Skype organisation - or others - to eavesdrop or record Skype conversations," the report says.
As well as hiding itself, Skype has also established a cycle of continuous upgrades that made effective detection and management tricky.
"The Skype program can update itself every time it runs, so the security over the overall system can change without warning or even a change in appearance. Systems could be brought down by an error in this download."
With last week's outages in mind, it concludes that even companies willing to use the program in a proxy setup, should assume a degree of unreliability and insecurity when compared to running standards-based VoIP over a VPN.
Other recommendations include being careful about the identity of apparently legitimate Skype contacts, using a login for Skype not used to access any other system, and making sure that Skype users inside companies don't identify the companies they are working for in their user names or Skype profiles.
Worries over Skype's security are nothing new, some thing the company has attempted to address with a business version that claims to be easier for IT administrators to manage and control.