The warning comes after the disclosure this week of two critical flaws in Skype's software, one of which could allow malicious hackers to take complete control of compromised systems.
One of the flaws is a buffer overflow error in Skype's user client for Windows that could allow attackers to execute arbitrary code on compromised systems, according to a statement from the company. The other vulnerability is a heap overflow flaw in a networking routine affecting Skype clients for all platforms. That flaw could crash the client software. Fixes for both problems have been released.
Skype, which was recently acquired by eBay for US$2.6 billion (£1.45 billion) offers downloadable software allowing PC users to make free internet telephone calls to each other and low-cost calls to telephone users.
So far, Skype has garnered more than 61 million registered users, approximately 30 percent of whom use it for business purposes, according to the company. Almost all of that adoption has been in Europe and Asia, though analysts expect Skype to eventually become widely accepted in the US as well.
According to Stamford Conn.-based analyst firm Gartner, eBay's purchase of Skype could result in a product more suited for corporate use.
In the meantime, business users should refrain from using "voice services based on proprietary protocols like Skype while on corporate networks because of network security issues," Gartner said in a Sept. 15 advisory.
There are several reasons for the concern, industry experts said. "Skype is VoIP on steroids", capable of punching holes through many of the network defenses that companies typically deploy, said Tom Newton, product manager at SmoothWall, a Leeds-based vendor of firewalls and other security products.
As with other other P2P technologies, Skype allows users to establish direct connections with each other. It's also ‘port agile’, meaning that if a firewall port is blocked Skype will look around for other open ports that it can use to establish a connection. "If you put Skype behind a firewall or Network Address Translation layer, 99 times out of 100 it will work" without any special configuration, he said.
As a result, Skype could provide a backdoor entry into otherwise secure networks for Trojans, worms and viruses, Newton said. It could also provide a channel for corporate data to be freely shared between users without any of the usual security considerations.
Also, as with other P2P applications such as KaaZaa, the connection sharing permitted by Skype makes the the host computer and the network available to others as well, said Robin Bloor, an analyst at Hurwitz & Associates in Waltham. Mass. As a result, "Skype can use a lot of network bandwidth, which may interfere with business applications and services," said Andrea Wuchner-Bruhl, head of global IT security at Novartis Pharma AG, in Basel, Switzerland.
The fact that Skype uses a proprietary protocol instead of a standard one such as the Session Initiation Protocol (SIP) also makes it an "unknown from the point of view of the vulnerabilities that might be there," said John Pescatore, a Gartner analyst.
"Every nonstandard application can add unnecessary risks to your environment", Wuchner-Bruhl said. "In the end no one really knows what all is built-in to such an application."
So far at least, there have been no major attacks directed against Skype. But its growing popularity and installed base will inevitably make it a hacker target, analysts said. Companies will need to keep a close eye on both the sanctioned and the unsanctioned use of Skype on their networks, Pescatore said.
IT administrators may also need to impose measures such as denying local administration rights on the desktop, content control and management at the network gateways. They may also need to lay out clear policies and procedures for users, Wuchner-Bruhl said.
In the end, the use of Skype needs to be resolved in the same way companies have gone about addressing other P2P applications, including instant messaging, Bloor said. "But you are probably going to have something bad happen to someone first”.
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