You've heard of data leakage, but what about data seepage?

That's when your desktop applications are set to connect to your internal mail server or shared folders -- for instance, when you boot up your machine -- and your corporate network is then exposed to a targeted attack.

"Data seepage is one of the things that most good sys admins or network admins know about, but it doesn't have a name or they don't realize it is a risk," says David Maynor, CTO of Errata Security. "It's a problem at the application level."

It's all about just how chatty your client machines are. And with the rise in targeted attacks and mobile enterprise users increasingly working off public WiFi connections while on the road or in the coffee shop, data seepage gets riskier.

An attacker using a sniffer on a wired or wireless LAN can "see" the victim's machine gathering DNS server names or other information on network resources and use that information to better hone in on her target.

The threat of data seepage may be insidious, but it's not new. "Sniffing network traffic and either stealing passwords or hijacking sessions are attacks that are as old as networks," says Marc Maiffret, CTO and chief hacking officer of eEye Digital Security. "The main thing is not how you protect against someone trying to sniff your network traffic, but what have you done to safeguard each individual asset to make sure that it cannot be compromised so that someone [can] sniff your traffic in the first place."

If the machine is too chatty when it connects to the resources, then it "seeps" data. "Outlook, for instance, will look for its Exchange server, Windows will try to reconnect to mapped drives, and AIM will try to connect to its network server," Maynor says. "This chattiness allows an attacker to gain information about you without your knowing it."

So any apps set to start up by default such as email, IM, and Windows file and print sharing can be the culprits. And Web browsers, too: "Many companies set their browser to their intranet home page," Maynor says. "Just looking at the outgoing request from the browser can give a remote user [attacker] a view into the enterprise network. These types of applications will literally bleed information about your internal network to anyone who is listening."

Automatic software updates are another potential seepage point. "Systems can be much more verbose when they reach out for software updates," says Ron Gula, CTO of Tenable Network Security.

The best defense is not to allow any network communications with a new or untrusted network, Maynor says. Some personal firewalls can be used to set restrictions on apps' access to specific network connections.

But Gula says there's no simple solution: "For the most part, your firewall won't stop your OS or applications from updating themselves. The best defense against this stuff is general education and physical control of the immediate network."

— Kelly Jackson Higgins, Senior Editor, Dark Reading