Holes Remain in SSL VPNs

Your Secure Sockets Layer (SSL) VPN still may not be as secure as you think, especially if your users don't always access the network via corporate-issue laptops.

Once they jump on an outside machine to Web browse or check their email, SSL VPN users can leave behind sensitive data or be vulnerable to man-in-the-middle attacks and keystroke loggers, experts say. An infected kiosk can infect your network, too. So even though they may be more convenient than their IPSec counterparts (SSL can be used by browsers anywhere without client software) these VPNs can also backfire if you're not careful in how you deploy them.

SSL VPNs are popular among enterprises that don't have the IT resources to support the administratively-heavy IPSec VPNs, which require client software. Unlike IPSec, which uses digital certifications on both the server and client side, SSL VPNs mostly use certs only on the server side. "With SSL mostly being done in this one-way mode, it opens you up to a man-in-the-middle" attack, says John Pescatore, a vice president with Gartner.

SSL VPN products have come a long way in the past year. Many come with features to prevent downloading files or ActiveX or Java applets, for instance, Pescatore observes.

But it's all about how you configure your SSL VPN. "It doesn't solve the right problems. It protects your information in transit but doesn't stop an end point from getting owned," says Dino Dai Zovi, a researcher with Matasano Security.

However, SSL VPNs are actually more secure than IPSec when the remote user's machine is connected to the network, he says. "With an IPSec VPN, the remote user's machine is fully connected to the remote network. Worms and other malware can connect directly to hosts on the corporate network over the VPN link," Dai Zovi says. "With an SSL VPN, software running on the remote user's machine does not have direct access to the corporate network."

The push for VPN access from any computer, anywhere, typically comes from the top, such as the CEO who wants to check his email from outside the office. "The basic problem is that enterprises have been forced on the business side to allow people to do their jobs from any computer -- a home PC, a PC at Kinko's, or one at a tradeshow conference," Pescatore says.

But accessing the SSL VPN from a home or contractor's machine, or a trade-show kiosk, can cause a user to leave a trail behind. Dai Zovi says it's best not to allow users to use a kiosk for anything other than a Google search. "Using a kiosk for sensitive communications is risky," he says. "You can leave a lot of traces in a Web browser and cache that can be recovered forensically."

And you just can't secure an Internet kiosk, anyway, says researcher Dan Kaminsky. "People keep trying to get around this fact," he says. "It can't work."

There are some products that clean up any tracks users leave in a kiosk. Cisco's WebVPN's Secure Desktop feature, for instance, eliminates any traces of sensitive data a user might inadvertently leave behind with a session "location" that gets rid of cookies, temporary files, and downloaded content after the VPN session is over. Secure Desktop runs with WebVPN a Cisco Catalyst switch.

Security experts say you should institute strong authentication for all users doing remote access. If they can't carry a laptop, users could take advantage of other devices, such as USB thumb drives like Route1's MobiKey's that contain a miniature of a user's PC hard drive, including OS, says Gartner's Pescatore.

But that type of device is better for convenience than security, notes Dai Zovi. A more secure approach would be the small, IPod-sized VMWare-based hard drives that plug into a third-party machine and keep everything on that device, he says.

Meanwhile, most SSL implementations still use only 128-bit encryption, which isn't airtight enough, says Sean Kelly, business technology consultant for Consilium1.

— Kelly Jackson Higgins, Senior Editor, Dark Reading