Backdoors In The Network: Modems, WiFi, & Cellular

War-dialing received a revival in March with HD Moore's release of WarVOX, a tool that leverages VoIP to speed up the calling of phone numbers to find modems, faxes, and voice systems. Finding modems can help enterprises find backdoors into their network setup by a rogue employee. Likewise, it can help penetration testers find forgotten or lesser-known ways into a target's network through a poorly secured modems.In a response to DarkReading's March coverage on WarVOX at the time of its release, the CFO and a co-founder of SecureLogix Corporation said a sampling of major security vendors showed only six, included war-dialing as a service. With speed and ease of use of WarVOX, I would expect those numbers to have grown in the past two months, but what other areas should be assessed in addition to the typical network-based assessment?

Provided that site audits are included in the assessment, wireless network analysis and detection is a natural inclusion, but are pen testers covering all the bases? There are many flavors of 802.11, so keep in mind when hiring a security vendor for a pen test or building your own pen-testing team that their methodology includes a check for all of them. Similarly, Bluetooth needs to be included because it can provide similar networking capabilities.

One area that has seen little coverage is the detection of cellular-based backdoors. 3G/EVDO modems are becoming very popular and inexpensive, making them likely methods for intentional, or unintentional, backdoors into your network. The question is, how do you defend against and detect them?

Policy is the important first step -- to say that while using the corporate network, cellular modems are not allowed to be connected or enabled. Of course, we all know that some users make mistakes, and some others are malicious, so the next step is to detect them. The first possibility is to do so from the internal network and query each machine regularly to see if it has multiple interfaces enabled, what IPs are assigned to the interfaces, etc.

Technically, finding rogue cellular modems is not hard from an internal perspective, but what about from the outside? As a pen tester, how would you go about finding your target's resources on a different network? Scanning that network isn't a good option because you might get a call from an unhappy ISP, but you could send the users e-mail messages with images linked from a Web server you control. Then, monitor any connections that don't come from the corporate IP space.

Targeting users who have computers that are multihomed between the corporate network (possibly on a VPN) and one using their 3G/EVDO modems may only happen in rare situations. It's not something I've come across because very few of the clients I've worked with have users that need 24/7 access capabilities. Still, I'm curious if any of you pen testers out there have encountered this situation, and how often? Leave a comment or send me an e-mail.

John H. Sawyer is a senior security engineer on the IT Security Team at the University of Florida. The views and opinions expressed in this blog are his own and do not represent the views and opinions of the UF IT Security Team or the University of Florida. When John's not fighting flaming, malware-infested machines or performing autopsies on blitzed boxes, he can usually be found hanging with his family, bouncing a baby on one knee and balancing a laptop on the other. Special to Dark Reading.