, SecurityFocus 2004-02-05
A small and diverse band of hobbyists steeped in the obscure languages of embedded systems has released its own custom firmware for a popular brand of cable modem, along with a technique for loading it -- a development that's already made life easier for uncappers and service squatters, and threatens to topple long-held assumptions about the privacy of cable modem communications.
The project is the work of a gang of
To make Sigma work,
But TCNiSO discovered that the Surfboard modem contains an undocumented vestigial console port, through which the boot process can be controlled. The group published a detailed tutorial for tapping into the serial port, which involves opening the modem and connecting two wires at particular points on the circuit board, then routing them through an inexpensive chip that converts the signal to RS-232 levels-- allowing the user to plug in a PC running a terminal program. Users less handy with a soldering iron can purchase a special conversion cable from TCNiSO's website to simplify the process.
From there, the user simply reboots the modem. A stream of text comes down the port describing the boot process, which the user can interrupt with a keystroke, then redirect by typing in a new boot string. The modem can be told to boot from any FTP server -- in this case, one running on the user's own PC, and serving up Sigma. Once the new firmware is loaded, it becomes part of the modem, and the process need not be repeated.
With Sigma installed, the cable modem lays exposed for what it really is: a versatile computer in its own right. Built on a powerful MIPS processor, the modem's native operating system is
While it's a boon to uncappers, the security implications of firmware hacking go beyond mere bandwidth-boosting and theft-of-service. The topography of cable modem networks typically puts between 500 and 1,000 homes in a neighborhood on the same circuit, their Internet traffic all mingled on the same co-ax cable. Subscribers are prevented from eavesdropping on their neighbors' traffic by their own modem, which is programmed to only pass packets destined for them. By building on TCNiSO's hacking technique, a malefactor could write custom code to forward all the raw network traffic to their PC.
Outside security experts have generally dismissed any eavesdropping threat on modern cable systems based on a belief that cable companies are encrypting customer traffic, a capability built into all DOCSIS-certified modems since 1999. But while encryption would indeed thwart any eavesdropping attempt, in the most commonly-deployed version of the DOCSIS standard, version 1.0, the encryption option is just that -- an option, and one that's turned off by default. "The security has to be there" in the modem, says Oscar Marcia, chief security architect at for CableLabs, the industry group responsible for DOCSIS. "But the [service provider] can decide when to turn it on."
And turning it on they are, Marcia says, but slowly, and in bits and pieces, even five years after the option became available. "It's kind of a gradual process... They want to make sure that they have all the kinks worked out of their system." He adds that he expects the process to accelerate as cable companies migrate to newer versions of the DOCSIS specifications, where encryption is "on" by default, instead of off.
SecurityFocus asked four U.S. cable modem service providers if they protected their customers with the encryption option. Comcast, Adelphia, and CableVision's Optimum Online declined comment; a spokesman for Time Warner's Road Runner service didn't return repeated phone calls on the question. Comcast's terms of service, however, acknowledges a risk of eavesdropping by "other subscribers," and Optimum Online's
The architecture of cable modem networks likely prevents eavesdropping of upstream traffic, liked typed passwords and credit card numbers, and websites using SSL would be immune from passive monitoring. "But downstream traffic is certainly visible to lots of people if crypto isn't used," said AT&T security researcher Steve Bellovin, in an e-mail interview.
The potential for spying and other mischief based on TCNiSO's research is not lost on "DerEngel" -- the 23-year-old unemployed programmer who heads the group. In an effort to be responsible, the group programmed Sigma to block execution of the VxWorks functions that change the modem's MAC address, a capability that could otherwise wreak havoc on a network in the wrong hands. And on the group's website, DerEngel offers to provide cable companies with a tool to detect Sigma in use. "If you're going to make the crack, might as well sell the glue," he says. So far, no one's taken him up on the offer.
DerEngel says he and a friend began hacking cable modems three years ago. Since then, the number of coders and researchers working on TCNiSO projects has grown to ten, each with specialized skill sets, hand-picked by DerEngel with the care of the roguish ringleader in a caper movie assembling a team for a big score. He has a C coder and a Windows programmer in Australia, a programmable memory expert in the U.K., testers in Europe and Canada, and an assembly language coder in Kentucky.
The latter is "Isabella," a 31-year-old programmer who coded Sigma from her home near Louisville. Isabella scratches out a living doing odd software and hardware jobs, like designing an electronic light toy, or writing the embedded code that operates the ghosts and goblins in a local haunted house attraction every Halloween. DerEngel approached her online last year, after hearing she was good with an assembler and might be interested in helping.
Underemployed and intrigued by the possibilities, Isabella wrote Sigma in three months of days-long spurts of creativity. She doesn't have cable modem service. "Everybody, it seems like, messes with PC-based stuff, but nobody that I know does the embedded thing," she says, explaining her interest in the project. "And Der is really nice. Some people think he's kind of crazy, but I figured out how to deal with it."
How crazy? When Isabella mentioned to DerEngel that she was looking for a better MIPS assembler for the job, she expected him to suggest one of the free programs already available. Instead, he wrote a new one from scratch, filling it with features particularly useful to firmware hacking. "He wrote a good assembler," she says. "Der was determined to do it."
Indeed, the accumulated talent of the group's members has begun to dwarf their raison d'être, and the coders seem to know it. DerEngel is barely interested in discussing uncapping, and speaks instead of the possibilities of writing plug-ins for Sigma -- extensible by design -- that would transform the capabilities of the Surfboard, turning it into a NAT box and a firewall. Isabella thinks they can program the modem to tune to the channels used by the cable companies' digital music feeds, which -- like TV programming -- share the co-ax with the cable modem service. The hack might let the modem send music to the user's PC, where it could be streamed in real time.
Ultimately, DerEngel and Isabella would even like to go legit, and turn the group into a research shop for cable system providers, or at least make a deal that allows TCNiSO to test their techniques in a sanctioned laboratory setting. But after three-years as the preeminent underground think tank for cable modem uncappers, DerEngel is realistic about the future. "In this industry you can't be the good guy and the bad guy," he says. "So I guess we have to hide for now, for that reason, because everyone will perceive us as the bad guys... I think they look at us as hacking something that we shouldn't, instead of just interested in electronics and trying to get better at what we do."
Of course, the cable industry has its own impossible dreams, which include preventing smart coders with lots of time and restless passion from hacking the next generation of cable modems. "What you're talking about only affects the DOCSIS 1.0 modems," says CableLabs' Marcia of the Surfboard hack. The DOCSIS 1.1 and 2.0 specifications only accept firmware that's been digitally signed by the cable company. "Once you move to a DOCSIS 1.1, and we already have some cable operators deploying 1.1, this hack is not a viable hack any more.... One mistake, and it turns the modem into a brick."
But DerEngel doesn't believe any cable modem is going to be immune from customization, and he says his team is ready to prove it. "If you have to, you can just change the [programmable memory chip] -- desolder it, put it back on there," he says. "As long as the customer has the actual hardware in their hands, the customer will always be able to change what he has."