Re: DOCSIS vulnerability

[Rob Koliha <rkoliha () charter net>]

This is quite an old issue.. There are half written documents on how to do it everywhere ;)
In most of the howtos I've seen a step or two is missing.. It won't work on a lot of the modems out there, I know it 
will work with motorola but it doesn't work on most toshiba and com21's.
Basically since the ethernet interface comes up before the rf interface, when you ping it your exploiting an arp table 
bug and fooling the modem into thinking that the tftp server lies on the wrong interface. This can be circumvented a 
few different ways.. Doing it with a packetshaper would be pretty expensive since you could possibly need thousands of 
flows (and you only get so many flows with each model) and packetshapers are not cheep. The best way to get around it 
is setting up the shared password stuff on the cmts (results in a little higher load, but prevents theft of service). A 
plaintext password or key is encoded into the .bin file that is downloaded and the cmts checks the key that the modem 
has before allowing it to go online. I know for a fact the docsis config decoder/encoder won't really decode the 
passphrase.. There may be other apps out there (or in the works) that will. More and more cable isp's will enable this 
as time goes on and hopefully the hardware manufacturers have fixes in place or in the works.. Firmware upgrades are 
done from the provider side, meaning it would be quite easy for affected modems to be fixed with a new release. It 
would be a little bit of a pain to prevent your modem from being patched. There are also QoS (quality of service) 
tables on each router (uBR) which your isp monitors.. If you push your modem higher than a speed level that your isp 
sells you stick out like a sore thumb. Once they find that you've hacked it you will either get one warning or they 
will disconnect you and refuse to serve you. If you have no other broadband alternatives it could really suck. It would 
also be bad if you enjoy vod and other 2 way cable services (as they could just as easily refuse data and tv both and 
cut/put a trap on your lines). Performing the hack could also probably land you in just as much hot water as the theft 
of tv services.



Rob Koliha
Charter Communications / Charter Pipeline
Hickory, NC

PS: in no way have my statements here represented Charter Communications, I am just a member of the list who happens to 
be happily employed for them ;)


On 12 Mar 2002 10:07:02 -0600
Justin Ellison <justin () techadvise com> wrote:

> I think you misunderstood his post.  These vendors have allowed him to
> spoof the tftp server.  He's not hacking the ISP's tftp server, he's
> creating his own files, placing them on his PC, and spoofing the ip of
> the ISP's tftp server.  This is a vendor problem, because they should
> only allow the tftp request to complete on the RF interface, not the
> ethernet...
>
> Justin
>
> On Tue, 2002-03-12 at 04:49, Rense Buijen wrote:
> > Maybe your posts were rejected because this is very old news.
> > This is known for ages, I have such a cable modem and indeed you can get
> > the config file by TFTP; decode, alter, encode and upload it, but the
> > ISP's are not stupid and most of the time this is NOT how they cap your
> > cable modem, they throw traffic into a packeteer or use other methods to
> > squeeze your bandwidth.
> >
> > All the info can be gathered by a tool like this:
> > http://www.weird-solutions.com/_bin/bootpq.exe
> >
> > And a simple google search shows up hundreds of articles explaining how
> > you can "hack" DOCSIS cable modems, unfortunately (unless you have a
> > completely clueless provider) all these tricks wont work.
> >
> > E.g: http://lists.wi2600.org/pipermail/2600/2001-October/008668.html
> >
> > Which dates from October 2001.
> >
> > (I tried it but my isp squeezes on the other end of the pipe, some
> > things that you can alter though is bypass restrictions of how many
> > computers could be connected right into the modem)
> >
> > With kind regards,
> >
> > Rense
> >
> > -----Original Message-----
> > From: Matthew S. Hallacy [mailto:poptix () techmonkeys org] 
> > Sent: dinsdag 12 maart 2002 4:55
> > To: vuln-dev () securityfocus com
> > Subject: DOCSIS vulnerability
> >
> > Hi,
> >
> > Apparently this isn't bugtraq worthy (my posts weren't rejected, they
> > were simply
> > deleted), so I'll send it here.
> >
> > ---
> >
> > Pre-ramble:
> >
> >     I've been debating this for a while, but now I'm sufficiently
> > agitated by dishonest cable ISP's to post it.
> >
> > Background:
> >
> >     DOCSIS was created to be a standard for data over cable systems
> > so
> > that a cable modem that worked on one system would work just as well on
> > the
> > next, this brings down hardware costs, as well as training costs.
> > Basicly
> > you plug the cable modem in, it acquires a data path to the ISP's
> > hardware,
> > and sends a BOOTP request. The BOOTP reply that it recieves contains a
> > few
> > items, a syslog server, a tftp server, a time server, and a config file
> > to
> > download from the TFTP server. Until now everyone has claimed that it's
> > impossible to disrupt this, 6 months ago I found a way to.
> >
> > Ramifications:
> >
> > Everything from 'uncapping' your cable modem to being able to destroy
> > the cable network you're connected to, this is how cable companies
> > rate limit their customers, it's how they keep their customers
> > DHCP servers from replying to DHCP requests from other customers,
> > it's also how they block everything from netbios to web servers.
> > this is also the method used to restrict customers to a certain
> > number of IP addresses.
> >
> > Details:
> >
> > It's a simple attack, while the modem is booting it looks for the
> > address
> > of the TFTP server, simply assaign that address to your system and ping
> > the cable modem on its management address (usually 192.168.100.1). It
> > will
> > then connect to your machine to download the TFTP configuration file.
> >
> > This is known to work on the following models:
> > Motorola (all models)
> > 3Com Sharkfin
> > Toshiba PCX 1100
> >
> > This is known to NOT work on these models:
> > RCA DCM235
> > 3Com CMX
> >
> >
> >
> > Copyright:
> > If you're redistributing this, keep it intact.
> > (c) 2002 Matthew S. Hallacy
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