SYSSEC: May 2008 Archives


May 10, 2008

Danny McPherson posts about his experience with the free WiFi in the Unied Red Carpet Club:
More interesting is perhaps the access model they employ. To login, all you need is the United Mileage Plus number of the primary Red Carpet Club account holder. Now, having long questioned the wisdom of a luggage tag that displays these numbers, be it a "hole-punched" Mileage Plus membership card, or a more obvious oval-shaped Red Carpet Club tag, I'm even more wary now. But if you're in bind and need your airport wireless fix, odds are you won't have to walk far to find one available for the taking. As a matter of fact, I see two from where I'm sitting right now.

I've yet to explore how difficult it would be to exhaustive search for valid numbers, or if multiple logins are permitted at a given time, or how far outside of the Red Carpet Club these numbers are valid, or... I also wonder how long it'll be until some poor schmuck is arrested for allegedly downloading child porn from an airport wireless network...

If this were a wired network this wouldn't be a security problem. After all, if you're inside the RCC, presumably you're an RCC member (unless you bought a day pass), in which case you should be entitled to use the network. But as Danny indicates, the wireless AP is probably accessible from outside the RCC, so if you sit outside the club, you should be able to get on the network, making it just a matter of having a valid mileage plus number, which you can get off of someone's luggage tag.

As far as exhaustive search goes, MP numbers are 11 digits long, but the first digit seems to always be zero, so this is a 10 digit space. I don't know how many RCC members there are, but Wikipedia claims that there are about 750,000 Premier and Premier Executive members, so let's say there are on the order of 200,000 RCC members, or 2*10^{-5} of the space. If the numbers are randomly distributed, you'd need to search about 100,000 numbers in order to find one. This could take quite some time (over a day at one per second). You might be able to get some leverage because the distribution isn't random. They seem to be issued in some kind of increasing sequence, though there seem to be too many numbers for it to be strictly sequential. If there's a check digit like in credit card numbers this would make the space a lot easier to search. (If someone knows the actual algorithm, please write in.) Of course, you only need to know a few valid numbers, so this might not be a totally prohibitive attack if reading it off someone's tag weren't so easy.

Three more thoughts:

  • RCC entry itself is a lot more valuable than access to the wireless, since the wireless access doesn't cost United much, but access to the club costs them food and (in the International terminal), free drinks. I assume it's not hard forge an MP card once you know a valid number. I'm not an RCC member so usually when I'm there it's on the "international ticket" + Star Alliance Gold exception, so they check my ticket, which is hard to forge. Do they insist on seeing your ticket if you're an RCC member? If not, this is actually a new attack vector on the RCC, since it would let you extract numbers even if it weren't easy to read them off other people's luggage.
  • There's actually a fairly easy way to secure the system against remote attacks (ones that don't involve somehow gaining access to the RCC interior) that wouldn't require lining the RCC walls with copper sheeting. For the first login to the RCC network, require not just your RCC #, but also a random passcode given to people on entrance (or maybe posted on the wall). After that, you can install a cookie on their computers and just let them on without a new login. 1
  • I'm a bit curious how the system checks for RCC number validity. Does it have a local copy of the RCC database? Is it connected to United's central systems? That could be interesting.

1 See draft-rescorla-stateless-tokens for a description of some techniques for avoiding the need for a centralized cookie database.


May 8, 2008

Even the most diehard TeXhead has moments when he needs to read some Word document. Tonight was such a night and I have Office 2004 on my machine for just such an eventuality (Please don't write in to tell me that I should run Pages. As I said, I don't want to run either of them, but I also don't want to deal with Pages/Word incompatibility.) Anyway, I boot up Word and the Leopard firewall asks me if I'd like to let Word listen for network connections. I go to click no and either manage to click it or raise some other window or something. The dialog disappears and when I check the firewall it sure does say to block MS Word. So, that's OK, I guess.

And then I get to thinking, "Why is Word opening up TCP listening ports anyway?" So, I run netstat -a | grep LISTEN and get:

[49] /usr/sbin/netstat -a | grep LISTEN
tcp4       0      0  *.3369                 *.*                    LISTEN

Hmmm. What's 3369? Google doesn't know, so that's not good. I close Word and the port goes away and lsof confirms it's Word:

[52] /usr/sbin/lsof -i TCP:3369

Word    8198  ekr   16u  IPv4 0x6c4d66c      0t0  TCP *:3369 (LISTEN)

I shut down Word and my WiFi and restart it, but it's not listening now. Maybe I need the network on. Sure enough, I bring the WiFi back up and restart Word and now it's listening, but on a different port: 3828 this time. Stranger and stranger. Now ordinarily this would only be about a 4.0 freakout on a scale of 1 to 10, but it turns out that I only recently installed Office on this machine and was unaware of the following delightful property of MS AutoUpdate: it only installs one update at a time, no matter how many updates are pending. So, when you have 10-20 updates to install, and you're just letting update run itself, it takes forever to get uprev. The consequence of this is that I was loading random people's documents with some two year old (and vulnerable) version of Word. Who knows what malware I've had the joy of installing. This jacks things up to a freakout factor of about 6.2.

Next step: compare to another machine. It shows up on my other Mac, which is a little comforting, but of course that machine could be infected too. I double check with Hovav, who is about as paranoid as I am, and his copy of Office is is listening, but on some other random port. That's sort of comforting. This is starting to look a lot less like malware and a lot more like a feature of Word. A little more digging tells us the process name that is actually doing the listening. It's Word (as I knew) but with some wacky argument starting with -psn_0_.... Searching on this, we find out that I'm not the only person who has had this question.

If you close UDP 2222, then no other computers will know which TCP port your copy of word has chosen to listen to (in the 3000-3999 range), because that info is broadcasted in the UDP packets. The protocol is thus: Your copy of word spews it's serial number (encoded) and the TCP port it is listening on in a packed on UDP 2222. Other copies of word on the network get this packet and then respond the your copy of word on the specified TCP port if they have the same serial. Then one copy shuts down.

I guess it was malware after all. Outstanding!