Some time ago there was a flap about whether or not DIVAD did or did not shoot at a latrine fan. [See Doug Schuler in RISKS-3.1, with subsequent discussion in RISKS-3.3, 4, 5.] I have documentation now from a person who should know: Richard DeLauer, former Undersecretaty of Defense for Research and Engineering in the first Reagan term. He says it did, and that it was supposed to do that. See [MIT] Technology Review, July 1986, page 64.
Phenomena like this are well known by the CEGB (Central Electricity Generating Board) engineers. Operation of a power grid assumes that the load does not change suddenly, indeed sudden changes can cause instability. Anyway, it is well known in the U.K. (I'm not sure about the U.S. and Canada) that the largest power surge is at the end of Coronation Street, or one of the other soaps, when everyone gets up from the Telly and plugs in the kettle to make tea. I assume that's what happened at the end of the wedding telecast. A similar thing happened in the U.S. a couple of years ago. I think it was somewhere in New Mexico or Arizona that there was a pause in the super bowl game so a lot of people got up, went to the bathroom (all that beer) and flushed at nearly the same time which caused some sewer backups. Don Chiasson
MSG: *MSG 5759 Date: 24 Jul 86 12:22:30 GMT From: frog!die at EDDIE.MIT.EDU (Dave Emery, Software) Re: Security and dialbacks DISTRIB: *BBOARD Summary: Dialbacks aren't very secure (repost of old article) Apparently-To: codebreakers In article <firstname.lastname@example.org> gnu@hoptoad.UUCP writes: >Here are the two messages I have archived on the subject... >[I believe the definitive article in that discussion was by Lauren Weinstein, >vortex!lauren; perhaps he has a copy. What follows is the original article that started the discussion. I do not know whether it qualifies as the "definitive article" as I think I remember Lauren and I both posted further comments. - Dave ** ARTICLE FOLLOWS ** ---------------------------------------------------------------------- An increasingly popular technique for protecting dial-in ports from the ravages of hackers and other more sinister system penetrators is dial back operation wherein a legitimate user initiates a call to the system he desires to connect with, types in his user ID and perhaps a password, disconnects and waits for the system to call him back at a prearranged number. It is assumed that a penetrator will not be able to specify the dial back number (which is carefully protected), and so even if he is able to guess a user-name/password pair he cannot penetrate the system because he cannot do anything meaningful except type in a user-name and password when he is connected to the system. If he has a correct pair it is assumed the worst that could happen is a spurious call to some legitimate user which will do no harm and might even result in a security investigation. Many installations depend on dial-back operation of modems for their principle protection against penetration via their dial up ports on the incorrect presumption that there is no way a penetrator could get connected to the modem on the call back call unless he was able to tap directly into the line being called back. Alas, this assumption is not always true - compromises in the design of modems and the telephone network unfortunately make it all too possible for a clever penetrator to get connected to the call back call and fool the modem into thinking that it had in fact dialed the legitimate user. The problem areas are as follows: Caller control central offices Many older telephone central office switches implement caller control in which the release of the connection from a calling telephone to a called telephone is exclusively controlled by the originating telephone. This means that if the penetrator simply failed to hang up a call to a modem on such a central office after he typed the legitimate user's user-name and password, the modem would be unable to hang up the connection. Almost all modems would simply go on-hook in this situation and not notice that the connection had not been broken. If the same line was used to dial out on as the call came in on, when the modem went to dial out to call the legitimate user back the it might not notice (there is no standard way of doing so electrically) that the penetrator was still connected on the line. This means that the modem might attempt to dial and then wait for an answerback tone from the far end modem. If the penetrator was kind enough to supply the answerback tone from his modem after he heard the system modem dial, he could make a connection and penetrate the system. Of course aome modems incorporate dial tone detectors and ringback detectors and in fact wait for dial tone before dialing, and ringback after dialing but fooling those with a recording of dial tone (or a dial tone generator chip) should pose little problem. Trying to call out on a ringing line Some modems are dumb enough to pick up a ringing line and attempt to make a call out on it. This fact could be used by a system penetrator to break dial back security even on joint control or called party control central offices. A penetrator would merely have to dial in on the dial-out line (which would work even if it was a separate line as long as the penetrator was able to obtain it's number), just as the modem was about to dial out. The same technique of waiting for dialing to complete and then supplying answerback tone could be used - and of course the same technique of supplying dial tone to a modem which waited for it would work here too. Calling the dial-out line would work especially well in cases where the software controlling the modem either disabled auto-answer during the period between dial-in and dial-back (and thus allowed the line to ring with no action being taken) or allowed the modem to answer the line (auto-answer enabled) and paid no attention to whether the line was already connected when it tried to dial out on it. The ring window However, even carefully written software can be fooled by the ring window problem. Many central offices actually will connect an incoming call to a line if the line goes off hook just as the call comes in without first having put the 20 hz. ringing voltage on the line to make it ring. The ring voltage in many telephone central offices is supplied asynchronously every 6 seconds to every line on which there is an incoming call that has not been answered, so if an incoming call reaches a line just an instant after the end of the ring period and the line clairvointly responds by going off hook it may never see any ring voltage. This means that a modem that picks up the line to dial out just as our penetrator dials in may not see any ring voltage and may therefore have no way of knowing that it is connected to an incoming call rather than the call originating circuitry of the switch. And even if the switch always rings before connecting an incoming call, most modems have a window just as they are going off hook to originate a call when they will ignore transients (such as ringing voltage) on the assumption that they originate from the going-off-hook process. [The author is aware that some central offices reverse battery (the polarity of the voltage on the line) in the answer condition to distinguish it from the originate condition, but as this is by no means universal few if any modems take advantage of the information supplied] In Summary It is thus impossible to say with any certainty that when a modem goes off hook and tries to dial out on a line which can accept incoming calls it really is connected to the switch and actually making an outgoing call. And because it is relatively easy for a system penetrator to fool the tone detecting circuitry in a modem into believing that it is seeing dial tone, ringback and so forth until he supplies answerback tone and connects and penetrates system security should not depend on this sort of dial-back. Some Recommendations Dial back using the same line used to dial in is not very secure and cannot be made completely secure with conventional modems. Use of dithered (random) time delays between dial in and dial back combined with allowing the modem to answer during the wait period (with provisions made for recognizing the fact that this wasn't the originated call - perhaps by checking to see if the modem is in originate or answer mode) will substantially reduce this window of vulnerability but nothing can completely eliminate it. Obviously if one happens to be connected to an older caller control switch, using the same line for dial in and dial out isn't secure at all. It is easy to experimentally determine this, so it ought to be possible to avoid such situations. Dial back using a separate line (or line and modem) for dialing out is much better, provided that either the dial out line is sterile (not readily traceable by a penetrator to the target system) or that it is a one way line that cannot accept incoming calls at all. Unfortunately the later technique is far superior to the former in most organizations as concealing the telephone number of dial out lines for long periods involves considerable risk. The author has not tried to order a dial out only telephone line, so he is unaware of what special charges might be made for this service or even if it is available. A final word of warning In years past it was possible to access telephone company test and verification trunks in some areas of the country by using mf tones from so called "blue boxes". These test trunks connect to special ports on telephone switches that allow a test connection to be made to a line that doesn't disconnect when the line hangs up. These test connections could be used to fool a dial out modem, even one on a dial out only line (since the telephone company needs a way to test it, they usually supply test connections to it even if the customer can't receive calls). Access to verification and test ports and trunks has been tightened (they are a kind of dial-a-wiretap so it ought to be pretty difficult) but in any as in any system there is always the danger that someone, through stupidity or ignorance if not mendacity will allow a system penetrator access to one. ** Some more recent comments ** Since posting this I have had several people suggest use of PBX lines that can dial out but not be dialed into or outward WATS lines that also cannot be dialed. Several people have also suggested use of call forwarding to forward incoming calls on the dial out line to the security office. [This may not work too well in areas served by certain ESS's which ring the number from which calls are being forwarded once anyway in case someone forgot to cancel forwarding. Forwarding is also subject to being cancelled at random times by central office software reboots.] And since posting this I actually tried making some measurements of how wide the incoming call window is for the modems we use for dial in at CRDS. It appears to be at least 2-3 seconds for US Robotics Courier 2400 baud modems. I found I could defeat same-line-for-dial-out dialback quite handily in a few dozen tries no matter what tricks I played with timing and watching modem status in the dial back login software. I eventually concluded that short of reprogramming the micro in the modem to be smarter about monitoring line state, there was little I could do at the login (getty) level to provide much security for same line dialback. Since it usually took a few tries to break in, it is possible to provide some slight security improvement by sharply limiting the number of unsucessful callbacks per user per day so that a hacker with only a couple of passwords would have to try over a significant period of time. Note that dialback on a dedicated dial-out only line is somewhat secure. David I. Emery Charles River Data Systems 617-626-1102 983 Concord St., Framingham, MA 01701. uucp: decvax!frog!die
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