Slashdot Mirror
The Next Step in Fighting Spam: Greylisting
Evan Harris writes "I've just published a paper on a new and unique spam blocking method called "Greylisting". The best thing about it other than achieving better than 97% effectiveness in blocking spam, is that it practically eliminates the main problem of other solutions: the false-positive. There's even source code for an example implementation written as a perl filter for sendmail, along with instructions for installing, so you can get up and running quickly."
How does the effectiveness of Greylisting compare with what others are seeing with existing techniques (such as Bayesian filtering)? Is it a false positives problem, such as digests and opt-in mailing lists getting incorrectly tagged as spam?
Dr. Rick
- "It's such a fine line between clever and stupid" (Nigel Tufnel)
- Zort! (Pinky)
The thing that is wrong is the SMTP protocol, and most people's conception of a spammer. Once you see a few "confessions of ex-spammers", everything changes.
There are people out there who pay $10000 in startup costs, and then make $2000/week for spamming. The $10000 gets them software written by knowledgable internet security experts. This software finds any and every way to anonymify the email spam, and finds lists of people to spam.
As long as knowledgable internet security experts are getting paid good cash to enable spammers, and SMTP doesn't change, spam will only continue to get worse. There needs to be a fundamental change in SMTP protocols. It oughta take the spammers about 2 days to fix their MTA bug to get around greylisting.
Doels this mean all public crypto algorithims are useless?
Keep the Classic Slashdot.
Trolling is a art,
The best idea I've seen in YEARS was to have people start using a specific, original poem as their signatures. Then, the author granted license to anyone who WASN'T sending spam. Therefore, they could sue any spammer for copyright infringement if they used it, and you could train your mail filter to look for the signature. Once spamassassin took it up, it pretty much snowballed. See story here
-Looking for a job as a materials chemist or multivariat
At USENIX '03 there was a paper presented on artificial intelligence techniques for spam detection. I can't provide a link since only USENIX members can download the paper (at this point, at least). I was a coauthor of that paper.
One of the things we've discovered in our research is that some classes of filters (most notably, the one I have been developing along with a few other individuals) are actually more effective at correctly classifying email than humans are. That is to say, you can train the learning algorithm on mostly-correctly-classified data, then re-run it over the training data, and almost miraculously, it discovers all kinds of email in the training set that was incorrectly classified.
I.e., this filter has discovered mail that I myself incorrectly thought was spam. It's scary, because there's a lot of it.
To assume that a human will always be 100% accurate at classifying their own email isn't just arrogant, it's plain wrong. Newer filters that will be introduced in the near future might possibly be more accurate than you, a frail human, could ever be.
The registrar I use (jumpdomain.com) has a clever hack for despamming WHOIS contact email. Basically they change your published contact address once a week. The published address i automatically generated, looks like gibberish, and forwards to your real address. If someone wants to contact you by looking up your address by WHOIS and writing to you, it works fine. But if they add the address to a mailing list, it stops working in a week. That has eliminated almost all my WHOIS spam. Good scheme.
I'm currently using Bogofilter (and looking into CRM114) and getting better than 99% accuracy (about 1 in 200 false negatives at the moment) and very very few false positives (maybe 2 in 5000 messages).
Of course these are MUA level filters (and yes, I know, I've already "paid" with bandwidth to download the spam) - however since the proposed "greylister" would have to be installed as the MTA at major ISPs (as the authors note) I'm not convinced that is more likely to get widespread adoption than the various sorts of adaptive client-based filtering now available, particularly as it requires a database to back the method up.
As far as I am concerned the major factor in a spam filter should be zero false positives - personally I don't mind reviewing one or two spams a week but I get really annoyed if I were to lose a real message (note the two false positives I have sent to date with bogofilter contained forwarded sales pitches along with a message).
-- Nothing unusual happened today
Not to mention that if this is used in conjunction with other collaborative tools (i.e. RBL, checksums), by the time that the spamming MTA can return its IP address will have been submitted to MAPS/etc. and the contents of the message will have been submitted to Razor/Pyzor/DCC.
I think that this greylisting idea will be pretty hard to beat by Joe spammer. Since the game of spam detection is pretty much an arms race, slowing him down will probably be enough to turn the battle in your favour.
Anything that makes spammers do extra work seems to make it more likley that they will have patterns that can be observed and then blocked before mail really goes anywhere...
"There is more worth loving than we have strength to love." - Brian Jay Stanley
I doubt it. I would assume the spam software would have a timeout, and I doubt it's ten minutes. If they want to hit-and-run and aren't even willing to make a second delivery attempt when an error code is returned, I doubt they're going to wait 10 minutes. I'm sure that within 30 seconds or less they'll consider it a dead connection and hang up.
Problem is, I used to have my sendmail HANG UP in real-time on an incoming connection as soon as it realized a message was spam. I.e., the incoming message was filtered in the DATA phase and if it was spam I hung up immediately. It worked great and it felt good, but there were many spam programs that took the disconnection as some kind of TCP/IP failure and immediatelty tried again. So I had one day where a single message was attempted to be delivered about 30,000 times as the spammer connected, I hung up, spammer software said "Oops, let me try again!" About one delivery attempt every second or so.
I'd be willing to bet if you put a 10 minute timeout in sendmail you'll see lots of spammer software disconnecting sooner and just trying again. It takes more of their resources, but takes more of yours, too.
There is no magical waiting period or re-try period that cannot be trivially coded around. And, with good money on the line, will be trivially coded around. You don't get it. Really smart people are getting paid a whole lot of money to make programs to exploit every possible crack in the way we send email.
Yeah, spammers are so clever. Well, the fact is if for every one of these "smart" (yeah, right) spammers who has the help of a network consultant that will work around greylisting there are 5 dumbasses who don't (and I think I'm being generous there), then if I greylist I'd think over 80% of my spam problem would be eliminated. What's wrong with that? What's to "get"? Looking through headers I see the same bulk mailers used over the years, probably passed around as warez in spammer circles.