Hashing Email Addresses For Web Considered Harmful

cce writes "The MicroID standard, despite getting thrashed soundly by Ben Laurie two years ago, has since been recommended by the DataPortability Project and published on the user profiles of millions of users at Digg and Last.fm. MicroID is basically a hash calculated using a user's profile page URL and registered email address, producing a token that makes the email address vulnerable to dictionary attacks. To see how easy it was to crack these tokens, I conducted a small study, choosing 56,775 random Digg users, and cracking the email addresses of 14,294 of them (25%) using just their MicroID, username, and a list of popular email domains. Digg has more than 2 million users, and that means half a million of them — mostly people who had never heard of MicroID, and had probably not logged in for a long time — had their email addresses exposed to this trivial attack. I also applied this attack to Last.fm (19%) and ClaimID (34%). Digg and Last.fm have since removed support for MicroID, but the lesson is clear: don't publish a hash of my email address online, guys!"

Comments

Re:What does MicroID actually do for the user?

[Fred Ferrigno]

I read up on it and I'm still confused, but I think this is the idea:

1. You set up an account at website Alpha.
2. You have a publicly-viewable profile page at Alpha. On the page is your MicroID.
3. You set up an account at website Beta.
4. You tell Beta about your Alpha profile page.
5. Beta verifies that your Alpha profile page is really yours by checking the MicroID.

Beta can't really do anything with your Alpha page except link to it. I guess the point would be to prevent people who aren't you from linking to your Alpha page on their Beta pages. That way, other people can be sure that the same person owns both accounts.

The attack mentioned in the article doesn't compromise the proper use of the MicroID, since Beta is assumed to have verified that you own your email address and you wouldn't link to a profile page claiming to be yours that wasn't. All it does is make it possible for spammers to harvest your email.

Re:Solution: salt your emails

[geekgirlandrea]

Except that lots and lots of web sites fail at RFC 822 and think + isn't a valid character in an e-mail address. Usually the same sort of maldesigned horrors that make you type your e-mail address twice even though, unlike your password, you can read it as you type to make sure it's correct, or have a single free-form blank for credit card numbers and enforce some idiosyncratic rule on separators (really, is $cc =~ s/-//g; that hard?), or enforce strong passwords and then cripple them with mandatory 'security' questions that allow anyone who knows you halfway well to reset your password.

Yeah, I use them too, and if web designers were a whole lot smarter they would be a better solution to things like this, but in practice lots of web sites just refuse to accept addresses like that. I should get around to making sendmail let me use an underscore instead of a + for that purpose.

Re:Solution: salt your emails

[statemachine]

Giving out e-mails with "+something" is worthless for spam. The malicious spammers will just strip the "+something" from address, as both can be delivered, but the short form will be less likely filtered, and you won't know which service it was sold/stolen from.

I actually make a separate alias for each site eg. name-something@example.com. If you shorten my alias to the part before the hyphen, it won't deliver. Yes, spammers have tried.

If you're using "+something" just know that you might as well not append that onto your e-mail address, for all the good that it does, as you're giving out your primary address anyway. Cat, bag, already open.

Re:Flawed study?

[QuantumG]

Offline attacks are better because they:

1. can't be monitored
2. can't be blocked
3. are not limited by bandwidth
4. can be sped up by throwing more hardware at them

This is basically why salting was added to the unix password file. And that failed.. so /etc/shadow was introduced. Revealing hashes is just unnecessary, so don't do it.

Re:Solution: salt your emails

[mi]

+ is a bad delimiter.

It is the delimiter, originally created as such by the authors of the very first MTA... There is no other character, that:

1. Can be part of an e-mail address.
2. Can not be part of a username.

Many web-forms don't accept email addresses with '+' in the username portion. Attempts to educate webmasters to the information in the relevant RFC's is usually met with silence or worse...

This is, unfortunately, the truth... Far too many programmer wannabees around... It is a good fight, however, and kudos to GMail for keeping support for it (unlike Yahoo! Mail).

I use this whenever I can, when giving my address to web-sites (including Slashdot)...

Re:Solution: salt your emails

[mcrbids]

Let's see... Large email provider, throwaway addresses, access until you don't want it anymore...

You mean, kinda like Mailinator??

There are others, Mailinator is the easiest.