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

Solution: salt your emails

[pwnies]

I suppose this is yet another reason why it's nice that a few email services (most notably gmail) allow you to append a string to your email address using the + symbol (e.g. youremail+string@gmail.com will go to the inbox of youremail@gmail.com). In effect it allows you to "salt" your email, which adds a layer of complexity when trying to match these hashes with valid email (not to mention it allows you to check which site compromised your email if you use different 'salts' for each site you use your address on). If more email services start to allow this (doubtful), more sites start realizing that a + in your email is still a valid email (more doubtful), and more users start using it effectively (even more doubtful still), then I don't think the MicroID will be a huge problem.

Re:Solution: salt your emails

[nblender]

+ is a bad delimiter. 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... I did manage to get a FOAF to fix dell.com though.

Re:Solution: salt your emails

[Anonymous Coward]

Except that once the salted email is found, everything between the @ and the + will just be discarded.

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:Solution: salt your emails

[geekgirlandrea]

Yeah, this can happen, but I dunno that this is as big a problem as you think. Spammers just plain aren't all that bright, and they don't care very much if they miss the tiny proportion of addresses that geeks try to protect like this when there are so many totally unprotected addresses so easy to obtain. It seems like a lot of the time, when they try to harvest addresses, the harvester doesn't realize + is a valid character in an address and only gets the part after the plus sign. I bounce a lot of spam sent to addresses like slashdot@persephoneslair.org and usenet@persephoneslair.org.

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

[daeg]

Spammers aren't bright? So spam filtering is easy, right?

One (partial) solution is to have large providers provide alternate domains that you can register throw-away addresses. For instance, under Google Account settings, you might have the option to generate an address from cephelo@gmail.com and assign d785jd47fj@southeast.gmail.com and allow you to record a note that you intend to use d785jd47fj@southeast.gmail.com as your Amazon.com user ID.

As time progresses, Gmail can show you stats that, for example, 100% of e-mail on d785jd47fj@southeast.gmail.com is spam - "Do you want to delete this account?" and poof - the spam stops. Now that address automatically becomes a honey pot.

They already have your email address

[RevDigger]

This concern that you may have your email address *discovered* by spammers because you post it on a web page is so 5-years-ago. They already have your email address, and they probably didn't get it by scraping web pages.

When you have sent a couple emails out with a given address, you can figure that at least one of them will to sit around in someone's Outlook mailstore for the next couple years. (Someone you know uses Windows!) When that person's computer gets infected with spam gang malware (as they all do), they have your address.

Once of them has it, they probably all have it.

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.

Why is this a big deal?

[Ed Avis]

You are worried because someone, if they really wanted to send you some mail, could go to the trouble of doing a CPU-intensive search against some hash shown on a website and find out that ultimate, embarassing secret: your *email address*??

What gives? Email addresses are designed to be public. If you don't want people you do not know to be able to contact you, then you are free to drop all mail from unrecognized addresses. If you want to set up some kind of secret knowledge that people must have in order to contact you, then ask them to put a particular word in the subject line when first sending you a message. Either of these does not rely on keeping the address secret, which just isn't likely to happen.

The only thing more broken than trying to keep an email address secret is trying to make a 'private' web page by keeping the URI secret. Again, the system is designed so that the address itself is not sensitive, but other information such as a password or PGP key can be.

Actually, what it reminds me of most is the crazy situation in the US where a basically public identifier, the social security number, is abused as some kind of secret token. Hence all the fuss made when it is possible to find out someone's SSN. The answer is not to add more and more baroque means to stop the SSN from leaking out: one breach, and it's no longer a secret.

I understand the desire to stop spam address harvesters, but really, there are hundreds of web sites which display email addresses with only light obfuscation, enough to stop a harvester bot but not a determined human being (or someone determined enough to use an OCR engine). The kind of hashing talked about here is way more difficult to undo than that. If you are even more paranoid, you need to revisit your assumptions of what is public and what is secret.