Obscure E-Mail Vulnerability

This vulnerability is a result of an interaction between two different ways of handling e-mail addresses. Gmail ignores dots in addresses, so bruce.schneier@gmail.com is the same as bruceschneier@gmail.com is the same as b.r.u.c.e.schneier@gmail.com. (Note: I do not own any of those email addresses — if they’re even valid.) Netflix doesn’t ignore dots, so those are all unique e-mail addresses and can each be used to register an account. This difference can be exploited.

I was almost fooled into perpetually paying for Eve’s Netflix access, and only paused because I didn’t recognize the declined card. More generally, the phishing scam here is: Hammer the Netflix signup form until you find a gmail.com address which is “already registered”. Let’s say you find the victim jameshfisher. Create a Netflix account with address james.hfisher. Sign up for free trial with a throwaway card number. After Netflix applies the “active card check”, cancel the card. Wait for Netflix to bill the cancelled card. Then Netflix emails james.hfisher asking for a valid card. Hope Jim reads the email to james.hfisher, assumes it’s for his Netflix account backed by jameshfisher, then enters his card **** 1234. Change the email for the Netflix account to eve@gmail.com, kicking Jim’s access to this account. Use Netflix free forever with Jim’s card **** 1234!

Obscure, yes? A problem, yes?

James Fisher, who wrote the post, argues that it’s Google’s fault. Ignoring dots might give people an enormous number of different email addresses, but it’s not a feature that people actually want. And as long as other sites don’t follow Google’s lead, these sorts of problems are possible.

I think the problem is more subtle. It’s an example of two systems without a security vulnerability coming together to create a security vulnerability. As we connect more systems directly to each other, we’re going to see a lot more of these. And like this Google/Netflix interaction, it’s going to be hard to figure out who to blame and who — if anyone — has the responsibility of fixing it.

Posted on April 9, 2018 at 6:30 AM • 107 Comments