Thursday, May 4, 2017

Mailinator and the Recent Google Docs Phishing Attack

Yesterday (Wednesday, May 3rd), someone launched a clever phishing attack against Google users.

They wrote a little application and falsely named it "Google Docs." Given the chance, it compromises your Gmail account and reads your contacts list. Then, using Google’s own email servers, it sends itself to all your contacts.

This kind of attack lives or dies by how successful it is at getting people to believe it and click through. If no one clicks, or if no one grants it the permissions it asks for, then it doesn't spread and nothing happens. But every time someone clicks through and grants the app the access it wants, the attack multiplies by the number of times it propagates.

The attack was very successful at getting people to click through and grant it permissions, and it spread very quickly.

But it did so in a quirky way, one that made Mailinator one of its victims. It emailed itself from one victim's account to the next set of targets by BCC (blind carbon-copy). If you received the email, it came to you that way. Your email address wasn't in the TO field, it was in the BCC field.

That's a little bit clever. Since the BCC: is blind, the victim doesn't see themselves included in a long list of recipients (many of whom they don't recognize), which is often a big clue that the email is nefarious (or some dumb joke that a clueless relative has forwarded to everyone they know). Any one attack email might have been BCC'd to dozens of new victims.


When you send an email, you can't just BCC people. To make it legit, there has to be something in the TO field. Since all the victim's contacts were in the BCC field, the address the emails are TO isn't part of the attack. The attacker just needs a dumping ground. Something that looks just-so to the target; not a name that stands out for being unfamiliar, but also a real address that works and won’t bounce. That will improve deliverability and believability. So who did the attacker choose to send these emails to?

hhhhhhhhhhhhhhhh@mailinator.com

What is the significance of that inbox? As far as we can tell, there isn't one. Since all addresses already exist at Mailinator, using your finger to hold down a key for a bit generates a completely innocuous - but usable - inbox name. The significance of using mailinator.com is clear: our well-deserved reputation for successfully handling high volumes of incoming email.

In effect, they were relying on Mailinator’s proven ability to receive lots of email.

From the attacker's perspective, it doesn't matter much what is in the TO field. They just need to make sure that the emails get out the door. Mailinator is very good at receiving emails, the attack spread very quickly, and very soon the hhhhhhhhhhhhhhhh inbox at Mailinator was getting thousands of emails.

We noticed the activity early on, and shut down the inbound stream of emails to the hhhhhhhhhhhhhhhh inbox. Unfortunately, this did nothing to stop the attack. That's because nothing about the attack was happening via Mailinator. Mailinator was simply another recipient of the email. The attack could not propagate itself via Mailinator but it sure could send us email when other people propagated it. By the time we shut down the inbound stream, hundreds of thousands of emails to hhhhhhhhhhhhhhhh had shoved their way through our system. Any one inbox at Mailinator only shows 50 emails, and that inbox was (at peak) receiving a few thousand emails per second. Because the volume was so large, it wasn’t possible to read any of these emails at Mailinator. Before anyone could have clicked on one to read it, Mailinator had expired it to make room for thousands more that were pouring in.

The emails to hhhhhhhhhhhhhhhh were of no consequence to the attack. Even the name was irrelevant.

Strangely, we've seen a few articles mentioning that that inbox was the sender. Most even showed a screenshot on the same page contradicting that idea with the Mailinator address clearly in the TO field and one of the victim's contacts as the sender.

Just to be clear: the Gmail phishing attack sent email to a victim’s contacts using Google’s email servers and all emails were FROM another Google user. Each time the attack propagated, it also emailed TO Mailinator as a receiver, nothing more. None of those emails came from Mailinator, they came from Gmail.

It's true that all of the attack emails were TO: hhhhhhhhhhhhhhhh@mailinator.com, but it seems pretty clear that if you're looking for where the emails came from, you'd want to look at the FROM: field.

Additionally, the emails can't be from Mailinator because Mailinator can't send email. It’s a receive-only service. The Mailinator system was not part of the attack - it was just a recipient like all the other victims. The only difference between us and the other recipients was that we received hundreds of thousands of these emails in a very short time.

If you visit that inbox now (here's a link to hhhhhhhhhhhhhhhh inbox - don't worry, it's safe to click, it will take you right to Mailinator), you’ll notice no such phishing emails. We turned the inbound stream of attack emails off very soon after the phishing scam started.

Service to our legitimate users was uninterrupted.

Mailinator remains, as always, the best place to get a free, disposable email. We can’t prevent people from sending email to us (receiving email is the whole point of the service!) and we still love our regular users: thousands of QA Teams that send us millions of test emails, and you - whenever you want to protect your real email address from spam.

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