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Some of the most successful and lucrative online scams employ a “low-and-slow” approach — avoiding detection or interference from researchers and law enforcement agencies by stealing small bits of cash from many people over an extended period. Here’s the story of a cybercrime group that compromises up to 100,000 email inboxes daily, and apparently does little else with this access except for siphon gift card and customer loyalty program data that can be sold online. The data in this story come from a trusted source in the security industry that has visibility into a network of hacked machines that fraudsters in just about every corner of the Internet are using to anonymize their malicious Web traffic. For the past three years, the source — we’ll call him “Bill” to preserve his requested anonymity — has been watching one group of threat actors that is mass-testing millions of usernames and passwords against the world’s major email providers day. Bill said he’s not sure where the passwords are coming from, but he assumes they are tied to various databases for compromised websites that get posted to password cracking and hacking forums on a regular basis. Bill said this criminal group averages between five and ten million email authentication attempts daily and comes away with anywhere from 50,000 to 100,000 working inbox credentials. In about half the cases the credentials are being checked via “IMAP,” which is an email standard used by email software clients like Mozilla’s Thunderbird and Microsoft Outlook. With his visibility into the proxy network, Bill can see whether or not an authentication attempt succeeds based on the network response from the email provider (e.g. mail server responds “OK” = successful access). You might think that whoever is behind such a sprawling crime machine would use their access to blast out spam, or conduct targeted phishing attacks against each victim’s contacts. But based on interactions that Bill has had with several large email providers so far, this crime gang merely uses a custom, automated scripts that periodically log in and search each inbox for digital items of value that can easily be resold. And they seem particularly focused on stealing gift card data. “Sometimes they’ll log in as much as two to three times a week for months at a time,” Bill said. “These guys are looking for low-hanging fruit — basically cash in your inbox. Whether it’s related to hotel or airline rewards or just Amazon gift cards after they successfully log in to the account their scripts start pilfering inboxes looking for things that could be of value.” How do the compromised email credentials break down in terms of ISPs and email providers? There are victims on nearly all major email networks, but Bill said several large Internet service providers (ISPs) in Germany and France are heavily represented in the compromised email account data. “With some of these international email providers we’re seeing something like 25,000 to 50,000 email accounts a day get hacked,” Bill said.  “I don’t know why they’re getting popped so heavily.” That may sound like a lot of hacked inboxes, but Bill said some of the bigger ISPs represented in his data have tens or hundreds of millions of customers. Measuring which ISPs and email providers have the biggest numbers of compromised customers is not so simple in many cases, nor is identifying companies with employees whose email accounts have been hacked. This kind of mapping is often more difficult than it used to be because so many organizations have now outsourced their email to cloud services like Gmail and Microsoft Office365 — where users can access their email, files, and chat records all in one place. In a December 2020 blog post about how Microsoft is moving away from passwords to more robust authentication approaches, the software giant said an average of one in every 250 corporate accounts is compromised each month. As of last year, Microsoft had nearly 240 million active users, according to this analysis. “To me, this is an important story because for years people have been like, yeah we know email isn’t very secure, but this generic statement doesn’t have any teeth to it,” Bill said. “I don’t feel like anyone has been able to call attention to the numbers that show why email is so insecure.” Bill says that in general companies have a great many more tools available for securing and analyzing employee email traffic when that access is funneled through a Web page or VPN, versus when that access happens via IMAP. “It’s just more difficult to get through the Web interface because on a website you have a plethora of advanced authentication controls at your fingertips, including things like device fingerprinting, scanning for HTTP header anomalies, and so on,” Bill said. “But what are the detection signatures you have available for detecting malicious logins via IMAP?” Microsoft declined to comment specifically on Bill’s research but said customers can block the overwhelming majority of account takeover efforts by enabling multi-factor authentication. 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