The cyber-criminals behind email attacks are well-researched and highly responsive to human behaviors and emotions, often seeking to evoke a specific reaction by leveraging topical information and current news. It’s therefore no surprise that attackers have attempted to latch onto COVID-19 in their latest effort to convince users to open their emails and click on seemingly benign links.
The latest email trend involves attackers who claim to be from the Center for Disease Control and Prevention, purporting to have emergency information about COVID-19. This is typical of a recent trend we’re calling ‘fearware’: cyber-criminals exploit a collective sense of fear and urgency, and coax users into clicking a malicious attachment or link. While the tactic is common, the actual campaigns contain terms and content that’s unique. There are a few patterns in the emails we’ve seen, but none reliably predictable enough to create hard and fast rules that will stop emails with new wording without causing false positives.
For example, looking for the presence of “CDC” in the email sender would easily fail when the emails begin to use new wording, like “WHO”. We’ve also seen a mismatch of links and their display text – with display text that reads “https://cdc.gov/[random-path]” while the actual link is a completely arbitrary URL. Looking for a pattern match on this would likely lead to false positives and would serve as a weak indicator at best.
The majority of these emails, especially the early ones, passed most of our customers’ existing defenses including Mimecast, Proofpoint, and Microsoft’s ATP, and were approved to be delivered directly to the end user’s inbox. Fortunately, these emails were immediately identified and actioned by Antigena Email, Darktrace’s Autonomous Response technology for the inbox.
Gateways: The Current Approach
Most organizations employ Secure Email Gateways (SEGs), like Mimecast or Proofpoint, which serve as an inline middleman between the email sender and the recipient’s email provider. SEGs have largely just become spam-detection engines, as these emails are obvious to spot when seen at scale. They can identify low-hanging fruit (i.e. emails easily detectable as malicious), but they fail to detect and respond when attacks become personalized or deviate even slightly from previously-seen attacks.
SEGs tend to use lists of ‘known-bad’ IPs, domains, and file hashes to determine an email’s threat level – inherently failing to stop novel attacks when they use IPs, domains, or files which are new and have not yet been triaged or reported as malicious.
When advanced detection methods are used in gateway technologies, such as anomaly detection or machine learning, these are performed after the emails have been delivered, and require significant volumes of near-identical emails to trigger. The end result is very often to take an element from one of these emails and simply blacklist it.
When a SEG can’t make the determination on these factors, they may resort to a technique known as sandboxing, which creates an isolated environment for testing links and attachments seen in emails. Alternatively, they may turn to basic levels of anomaly detection that are inadequate due to their lack of context of data outside of emails. For sandboxing, most advanced threats now typically employ evasion techniques like an activation time that waits until a certain date before executing. When deployed, the sandboxing attempts see a harmless file, not recognizing the sleeping attack waiting within.
Taking a sample COVID-19 email seen in a Darktrace customer’s environment, we saw a mix of domains used in what appears to be an attempt to avoid pattern detection. It would be improbable to have the domains used on a list of ‘known-bad’ domains anywhere at the time of the first email, as it was received a mere two hours after the domain was registered.
Antigena Email sits behind all other defenses, meaning we only see emails when those defenses fail to block a malicious email or deem an email is safe for delivery. In the above COVID-19 case, the first 5 emails were marked by MS ATP with a spam confidence score of 1, indicating Microsoft scanned the email and it was determined to be clean – so Microsoft took no action whatsoever.
Dan Fein. “How Antigena Email caught a fearware attack that bypassed the gateway”, Originally published on Darktrace, March 12, 2020. Accessed 3/12/20
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