You probably know MailChimp either as an email newsletter service, or the company that seems to have adverts on every single podcast you've ever listened to. Hackers recently jumped on that popularity, and managed to send out emails containing malicious links to subscribers of various different companies.
The incident shows that hackers will likely use whatever distribution channels they can in an attempt to spread their malware and turn a profit.
"Here's your invoice! We appreciate your prompt payment," one email sent by news site Business News Australia reads, and claims to be affiliated with accounting software Quickbooks.
Troy Hunt, an Australian security researcher and owner of breach notification site Have I Been Pwned?, sent Motherboard a copy of the email that he had received from a source. According to the email, it was sent by an administrator account at the news website.
The "View Invoice" button leads to a .zip file, which, according to scans on malware analysis site Virus Total, is malicious.
Companies and websites sometimes outsource their newsletter distribution to another company, to handle the infrastructure and headaches of firing out tens or hundreds of thousands of emails at a time. In this case, that was MailChimp, according to another apparent email from Business News Australia.
"This morning our MailChimp subscriber database was hacked and a fake invoice (Inoice 00317) [sic] was sent to our list," the email reads, according to a screenshot tweeted by Hunt.
"Please disregard and delete this email. You have not been charged," it adds. Camilla Jansen, managing editor of Business News Australia, told Motherboard in an email "We're waiting to find out more."
But it seems other companies have been affected too. One Twitter user uploaded an apparent screenshot of a near identical email sent to subscribers of the Sit Down Comedy Club in Brisbane's mailing list.
Motherboard sent an email to The Sit Down Comedy Club, asking for comment, and immediately received the following, perhaps automated, reply.
"IF YOU RECEIVE AN EMAIL WITH THE TITLE - Inoice 00317 from Sit Down Comedy Club Pty Ltd - PLEASE DELETE the email you received, we do not use Quickbooks. It is SPAM and do not open it," the email reads.
"We are trying to get to the bottom of this at the moment," it adds.
Another Twitter user uploaded a screenshot of an apparent email from Jim's Building Inspections, also an Australia-based company. The firm blamed the issue, without any evidence, on a "known cyber terrorist."
MailChimp told Motherboard in a statement that "Early this morning MailChimp's normal compliance processes identified and disabled a small number of individual accounts sending fake invoices. We have investigated the situation and have found no evidence that MailChimp has been breached. The affected accounts have been disabled, and fraudulent activity has stopped."
The company would not say what the exact issue was, but MailChimp's statement also strong encouraged users to setup two-factor authentication, implying that the problem might have been password reuse.
Update: This article has been updated to include MailChimp's statement, which was sent to Motherboard after publication.