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A recent data breach at the email marketing platform vendor Mailchimp has prompted a warning from the Department of Health and Human Services’ Health Sector Cybersecurity Coordination Center (HC3) about the risk of phishing attacks using the platform.
The breach came to light when the cryptocurrency hardware wallet provider, Trezor, investigated a phishing campaign targeting its customers that used the email addresses registered to Trezor accounts, which uncovered a data breach at Mailchimp.
Mailchimp’s investigation confirmed that threat actors had successfully compromised internal accounts of its customer support and account administration teams, and while those accounts have now been secured, the attackers were able to gain access to the accounts of 300 Mailchimp users and were able to extract audience data from 102 of those accounts. API keys were also obtained by the attackers that allow them to create email campaigns for use in phishing attacks without having to access customer portals.
Since accounts used by Mailchimp customers to send marketing campaigns such as newsletters may be whitelisted by subscribers, any phishing campaigns conducted using the compromised accounts may see the emails delivered to inboxes. HC3 says it is only aware of one phishing campaign being conducted using a compromised account, which targeted users in the cryptocurrency and financial sectors, but there is a risk that campaigns could also be conducted targeting users in the healthcare and public health (HPH) sector.
HC3 has recommended organizations in the HPH sector take steps to mitigate the threat. HC3 says the best defense is user awareness training since phishing emails will come from a legitimate and trusted sender. Employees should be made aware of the threat and be instructed to be wary of any emails sent via Mailchimp. While phishing emails could be sent, malware may also be delivered. Antivirus software should be implemented, network intrusion prevention systems are beneficial, and HC3 also suggests using web filters to restrict access to web content that is not necessary for business operations.
Anti-spoofing and other email authentication mechanisms are also recommended. These include performing validity checks of the sender domain using SPK, checking the integrity of messages using DKIM, and checking to make sure the sender is authorized to use the domain using DMARC.