A massive email spamming campaign has been detected that is generating up to 5 million emails per day that direct recipients of the emails to a variety of scam sites. The emails are sent through hijacked subdomains and domains of trusted companies, which help these emails evade email security solutions and be delivered to inboxes. Companies that have had domains and subdomains hijacked include eBay, CBS, McAfee, MSN, and Symantec.

Email security solutions perform a range of checks on inbound emails, including reputation checks on the senders of emails. If a domain is trusted and has not previously been associated with spamming, these checks – using SPK, DKIM, and DMARC – are likely to be passed, resulting in the emails being delivered to end users. The use of these legitimate domains also makes it harder for end users to determine whether the messages are genuine. Security awareness training programs often teach end users to check the sender of the email and make sure that it matches the company being spoofed. If the domain is eBay, and the email uses eBay branding, end users are likely to think that the communication is genuine. These emails include links to websites that generate fraudulent ad revenue, and often several redirects occur before the user lands on the destination scam or phishing site.

The ‘SubdoMailing’ campaign was identified by researchers at Guardio Labs, with the legitimate domains typically hijacked through SPF record exploitation or CNAME hijacking. The former involves searching for domains that use the ‘include’ configuration option that points to external domains that are no longer registered. Those domains are then registered by the threat actor and the SPF records are changed to authorize the use of their own email servers. When those servers are used to send emails, they appear to have been sent by the targeted brand, such as eBay.

With CNAME hijacking, scans are conducted to identify subdomains of reputable brands with CNAME records that point to external domains that are no longer registered. The threat actor then registers those domains, SPF records are injected, and emails can be sent from their email servers to show that they have been sent by a legitimate company. By hijacking huge numbers of domains and subdomains, the threat actor is able to conduct massive spamming campaigns. The researchers identified more than 13,000 subdomains and more than 8,000 domains that were used in the campaign, with more than 1000 residential lines used and almost 22,000 unique IPs. The researchers developed a tool to allow domain owners to check whether their own domains have been hijacked and take action to stop that abuse. An advanced spam filter is required to block the messages that are set from these hijacked domains and subdomains – one that does not rely on SPF, DKIM, and DMARC for identifying spam emails.