The malicious actors behind a recently launched phishing campaign aimed at American Express customers leveraged a weakness in email authentication systems to increase the chances of their bogus messages reaching their destination.
According to Trend Micro, the campaign started on March 4 and it ended on March 11. Over 430,000 phishing emails purporting to come from American Express had been sent out from a total of more than 4.600 IP addresses spread out across 120 countries. Nearly two thirds of the spam-sending IP addresses were located in the United States.
Phishing campaigns targeting the customers of financial organizations are not uncommon. However, this particular spam run might have been more successful than others thanks to a loophole in the way email verification systems handle messages from .gov top-level domains (TLDs).
DomainKeys Identified Mail (DKIM) and Sender Policy Framework (SPF) are email validation systems designed to detect email spoofing by allowing mail exchangers to ensure that incoming emails from a domain are authorized by that particular domain’s administrator. SPF and DKIM are configured for each domain through DNS TXT records.
Organizations often register their domain name on several extensions (.net, .org, .info) and announce SPF/DKIM records to prevent abuse.
The phishing emails sent out in the American Express campaign appeared to originate from email addresses such as AmericanExpress(at)welcome.aexp.com, fraud(at)americanexpress.com, and noreplay(at)americanexpress.gov.
Both aexp.com and americanexpress.com are registered by American Express and spoofed messages from these domains will likely not get past email authentication systems because SPF/DKIM records are published for them.
However, americanexpress.gov is not owned by the financial services company because only government organizations can register .gov domains. The problem, according to experts, is that spoofed emails coming from .gov domains are not handled properly by the email validation system.
“An SPF verification attempt would return none instead of fail, as there is no SPF record to authenticate at all (the domain is not even registered). Therefore, an email system checking for SPF records would not rule this message to be spam on those grounds alone. This may increase the risk that users would receive these spammed messages,” Trend Micro researchers explained in a blog post.
Experts have pointed out that while DKIM and SPF can be highly useful for blocking malicious emails, organizations should not rely solely on these systems for email security.