A novel exploit method named Simple Mail Transfer Protocol (SMTP) smuggling has surfaced, enabling threat actors to send deceptive emails with fabricated sender addresses while circumventing security measures. This technique leverages vulnerabilities in SMTP servers globally, allowing threat actors to conduct targeted phishing attacks by sending malicious emails from arbitrary addresses. SMTP, a TCP/IP protocol for email communication, is exploited through inconsistencies in handling end-of-data sequences by outbound and inbound SMTP servers. This vulnerability has been identified in messaging servers from Microsoft, GMX, and Cisco, impacting SMTP implementations in Postfix and Sendmail.
The crux of SMTP smuggling lies in exploiting variations in how outbound and inbound SMTP servers handle end-of-data sequences, enabling threat actors to break out of the message data, insert arbitrary SMTP commands, and send separate emails. This technique draws inspiration from HTTP request smuggling, capitalizing on disparities in interpreting “Content-Length” and “Transfer-Encoding” HTTP headers. Security flaws in messaging servers from Microsoft, GMX, and Cisco are exploited, allowing threat actors to send emails that appear to originate from legitimate senders, eluding authenticity checks like DomainKeys Identified Mail (DKIM), Domain-based Message Authentication, Reporting and Conformance (DMARC), and Sender Policy Framework (SPF).
While Microsoft and GMX have addressed the issues, Cisco considers the findings a “feature” and maintains default configurations. As a result, inbound SMTP smuggling to Cisco Secure Email instances remains possible with default settings. To mitigate this, SEC Consult recommends Cisco users adjust their settings from “Clean” to “Allow” to prevent receiving spoofed emails with valid DMARC checks. The emergence of SMTP smuggling underscores the continuous evolution of cyber threats and the need for organizations to bolster email security measures against innovative attack vectors.
