Component-based software design has been widely adopted as a way to manage complexity and improve reusability. The approach divides complex systems into smaller modules that can be independently created and reused in different systems. One then combines these components together to achieve desired functionality. Modern software systems are commonly built using components made by different developers who work independently.
While having wide-ranging benefits, the security research community has recognized that this practice also introduces security concerns. In particular, when faced with crafted adversarial inputs, different components can have inconsistent interpretations when operating on the input in sequence. Attackers can exploit such inconsistencies to bypass security policies and subvert the system’s operation.
In this work we provide a case study of such composition issues in the context of email (SMTP) sender authentication. We present 18 attacks for widely used email services to bypass their sender authentication checks by misusing combinations of SPF, DKIM and DMARC, which are crucial defenses against email phishing and spear-phishing attacks. Leveraging these attack techniques, an attacker can impersonate arbitrary senders without breaking email authentication, and even forge DKIM-signed emails with a legitimate site’s signature.
