The Unintended Consequences of Email Spam Prevention

To combat Domain Name System (DNS) cache poisoning attacks and exploitation of the DNS as amplifier in denial of service (DoS) attacks, many recursive DNS resolvers are configured as “closed” and refuse to answer queries made by hosts outside of their organization. In this work, we present a technique to induce DNS queries within an organization, using the organization’s email service and the Sender Policy Framework (SPF) spam-checking mechanism. We use our technique to study closed resolvers. Our study reveals that most closed DNS resolvers have deployed common DNS poisoning defense techniques such as source port and transaction ID randomization. However, we also find that SPF is often deployed in a way that allows an external attacker to cause the organization’s resolver to issue numerous DNS queries to a victim IP address by sending a single email to any address within the organization’s domain, thereby providing a potential DoS vector.