Cloud Strife: Mitigating the Security Risks of Domain-Validated Certificates

Infrastructure-as-a-Service (IaaS), and more generally the “cloud,” like Amazon Web Services (AWS) or Microsoft Azure, have changed the landscape of system operations on the Internet. Their elasticity allows operators to rapidly allocate and use resources as needed, from virtual machines, to storage, to bandwidth, and even to IP addresses, which is what made them popular and spurred innovation.

In this paper, we show that the dynamic component paired with recent developments in trust-based ecosystems (e.g., TLS certificates) creates so far unknown attack vectors. We discover a substantial number of stale DNS records that point to available IP addresses in clouds, yet, are still actively attempted to be accessed. We demonstrate that it is practical, and time and cost efficient for attackers to allocate IP addresses to which stale DNS records point. Considering the ubiquity of domain validation in trust ecosystems, like TLS certificates, an attacker can impersonate the service using a valid certificate trusted by all major operating systems and browsers. The attacker can then also exploit residual trust in the domain name for phishing, receiving and sending emails, or possibly distribute code to clients that load remote code from the domain (e.g., loading of native code by mobile apps, or JavaScript libraries by websites).

Even worse, an aggressive attacker could execute the attack in less than 70 seconds, well below common time-to-live (TTL) for DNS records. In turn, it means an attacker could exploit normal service migrations in the cloud to obtain a valid TLS certificate for domains owned and managed by others, and, worse, that she might not actually be bound by DNS records being (temporarily) stale, but that she can exploit caching instead.

We introduce a new authentication method for trust-based domain validation that mitigates staleness issues without incurring additional certificate requester effort by incorporating existing trust of a name into the validation process. Furthermore, we provide recommendations for domain name owners and cloud operators to reduce their and their clients’ exposure to DNS staleness issues and the resulting domain takeover attacks.