On Using Application-Layer Middlebox Protocols for Peeking Behind NAT Gateways

—Typical port scanning approaches do not achieve a full coverage of all devices connected to the Internet as not all devices are directly reachable via a public (IPv4) address: due to IP address space exhaustion, firewalls, and many other reasons, an end-to-end connectivity is not achieved in today’s Internet anymore. Especially Network Address Translation (NAT) is widely deployed in practice and it has the side effect of “hiding” devices from being scanned. Some protocols, however, require end-to-end connectivity to function properly and hence several methods were developed in the past to enable crossing network borders. In this paper, we explore how an attacker can take advantage of such application-layer middlebox protocols to access devices located behind these gateways. More specifically, we investigate different methods for identifying such devices by using only legitimate protocol features. We categorize the available protocols into two classes: First, there are persistent protocols that are typically port-forwarding based. Such protocols are used to allow local network devices to open and forward external ports to them. Second, there are non-persistent protocols that are typically proxy-based to route packets between network edges, such as HTTP and SOCKS proxies. We perform a comprehensive, Internet-wide analysis to obtain an accurate overview of how prevalent and widespread such protocols are in practice. Our results indicate that hundreds of thousands of hosts are vulnerable for different types of attacks, e.g., we detect over 400,000 hosts that are likely vulnerable for attacks involving the UPnP IGD protocol. More worrisome, we find empirical evidence that attackers are already actively exploiting such protocols in the wild to access devices located behind NAT gateways. Amongst other findings, we discover that at least 24% of all open Internet proxies are misconfigured to allow accessing hosts on non-routable addresses.