Since ZMap's debut in 2013, networking and security researchers have used the open-source scanner to write hundreds of research papers that study Internet behavior. In addition, ZMap has been adopted by the security industry to build new classes of enterprise security and compliance products. Over the past decade, much of ZMap's behavior---ranging from its pseudorandom IP generation to its packet construction---has evolved as we have learned more about how to scan the Internet. In this work, we quantify ZMap's adoption over the ten years since its release, describe its modern behavior (and the measurements that motivated changes), and offer lessons from releasing and maintaining ZMap for future tools.

Active DNS measurement is fundamental to understanding and improving the DNS ecosystem. However, the absence of an extensible, high-performance, and easy-to-use DNS toolkit has limited both the reproducibility and coverage of DNS research. In this paper, we introduce ZDNS, a modular and open-source active DNS measurement framework optimized for large-scale research studies of DNS on the public Internet. We describe ZDNS's architecture, evaluate its performance, and present two case studies that highlight how the tool can be used to shed light on the operational complexities of DNS. We hope that ZDNS will enable researchers to better---and in a more reproducible manner---understand Internet behavior.

Fast IPv4 scanning has enabled researchers to answer a wealth of security and networking questions. Yet, despite widespread use, there has been little validation of the methodology's accuracy, including whether a single scan provides sufficient coverage. In this paper, we analyze how scan origin affects the results of Internet-wide scans by completing three HTTP, HTTPS, and SSH scans from seven geographically and topologically diverse networks. We find that individual origins miss an average 1.6-8.4% of HTTP, 1.5-4.6% of HTTPS, and 8.3-18.2% of SSH hosts. We analyze why origins see different hosts, and show how permanent and temporary blocking, packet loss, geographic biases, and transient outages affect scan results. We discuss the implications for scanning and provide recommendations for future studies.

Certificate Authorities (CAs) regularly make mechanical errors when issuing certificates. To quantify these errors, we introduce ZLint, a certificate linter that codifies the policies set forth by the CA/Browser Forum Baseline Requirements and RFC 5280 that can be tested in isolation. We run ZLint on browser-trusted certificates in Censys and systematically analyze how well CAs construct certificates. We find that the number errors has drastically reduced since 2012. In 2017, only 0.02% of certificates have errors. However, this is largely due to a handful of large authorities that consistently issue correct certificates. There remains a long tail of small authorities that regularly issue non-conformant certificates. We further find that issuing certificates with errors is correlated with other types of mismanagement and for large authorities, browser action. Drawing on our analysis, we conclude with a discussion on how the community can best use lint data to identify authorities with worrisome organizational practices and ensure long-term health of the Web PKI.

Industrial control systems have become ubiquitous, enabling the remote, electronic control of physical equipment and sensors. Originally designed to operate on closed networks, the protocols used by these devices have no built-in security. However, despite this, an alarming number of systems are connected to the public Internet and an attacker who finds a device often can cause catastrophic damage to physical infrastructure. We consider two aspects of ICS security in this work: (1) what devices have been inadvertently exposed on the public Internet, and (2) who is searching for vulnerable systems. First, we implement five common SCADA protocols in ZMap and conduct a survey of the public IPv4 address space finding more than 60K publicly accessible systems. Second, we use a large network telescope and high-interaction honeypots to find and profile actors searching for devices. We hope that our findings can both motivate and inform future work on securing industrial control systems.

We investigate the security of Diffie-Hellman key exchange as used in popular Internet protocols and find it to be less secure than widely believed. First, we present Logjam, a novel flaw in TLS that lets a man-in-the-middle downgrade connections to "export-grade" Diffie-Hellman. To carry out this attack, we implement the number field sieve discrete log algorithm. After a week-long precomputation for a specified 512-bit group, we can compute arbitrary discrete logs in that group in about a minute. We find that 82% of vulnerable servers use a single 512-bit group, allowing us to compromise connections to 7% of Alexa Top Million HTTPS sites. In response, major browsers are being changed to reject short groups. We go on to consider Diffie-Hellman with 768- and 1024-bit groups. We estimate that even in the 1024-bit case, the computations are plausible given nation-state resources. A small number of fixed or standardized groups are used by millions of servers; performing precomputation for a single 1024-bit group would allow passive eavesdropping on 18% of popular HTTPS sites, and a second group would allow decryption of traffic to 66% of IPsec VPNs and 26% of SSH servers. A close reading of published NSA leaks shows that the agency's attacks on VPNs are consistent with having achieved such a break. We conclude that moving to stronger key exchange methods should be a priority for the Internet community.

Fast Internet-wide scanning has opened new avenues for security research, ranging from uncovering widespread vulnerabilities in random number generators to tracking the evolving impact of Heartbleed. However, this technique still requires significant effort: even simple questions, such as, "What models of embedded devices prefer CBC ciphers?", require developing an application scanner, manually identifying and tagging devices, negotiating with network administrators, and responding to abuse complaints. In this paper, we introduce Censys, a public search engine and data processing facility backed by data collected from ongoing Internet-wide scans. Designed to help researchers answer security-related questions, Censys supports full-text searches on protocol banners and querying a wide range of derived fields (e.g., 443.https.cipher). It can identify specific vulnerable devices and networks and generate statistical reports on broad usage patterns and trends. Censys returns these results in sub-second time, dramatically reducing the effort of understanding the hosts that comprise the Internet. We present the search engine architecture and experimentally evaluate its performance. We also explore Censys's applications and show how questions asked in recent studies become simple to answer.

The SMTP protocol is responsible for carrying some of users' most intimate communication, but like other Internet protocols, authentication and confidentiality were added only as an afterthought. In this work, we present the first report on global adoption rates of SMTP security extensions, including: STARTTLS, SPF, DKIM, and DMARC. We present data from two perspectives: SMTP server configurations for the Alexa Top Million domains, and over a year of SMTP connections to and from Gmail. We find that the top mail providers (e.g., Gmail, Yahoo, and Outlook) all proactively encrypt and authenticate messages. However, these best practices have yet to reach widespread adoption in a long tail of over 700,000 SMTP servers, of which only 35% successfully configure encryption, and 1.1% specify a DMARC authentication policy. This security patchwork---paired with SMTP policies that favor failing open to allow gradual deployment---exposes users to attackers who downgrade TLS connections in favor of cleartext and who falsify MX records to reroute messages. We present evidence of such attacks in the wild, highlighting seven countries where more than 20% of inbound Gmail messages arrive in cleartext due to network attackers.

Delay Tolerant Networks (DTNs) composed of commodity mobile devices have the potential to support communication applications resistant to blocking and censorship, as well as certain types of surveillance. We analyze the performance and energy consumption of such a network, and consider the impact of random and targeted denial-of-service and censorship attacks. To gather wireless connectivity traces for a DTN composed of human-carried commodity smartphones, we implemented and deployed a prototype DTN-based micro-blogging application, called 1am, in a college town. We analyzed the system during a time period with 111 users. Although the study provided detailed enough connectivity traces to enable analysis, message posting was too infrequent to draw strong conclusions based on user-initiated messages, alone. We therefore simulated more frequent message initiations and used measured connectivity traces to analyze message propagation. Using a flooding protocol, we found that with an adoption rate of 0.2% of a college town's student and faculty population, the median one-week delivery rate is 85% and the median delivery delay is 13 hours. We also found that the network delivery rate and delay are robust to denial-of service and censorship attacks eliminating more than half of the participants. Using a measurement-based energy model, we also found that the DTN system would use less than 10.0% of a typical smartphone's battery energy per day in a network of 2,500 users.

The Heartbleed vulnerability took the Internet by surprise in April 2014. The vulnerability, one of the most consequential since the advent of the commercial Internet, allowed attackers to remotely read protected memory from an estimated 24--55% of popular HTTPS sites. In this work, we perform a comprehensive, measurement-based analysis of the vulnerability's impact, including (1) tracking the vulnerable population, (2) monitoring patching behavior over time, (3) assessing the impact on the HTTPS certificate ecosystem, and (4) exposing real attacks that attempted to exploit the bug. Furthermore, we conduct a large-scale vulnerability notification experiment involving 150,000 hosts and observe a nearly 50% increase in patching by notified hosts. Drawing upon these analyses, we discuss what went well and what went poorly, in an effort to understand how the technical community can respond more effectively to such events in the future.