Free and open exchange of information on the Internet is at risk: more than 60 countries practice some form of Internet censorship, and both the number of countries practicing censorship and the proportion of Internet users who are subject to it are on the rise. Understanding and mitigating these threats to Internet freedom is a continuous technological arms race with many of the most influential governments and corporations. By its very nature, Internet censorship varies drastically from region to region, which has impeded nearly all efforts to observe and fight it on a global scale. Researchers and developers in one country may find it very difficult to study censorship in another; this is particularly true for those in North America and Europe attempting to study notoriously pervasive censorship in Asia and the Middle East. This dissertation develops techniques and systems that empower users in one country, or bystanders, to assist in the measurement and circumvention of Internet censorship in another. Our work builds from the observation that there are people everywhere who are willing to help us if only they knew how. First, we develop Encore, which allows webmasters to help study Web censorship by collecting measurements from their sites' visitors. Encore leverages weaknesses in cross-origin security policy to collect measurements from a far more diverse set of vantage points than previously possible. Second, we build Collage, a technique that uses the pervasiveness and scalability of user-generated content to disseminate censored content. Collage's novel communication model is robust against censorship that is significantly more powerful than governments use today. Together, Encore and Collage help people everywhere study and circumvent Internet censorship.

A significant fraction of today's Internet traffic is associated with popular web sites such as YouTube, Netflix or Facebook. In recent years, major Internet websites have become more complex as they incorporate a larger number and more diverse types of objects (e.g. video, audio, code) along with more elaborate ways from multiple servers. These not only affect the loading time of pages but also determine the pattern of resulting traffic on the Internet. In this thesis, we characterize the complexity of major Internet websites through large-scale measurement and analysis. We identify thousands of the most popular Internet websites from multiple locations and characterize their complexities. We examine the effect of the relative popularity ranking and business type of the complexity of websites. Finally we compare and contrast our results with a similar study conducted 4 years earlier and report on the observed changes in different aspects.

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The Domain Name System (DNS) has been frequently abused for Distributed Denial of Service (DDoS) attacks and cache poisoning because it relies on the User Datagram Protocol (UDP). Since UDP is connection-less, it is trivial for an attacker to spoof the source of a DNS query or response. DNS Cookies, a protocol standardized in 2016, add pseudo-random values to DNS packets to provide identity management and prevent spoofing attacks. This work finds that 30% of popular authoritative servers and open recursive resolvers fully support cookies and that 10% of recursive clients send cookies. Despite this, DNS cookie use is rarely enforced as it is non-trivial to ascertain whether a given client intends to fully support cookies. We also show that 80% of clients and 99% of servers do not change their behavior when encountering a missing or illegitimate cookie. This paper presents a new protocol to allow cookie enforcement: DNS Protocol Advertisement Records (DPAR). Advertisement records allow DNS clients intending to use cookies to post a public record in the reverse DNS zone stating their intent. DNS servers may then lookup this record and require a client to use cookies as directed, in turn preventing an attacker from sending spoofed messages without a cookie. In this paper, we define the specification for DNS Protocol Advertisement Records, considerations that were made, and comparisons to alternative approaches. We additionally estimate the effectiveness of advertisements in preventing DDoS attacks and the expected burden to DNS servers. Advertisement records are designed as the next step to strengthen the existing support of DNS Cookies by enabling strict enforcement of client cookies.

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Following proper guidelines and recommendations are crucial in software security, which is mostly obstructed by accidental human errors. Automatic screening tools have great potentials to reduce the gap between the theory and the practice. However, the goal of scalable automated code screening is largely hindered by the practical difficulty of reducing false positives without compromising analysis quality. To enable compile-time security checking of cryptographic vulnerabilities, I developed highly precise static analysis tools (CryptoGuard and TaintCrypt) that developers can use routinely. The main technical enabler for CryptoGuard is a set of detection algorithms that refine program slices by leveraging language-specific insights, where TaintCrypt relies on symbolic execution-based path-sensitive analysis to reduce false positives. Both CryptoGuard and TaintCrypt uncovered numerous vulnerabilities in real-world software, which proves the effectiveness. Oracle has implemented our cryptographic code screening algorithms for Java in its internal code analysis platform, Parfait, and detected numerous vulnerabilities that were previously unknown. I also designed a specification language named SpanL to easily express rules for automated code screening. SpanL enables domain experts to create domain-specific security checking. Unfortunately, tools and guidelines are not sufficient to ensure baseline security in internet-wide ecosystems. I found that the lack of proper compliance checking induced a huge gap in the payment card industry (PCI) ecosystem. I showed that none of the PCI scanners (out of 6), we tested are fully compliant with the guidelines, issuing certificates to merchants that still have major vulnerabilities. Consequently, 86% (out of 1,203) of the e-commerce websites we tested, are non-compliant. To improve the testbeds in the light of our work, the PCI Security Council shared a copy of our PCI measurement paper to the dedicated companies that host, manage, and maintain the PCI certification testbeds. / Doctor of Philosophy / Automatic screening tools have great potentials to reduce the gap between the theory and the practice of software security. However, the goal of scalable automated code screening is largely hindered by the practical difficulty of reducing false positives without compromising analysis quality. To enable compile-time security checking of cryptographic vulnerabilities, I developed highly precise static analysis tools (CryptoGuard and TaintCrypt) that developers can use routinely. Both CryptoGuard and TaintCrypt uncovered numerous vulnerabilities in real-world software, which proves the effectiveness. Oracle has implemented our cryptographic code screening algorithms for Java in its internal code analysis platform, Parfait, and detected numerous vulnerabilities that were previously unknown. I also designed a specification language named SpanL to easily express rules for automated code screening. SpanL enables domain experts to create domain-specific security checking. Unfortunately, tools and guidelines are not sufficient to ensure baseline security in internet-wide ecosystems. I found that the lack of proper compliance checking induced a huge gap in the payment card industry (PCI) ecosystem. I showed that none of the PCI scanners (out of 6), we tested are fully compliant with the guidelines, issuing certificates to merchants that still have major vulnerabilities. Consequently, 86% (out of 1,203) of the e-commerce websites we tested, are non-compliant. To improve the testbeds in the light of our work, the PCI Security Council shared a copy of our PCI measurement paper to the dedicated companies that host the PCI certification testbeds.