Benjamin VanderSloot, Sergey Frolov, Jack Wampler, Sze Chuen Tan, Irv Simpson, Michalis Kallitsis, J. Alex Halderman, Nikita Borisov, and Eric Wustrow

Refraction networking is a next-generation censorship circumvention approach that locates proxy functionality in the network itself, at participating ISPs or other network operators. Following years of research and development and a brief pilot, we established the world’s first production deployment of a Refraction Networking system. Our deployment uses a highperformance implementation of the TapDance protocol and is enabled as a transport in the popular circumvention app Psiphon. It uses TapDance stations at four physical uplink locations of a mid-sized ISP, Merit Network, with an aggregate bandwidth of 140 Gbps. By the end of 2019, our system was enabled as a transport option in 559,000 installations of Psiphon, and it served upwards of 33,000 unique users per month. This paper reports on our experience building the deployment and operating it for the first year. We describe how we overcame engineering challenges, present detailed performance metrics, and analyze how our system has responded to dynamic censor behavior. Finally, we review lessons learned from operating this unique artifact and discuss prospects for further scaling Refraction Networking to meet the needs of censored users.

Sergey Frolov, Jack Wampler, Eric Wustrow

Censorship circumvention proxies have to resist active probing attempts, where censors connect to suspected servers and attempt to communicate using known proxy protocols. If the server responds in a way that reveals it is a proxy, the censor can block it with minimal collateral risk to other non-proxy services. Censors such as the Great Firewall of China have previously been observed using basic forms of this technique to find and block proxy servers as soon as they are used. In response, circumventors have created new “probe-resistant” proxy protocols, including obfs4, Shadowsocks, and Lampshade, that attempt to prevent censors from discovering them. These proxies require knowledge of a secret in order to use, and the servers remain silent when probed by a censor that doesn’t have the secret in an attempt to make it more difficult for censors to detect them. In this paper, we identify ways that censors can still distinguish such probe-resistant proxies from other innocuous hosts on the Internet, despite their design. We discover unique TCP behaviors of five probe-resistant protocols used in popular circumvention software that could allow censors to effectively confirm suspected proxies with minimal false positives. We evaluate and analyze our attacks on hundreds of thousands of servers collected from a 10 Gbps university ISP vantage point over several days as well as active scanning using ZMap. We find that our attacks are able to efficiently identify proxy servers with only a handful of probing connections, with negligible false positives. Using our datasets, we also suggest defenses to these attacks that make it harder for censors to distinguish proxies from other common servers, and we work with proxy developers to implement these changes in several popular circumvention tools.

Sergey Frolov, Jack Wampler, Sze Chuen Tan, J. Alex Halderman, Nikita Borisov, Eric Wustrow

Refraction Networking (formerly known as “Decoy Routing”) has emerged as a promising next-generation approach for circumventing Internet censorship. Rather than trying to hide individual circumvention proxy servers from censors, proxy functionality is implemented in the core of the network, at cooperating ISPs in friendly countries. Any connection that traverses these ISPs could be a conduit for the free flow of information, so censors cannot easily block access without also blocking many legitimate sites. While one Refraction scheme, TapDance, has recently been deployed at ISP-scale, it suffers from several problems: a limited number of “decoy” sites in realistic deployments, high technical complexity, and undesirable tradeoffs between performance and observability by the censor. These challenges may impede broader deployment and ultimately allow censors to block such techniques. We present Conjure, an improved Refraction Networking approach that overcomes these limitations by leveraging unused address space at deploying ISPs. Instead of using real websites as the decoy destinations for proxy connections, our scheme connects to IP addresses where no web server exists leveraging proxy functionality from the core of the network. These phantom hosts are difficult for a censor to distinguish from real ones, but can be used by clients as proxies. We define the Conjure protocol, analyze its security, and evaluate a prototype using an ISP testbed. Our results suggest that Conjure can be harder to block than TapDance, is simpler to maintain and deploy, and offers substantially better network performance.

Jack Wampler, Ian Martiny, Eric Wustrow

Recently, the Spectre and Meltdown attacks revealed serious vulnerabilities in modern CPU designs, allowing an attacker to exfiltrate data from sensitive programs. These vulnerabilities take advantage of speculative execution to coerce a processor to perform computation that would otherwise not occur, leaking the resulting information via side channels to an attacker. In this paper, we extend these ideas in a different direction, and leverage speculative execution in order to hide malware from both static and dynamic analysis. Using this technique, critical portions of a malicious program’s computation can be shielded from view, such that even a debugger following an instruction-level trace of the program cannot tell how its results were computed. We introduce ExSpectre, which compiles arbitrary malicious code into a seemingly-benign payload binary. When a separate trigger program runs on the same machine, it mistrains the CPU’s branch predictor, causing the payload program to speculatively execute its malicious payload, which communicates speculative results back to the rest of the payload program to change its real-world behavior. We study the extent and types of execution that can be performed speculatively, and demonstrate several computations that can be performed covertly. In particular, within speculative execution we are able to decrypt memory using AES-NI instructions at over 11 kbps. Building on this, we decrypt and interpret a custom virtual machine language to perform arbitrary computation and system calls in the real world. We demonstrate this with a proof-of-concept dial back shell, which takes only a few milliseconds to execute after the trigger is issued. We also show how our corresponding trigger program can be a pre-existing benign application already running on the system, and demonstrate this concept with OpenSSL driven remotely by the attacker as a trigger program. ExSpectre demonstrates a new kind of malware that evades existing reverse engineering and binary analysis techniques. Because its true functionality is contained in seemingly unreachable dead code, and its control flow driven externally by potentially any other program running at the same time, ExSpectre poses a novel threat to state-of-the-art malware analysis techniques.

Aimee Coughlin, Greg Cusack, Jack Wampler, Eric Keller, Eric Wustrow

Modern CPU designs are beginning to incorporate secure hardware features, but leave developers with little control over both the set of features and when and whether updates are available. Reconfigurable logic (e.g., FPGAs) has been proposed as an alternative as it is both hardware, so can have similar capabilities at a reasonable performance degradation, and programmable, allowing customization of the secure hardware. This programmability, however, opens new attack vectors that allow an adversary to re-program the FPGA. Past attempts to solve this rely on a party maintaining a shared key with the FPGA, but these business processes to keep that key secret have been shown to be quite vulnerable. In this paper, we propose a new mechanism which eliminates the trust dependence on third party processes. This new mechanism consists of a self-provisioning stage, where keys are generated internal to the FPGA and never exposed externally, coupled with a secure update mechanism which allows updates to be governed by a policy defined by the secure hardware application. To demonstrate, we fully implemented these mechanisms on a Xilinx Zynq UltraScale+ FPGA along with an example secure co-processor with remote attestation with a flexible root of trust (in contrast to Intel SGX which fixes the root of trust to be Intel). Our performance evaluation of two applications, a password manager and a contact matching application, illustrates using FPGAs is practical.