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✏Poster: Circumventing the GFW with TLS Record Fragmentation
ACM CCS, 2023
Abstract
State actors around the world censor the HTTPS protocol to block access to certain websites. While many circumvention strategies utilize the TCP layer only little emphasis has been placed on the analysis of TLS-a complex protocol and integral building block of HTTPS. In contrast to the TCP layer, circumvention methods on the TLS layer do not require root privileges since TLS operates on the application layer. With this proposal, we want to motivate a deeper analysis of TLS in regard to censorship circumvention techniques. To prove the existence of such techniques, we present TLS record fragmentation as a novel circumvention technique and circumvent the Great Firewall of China (GFW) using this technique. We hope that our research fosters collaboration between censorship and TLS researchers.Turning Attacks into Advantages: Evading HTTP Censorship with HTTP Request Smuggling
Free and Open Communications on the Internet (FOCI), 2024
Abstract
Many countries limit their residents' access to various websites. As a substantial number of these websites do not support TLS encryption, censorship of unencrypted HTTP requests remains prevalent. Accordingly, circumvention techniques can and have been found for the HTTP protocol. In this paper, we infer novel circumvention techniques on the HTTP layer from a web security vulnerability by utilizing HTTP request smuggling (HRS). To demonstrate the viability of our techniques, we collected various test vectors from previous work about HRS and evaluated them on popular web servers and censors in China, Russia, and Iran. Our findings show that HRS can be successfully employed as a censorship circumvention technique against multiple censors and web servers. We also discover a standard-compliant circumvention technique in Russia, unusually inconsistent censorship in China, and an implementation bug in Iran. The results of this work imply that censorship circumvention techniques can successfully be constructed from existing vulnerabilities. We conjecture that this implication provides insights to the censorship circumvention community beyond the viability of specific techniques presented in this work.TLS-Attacker: A Dynamic Framework for Analyzing TLS Implementations (2nd Place Impact Award)
ACSAC, 2024
Abstract
TLS-Attacker is an open-source framework for analyzing Transport Layer Security (TLS) implementations. The framework allows users to specify custom protocol flows and provides modification hooks to manipulate message contents. Since its initial publication in 2016 by Juraj Somorovsky, TLS-Attacker has been used in numerous studies published at well-established conferences and helped to identify vulnerabilities in well-known open-source TLS libraries. To enable automated analyses, TLS-Attacker has grown into a suite of projects, each designed as a building block that can be applied to facilitate various analysis methodologies. The framework still undergoes continuous improvements with feature extensions, such as DTLS 1.3 or the addition of new dialects such as QUIC, to continue its effectiveness and relevancy as a security analysis framework.I(ra)nconsistencies: Novel Insights into Iran’s Censorship
Free and Open Communications on the Internet (FOCI), 2025
Abstract
Iran employs one of the most prominent Internet censors in the world. An important part of Iran’s censorship apparatus is its analysis of unencrypted protocols such as HTTP and DNS. During routine evaluations of Iran’s HTTP and DNS censorship, we noticed several properties we believe to be unknown today. For instance, we found injections of correct static IPs for some domains such as google.com on the DNS level, unclear HTTP version parsing, and correlations between DNS and HTTP censorship. In this paper, we present our findings to the community and discuss possible takeaways for affected people and the censorship circumvention community. As some of our findings left us bewildered, we hope to ignite a discussion about Iran’s censorship behavior. We aim to use the discussion of our work to execute a thorough analysis and explanation of Iran’s censorship behavior in the future.✏Transport Layer Obscurity: Circumventing SNI Censorship on the TLS-Layer (Distinguished Paper Award)
IEEE Symposium on Security and Privacy (SP), 2025
Abstract
HTTPS composes large parts of today’s Internet traffic and has long been subject to censorship efforts in different countries. While censors analyze the Transport Layer Security (TLS) protocol to block encrypted HTTP traffic, censorship circumvention efforts have primarily focused on other protocols such as TCP. In this paper, we hypothesize that the TLS protocol offers previously unseen opportunities for censorship circumvention techniques. We tested our hypothesis by proposing possible censorship circumvention techniques that act on the TLS protocol. To validate the effectiveness of these techniques, we evaluate their acceptance by popular TLS servers and successfully demonstrate that these techniques can circumvent censors in China and Iran. In our evaluations, we discovered 38—partially standard-compliant—distinct censorship circumvention techniques, which we could group into 11 unique categories. Additionally, we provide novel insights into how China censors TLS traffic by presenting evidence of at least three distinct censorship appliances. We suspect that other parts of China’s censorship apparatus and other censors exhibit similar structures and advocate future censorship research to anticipate them. With this work, we hope to aid people affected by censorship and stimulate further research into censorship circumvention using cryptographic protocols.✏Encrypted Client Hello (ECH) in Censorship Circumvention
Free and Open Communications on the Internet (FOCI), 2025
Abstract
Censors have long censored Transport Layer Security (TLS) traffic by inspecting the domain name in the unencrypted Server Name Indication (SNI) extension. By encrypting the SNI extension, the Encrypted ClientHello (ECH) prevents censors from blocking TLS traffic to certain domains. Despite this promising outlook, ECH’s current capability to contest TLS censorship is unclear; for instance, Russia has started censoring ECH connections successfully. This paper clarifies ECH’s current role for TLS censorship. To this end, we evaluate servers’ support for ECH and its analysis and subsequent blocking by censors. We determine Cloudflare as the only major provider supporting ECH. Additionally, we affirm previously known ECH censorship in Russia and uncover indirect censorship of ECH through encrypted DNS censorship in China and Iran. Our findings suggest that ECH’s contribution to censorship circumvention is currently limited: we consider ECH’s dependence on encrypted DNS especially challenging for ECH’s capability to circumvent censorship. We stress the importance of censorship-resistant ECH to solve the long-known problem of SNI-based TLS censorship.Towards Automated DNS Censorship Circumvention
Free and Open Communications on the Internet (FOCI), 2026
Abstract
Censorship is employed by many governments and ISPs worldwide, with an increasing trend in recent years. One of the most censored protocols is DNS: censors target unencrypted and encrypted DNS to prevent clients from resolving the domain name of unwanted websites. Despite much research on DNS censorship, only a few tools can circumvent it.To support users affected by DNS censorship, we present DPYProxy-DNS, a DNS resolver that automatically detects and employs a working DNS censorship circumvention. We demonstrate the effectiveness of DPYProxy-DNS by automatically circumventing DNS censorship in China and Iran and analyzing DNS censorship mechanisms in these countries. Our analyses re veal that DNS censorship in Iran is ineffective against encrypted DNS. In China, DPYProxy-DNS revealed two consistently working circumvention techniques for unencrypted DNS: TCP segmentation for DNS over TCP and ignoring DNS responses injected by the Great Firewall of China (GFW). Our findings reveal varying levels of DNS censorship across different countries, underscoring the importance of the automated circumvention approach we provide with DPYProxy-DNS.Insights into an Iranian Internet Shutdown
Free and Open Communications on the Internet (FOCI), 2026
Abstract
In June 2025, Iran enacted a nationwide Internet shutdown, culminating its already strict censorship apparatus. While Internet shutdowns happen regularly, insights into these shutdowns are notoriously difficult to obtain: their timing is hard to predict, and measurements are often impossible. In this paper, we present unique measurements surrounding Iran’s 2025 Internet shutdown in June, which we acquired during a regular analysis of Iran’s censorship apparatus. We contextualize our findings of Iranian DNS, HTTP, TLS, and QUIC censorship during the shutdown with measurements from platforms such as Cloudflare Radar and user reports. Our measurements show that Iranian censorship changed before and after the shutdown, marking preparation and recovery periods. For instance, QUIC censorship went into effect before and stayed in effect after the shutdown, while DNS over TCP censorship was only present briefly before the shutdown and resumed working afterwards. We also measured general network instabilities, especially for UDP, after the shutdown and the disabling of certain middleboxes. Our findings indicate that the Iranian censor enforced its shutdown using fine-grained techniques instead of relying on an all-or-nothing blackout. We advertise for continued measurements of the Iranian censor and hypothesize that future shutdowns in censoring countries could be detected during their preparation phase.On Russia’s Early Introduction of QUIC SNI Censorship
Free and Open Communications on the Internet (FOCI), 2026