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Internet traffic is increasingly encrypted. While this protects the confidentiality and integrity of communication, it prevents network monitoring systems (NMS) and intrusion detection systems (IDSs) from effectively analyzing the now encrypted payloads. Therefore, many enterprise networks have deployed man-in-the-middle (MitM) proxies that intercept TLS connections at the network border to examine packet payloads and thus retain some visibility. However, recent studies have shown that TLS interception often reduces connection security and potentially introduces additional attack vectors to the network. In this paper, we present a cooperative approach in which end-hosts as cryptographic endpoints selectively provide TLS key material to NMS for decryption. This enables endpoints to control who can decrypt which content and lets users retain privacy for chosen connections. We implement a prototype based on the Zeek NMS that is able to receive key material from hosts, decrypt TLS connections and perform analyzes on the cleartext. The patch is freely available and we plan to upstream our changes to Zeek once they are mature enough. In our evaluation, we discuss how our approach conceptually requires significantly less computational resources compared to the commonly deployed MitM proxies. Our experimental results indicate, that TLS decryption increases a runtime overhead of about 2.5 times of the original runtime on cleartext. Additionally, we show that the latency for transmitting keys between hosts and the NMS can be effectively addressed by buffering traffic at the NMS for at least 40ms, allowing successful decryption of 99.99% of all observed TLS connections.

Public Key Infrastructures (PKIs) with their trusted Certificate Authorities (CAs) provide the trust backbone for the Internet: CAs sign certificates which prove the identity of servers, applications, or users. To be trusted by operating systems and browsers, a CA has to undergo lengthy and costly validation processes. Alternatively, trusted CAs can cross-sign other CAs to extend their trust to them. In this paper, we systematically analyze the present and past state of cross-signing in the Web PKI. Our dataset (derived from passive TLS monitors and public CT logs) encompasses more than 7 years and 225 million certificates with 9.3 billion trust paths. We show benefits and risks of cross-signing. We discuss the difficulty of revoking trusted CA certificates where, worrisome, cross-signing can result in valid trust paths to remain after revocation; a problem for non-browser software that often blindly trusts all CA certificates and ignores revocations. However, cross-signing also enables fast bootstrapping of new CAs, e.g., Let's Encrypt, and achieves a non-disruptive user experience by providing backward compatibility. In this paper, we propose new rules and guidance for cross-signing to preserve its positive potential while mitigating its risks., Comment: Extended version of paper accepted at 2020 ACM SIGSAC Conference on Computer and Communications Security (CCS '20)

In late 2017, a sudden proliferation of malicious JavaScript was reported on the Web: browser-based mining exploited the CPU time of website visitors to mine the cryptocurrency Monero. Several studies measured the deployment of such code and developed defenses. However, previous work did not establish how many users were really exposed to the identified mining sites and whether there was a real risk given common user browsing behavior. In this paper, we present a retroactive analysis to close this research gap. We pool large-scale, longitudinal data from several vantage points, gathered during the prime time of illicit cryptomining, to measure the impact on web users. We leverage data from passive traffic monitoring of university networks and a large European ISP, with suspected mining sites identified in previous active scans. We corroborate our results with data from a browser extension with a large user base that tracks site visits. We also monitor open HTTP proxies and the Tor network for malicious injection of code. We find that the risk for most Web users was always very low, much lower than what deployment scans suggested. Any exposure period was also very brief. However, we also identify a previously unknown and exploited attack vector on mobile devices.

TLS 1.3 marks a significant departure from previous versions of the Transport Layer Security protocol (TLS). The new version offers a simplified protocol flow, more secure cryptographic primitives, and new features to improve performance, among other things. In this paper, we conduct the first study of TLS 1.3 deployment and use since its standardization by the IETF. We use active scans to measure deployment across more than 275M domains, including nearly 90M country-code top-level domains. We establish and investigate the critical contribution that hosting services and CDNs make to the fast, initial uptake of the protocol. We use passive monitoring at two positions on the globe to determine the degree to which users profit from the new protocol and establish the usage of its new features. Finally, we exploit data from a widely deployed measurement app in the Android ecosystem to analyze the use of TLS 1.3 in mobile networks and in mobile browsers. Our study shows that TLS 1.3 enjoys enormous support even in its early days, unprecedented for any TLS version. However, this is strongly related to very few global players pushing it into the market and sustaining its growth.

In this paper, we analyze the evolution of Certificate Transparency (CT) over time and explore the implications of exposing certificate DNS names from the perspective of security and privacy. We find that certificates in CT logs have seen exponential growth. Website support for CT has also constantly increased, with now 33% of established connections supporting CT. With the increasing deployment of CT, there are also concerns of information leakage due to all certificates being visible in CT logs. To understand this threat, we introduce a CT honeypot and show that data from CT logs is being used to identify targets for scanning campaigns only minutes after certificate issuance. We present and evaluate a methodology to learn and validate new subdomains from the vast number of domains extracted from CT logged certificates., Comment: To be published at ACM IMC 2018

The majority of electronic communication today happens either via email or chat. Thanks to the use of standardised protocols electronic mail (SMTP, IMAP, POP3) and instant chat (XMPP, IRC) servers can be deployed in a decentralised but interoperable fashion. These protocols can be secured by providing encryption with the use of TLS---directly or via the STARTTLS extension---and leverage X.509 PKIs or ad hoc methods to authenticate communication peers. However, many combination of these mechanisms lead to insecure deployments. We present the largest study to date that investigates the security of the email and chat infrastructures. We used active Internet-wide scans to determine the amount of secure service deployments, and passive monitoring to investigate if user agents actually use this opportunity to secure their communications. We addressed both the client-to-server interactions as well as server-to-server forwarding mechanisms that these protocols offer, and the use of encryption and authentication methods in the process. Our findings shed light on an insofar unexplored area of the Internet. The truly frightening result is that most of our communication is poorly secured in transit., Comment: NDSS 2016