The Security Impact of HTTPS Interception

As HTTPS deployment grows, middlebox and antivirus products are increasingly intercepting TLS connections to retain visibility into network traffic. In this work, we present a comprehensive study on the prevalence and impact of HTTPS interception. First, we show that web servers can detect interception by identifying a mismatch between the HTTP User-Agent header and TLS client behavior. We characterize the TLS handshakes of major browsers and popular interception products, which we use to build a set of heuristics to detect interception and identify the responsible product. We deploy these heuristics at three large network providers: (1) Mozilla Firefox update servers, (2) a set of popular e-commerce sites, and (3) the Cloudflare content distribution network. We find more than an order of magnitude more interception than previously estimated and with dramatic impact on connection security. To understand why security suffers, we investigate popular middleboxes and clientside security software, finding that nearly all reduce connection security and many introduce severe vulnerabilities. Drawing on our measurements, we conclude with a discussion on recent proposals to safely monitor HTTPS and recommendations for the security community. I . I N T R O D U C T I O N When it comes to HTTPS, the security community is working at cross purposes. On the one hand, we are striving to harden and ubiquitously deploy HTTPS in order to provide strong endto-end connection security [5], [20], [22], [23], [34], [51]. At the same time, middlebox and antivirus products increasingly intercept (i.e., terminate and re-initiate) HTTPS connections in an attempt to detect and block malicious content that uses the protocol to avoid inspection [6], [12], [15], [27]. Previous work has found that some specific HTTPS interception products dramatically reduce connection security [7], [12], [58]; however, the broader security impact of such interception remains unclear. In this paper, we conduct the first comprehensive study of HTTPS interception in the wild, quantifying both its prevalence in traffic to major services and its effects on real-world security. We begin by introducing a novel technique for passively detecting HTTPS interception based on handshake characteristics. HTTPS interception products typically function as transparent proxies: they terminate the browser’s TLS connection, inspect the HTTP plaintext, and relay the HTTP data over a new TLS connection to the destination server. We show that web servers can detect such interception by identifying a mismatch between the HTTP User-Agent header and the behavior of the TLS client. TLS implementations display varied support (and preference order) for cipher suites, extensions, elliptic curves, compression methods, and signature algorithms. We characterize these variations for major browsers and popular interception products in order to construct heuristics for detecting interception and identifying the responsible product. Next, we assess the prevalence and impact of HTTPS interception by applying our heuristics to nearly eight billion connection handshakes. In order to avoid the bias inherent in any single network vantage point, we analyzed connections for one week at three major Internet services: (1) Mozilla Firefox update servers, (2) a set of popular e-commerce websites, and (3) the Cloudflare content distribution network. These providers serve different types of content and populations of users, and we find differing rates of interception: 4.0% of Firefox update connections, 6.2% of e-commerce connections, and 10.9% of U.S. Cloudflare connections were intercepted. While these rates vary by vantage point, all are more than an order of magnitude higher than previous estimates [27], [46]. To quantify the real-world security impact of the observed interception, we establish a grading scale based on the TLS features advertised by each client. By applying the metric to unmodified browser handshakes and to the intercepted connections seen at each vantage point, we calculate the change in security for intercepted connections. While for some older clients, proxies increased connection security, these improvements were modest compared to the vulnerabilities introduced: 97% of Firefox, 32% of e-commerce, and 54% of Cloudflare connections that were intercepted became less secure. Alarmingly, not only did intercepted connections use weaker cryptographic algorithms, but 10–40% advertised support for known-broken ciphers that would allow an active man-in-the-middle attacker to later intercept, downgrade, and decrypt the connection. A large number of these severely broken connections were due to network-based middleboxes rather than client-side security software: 62% of middlebox connections were less secure and an astounding 58% had severe vulnerabilities enabling later interception. Finally, we attempt to understand why such a large number of intercepted connections are vulnerable by testing the security of a range of popular corporate middleboxes, antivirus products, and other software known to intercept TLS. The default settings for eleven of the twelve corporate middleboxes we evaluated expose connections to known attacks, and five introduce severe vulnerabilities (e.g., incorrectly validate certificates). Similarly, 24 of the 26 client-side security products we tested Permission to freely reproduce all or part of this paper for noncommercial purposes is granted provided that copies bear this notice and the full citation on the first page. Reproduction for commercial purposes is strictly prohibited without the prior written consent of the Internet Society, the first-named author (for reproduction of an entire paper only), and the author’s employer if the paper was prepared within the scope of employment. NDSS’17, 26 February–1 March, 2017, San Diego, CA, USA http://dx.doi.org/10.14722/ndss.2017.23456 reduce connection security, and two thirds introduce severe vulnerabilities. In some cases, manufacturers attempted to customize libraries or re-implement TLS, introducing negligent vulnerabilities. In other cases, products shipped with libraries that were years out of date. Across the board, companies are struggling to correctly deploy the base TLS protocol, let alone implement modern HTTPS security features. Our results indicate that HTTPS interception has become startlingly widespread, and that interception products as a class have a dramatically negative impact on connection security. We hope that shedding light on this state of affairs will motivate improvements to existing products, advance work on recent proposals for safely intercepting HTTPS [26], [38], [44], [54], and prompt discussion on long-term solutions. I I . B A C K G R O U N D In this section, we provide a brief background on HTTPS interception and describe the aspects of HTTP and TLS that are relevant to our fingerprinting techniques. We refer the reader to RFC 5280 [14] for a detailed description of TLS.