Poster: Exploiting Timing Side-Channel Leaks in Web Applications that Tell on Themselves Vik Vanderlinden, Tom Van Goethem, Wouter Joosen and Mathy Vanhoef imec-DistriNet, KU Leuven {firstname.lastname}@kuleuven.be Abstract—The performance of remote timing attacks is highly dependent on the network connection that the attack is executed over, where jitter in both the up- and downstream direction can significantly deteriorate an attack’s perfor- mance. Traditional timing attacks overcome this problem by obtaining a large number of measurements. In this poster, we present a technique to remove the inaccuracies caused by downstream jitter in a remote timing attack, which we expect to reduce the number of measure- ments required to perform a successful timing attack. Our core idea is to exploit timestamps in HTTP responses, whose values are independent of the downstream jitter. To abuse these timestamps, the adversary synchronizes with the target web server’s clock edge, after which the observed timestamps allow the adversary to infer secret information. We present a method to synchronize with the server’s clock and discuss how to compensate for the clock drift between the attacker and target machines. To evaluate the feasibility of our technique, we also investigate the occur- rence of timestamps in HTTP responses for the top 10,000 sites according to the Tranco list. Index Terms—Side-channel attacks, timing attacks, web- based attacks, network security 1. Introduction The first remote timing attack was performed in 2005 by Brumley and Boneh, who used more than 1.4 million samples to leak a 1024-bit RSA key from a server [1]. Historically, to exploit a remote timing attack an adversary has to obtain many samples to be able to differentiate requests in a statistically significant way. Collecting a high number of samples makes the attack more robust against the jitter imposed by the network. By performing a test like the box test proposed by Crosby, Wallach and Riedi, an attacker can confidently differentiate between operations on a server using a quantifiable metric and leak private data [3]. In order to reduce the need for obtaining such a high amount of samples, the network jitter that is present has to be reduced or removed. By eliminating the dependence on one or both network paths, the jitter can be elimi- nated from the obtained samples. Previous work showed that both up- and downstream jitter can be removed by coalescing multiple requests into a single TCP segment and looking at the order in which the responses are being returned [5]. However, their technique only works over HTTP/2 and requires that the server uses concurrent processing. We propose a new sequential timing attack that eliminates downstream jitter and can leak sensitive information under less strict prerequisites. First, the core concept of the attack and the prereq- uisites will be presented. Second, some necessary opti- mizations to make the attack feasible will be discussed. Specifically, the clocks of the attacker and target machines have to be synchronized in order to leak information from the target. Adding to the complexity, these clocks will experience a relative drift between them over time, due to the minor inaccuracies in the physical hardware- clocks they use. Because the attack takes a non-negligible amount of time, the relative drift between machines should be compensated for in order to keep the synchronization valid throughout the attack. Finally, the occurrence of timing information on the web is discussed due to its vital importance to a successful attack. 2. Proposed Attack Consider two requests, one of which includes a secret operation that takes additional processing time (the ‘tar- get’ request), the other does not (the ‘baseline’ request). When these two requests are sent to a server at exactly the same moment, the expected outcome would be that the response to the target request is returned after the response to the baseline request be