Practical Covert Channel Across Isolated Browser Instances via GPU Command Queue Contention

Web storages, e.g., cookies, are isolated across different browser modes (e.g., normal vs. incognito) and browsers (e.g., Chrome vs. Safari) to ensure user privacy. Despite such browser-level isolation, researchers have found covert communication channels between different modes and browsers, which can break such a security mechanism. However, existing covert communications are impractical and unreliable in the real world scenarios, where noise is prevalent. Furthermore, multiple prior covert channels only support cross-mode-not cross-browser-communications, because the relevant resources are shared within a single browser. In this paper, we discover a novel covert channel due to GPU command queue contention and then design a framework, called $\mathbf{C}^{3} \mathbf{F}$, for both cross-mode and cross-browser covert communication. $\mathbf{C}^{3} \mathbf{F}$ negotiates an adaptive communication speed between the sender and the receiver and then facilitates the covert communication. We evaluated $\mathbf{C}^{3} \mathbf{F}$ in real-world settings by deploying it to crowd-sourced workers on Amazon Mechanical Turk, simply asking them to visit two websites without imposing additional constraints. Our evaluation shows that covert communication achieves a 100 % accuracy once the data transmission is completed, with an overall completion rate of 92.81 % across 1,434 AMT experiments. So far, Firefox, Safari and Tor developers have confirmed our attack and are working with us on practical defenses.