A revisit of remote Spectre attacks on Cloudflare Workers

In 2021, we assessed remote Spectre attacks against Cloudflare Workers. Based on the results, we shipped a production defense called Dynamic Process Isolation (DyPrIs), which identifies maliciously looking scripts and isolates them into separate processes. Since then, newer techniques in the area of stabilizing Spectre attacks have been discovered. To understand if these techniques posed a threat to our Workers production environment, we decided to internally reassess the remote Spectre attack. Building an updated proof-of-concept on the production environment allowed us to empirically assess the risk of Spectre attacks under production workloads. 

To mount a successful side-channel attack in production, an external attacker has to overcome additional obstacles such as activity on shared hardware resources, interrupts, context switches, and coarse-grained timers. Our research uncovered a limitation in the implementation of DyPrIs and we managed to demonstrate a remote Spectre attack reliably leaking up to 12 bit/s with a 99% accuracy in the production environment of Cloudflare Workers. As a consequence of this research, we improved DyPrIs, integrated the V8 Sandbox and an in-process isolation mechanism to further reduce the risk of memory disclosure attacks. 

Today we are publishing a paper describing our findings, co-authored by Albert Pedersen, Haocheng Xiao, Sam Ainsworth, Nigel Topham, and Martin Schwarzl. This paper covers research done in 2024 and early 2025.

Note that the presented attack is mitigated already in the production system due to countermeasures applied by Cloudflare Workers Runtime team. We did not find any indicators of active exploitation over the last three years.

Cloudflare Workers security model