--- title: "A revisit of remote Spectre attacks on Cloudflare Workers" url: https://daily.dev/posts/a-revisit-of-remote-spectre-attacks-on-cloudflare-workers-abo8s6gpu source_url: https://blog.cloudflare.com/revisiting-spectre-attacks-on-workers type: article source: "Cloudflare" published: 2026-08-19T16:04:22.776Z updated: 2026-08-19T16:18:35.292Z tags: ["cloudflare", "appsec", "v8"] reading_time: 17 upvotes: 1 comments: 0 language: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # A revisit of remote Spectre attacks on Cloudflare Workers **[Cloudflare](https://daily.dev/sources/cloudflare)** · 17 min read · 1 upvotes · 0 comments ## Summary Cloudflare researchers reassessed remote Spectre attacks against its Workers platform, uncovering a limitation in the original Dynamic Process Isolation (DyPrIs) defense. Using new attack primitives - a speculative type-confusion gadget, tree-based PLRU cache amplification, remote WebSocket timers, and Durable Objects to keep isolates alive - they achieved reliable cross-tenant memory leakage of up to 12 bit/s at over 99% accuracy in production, demonstrated end-to-end by exfiltrating a JWT token bit by bit from a co-located victim Worker. No evidence of active exploitation in the wild was found over the past three years, and Cloudflare has since hardened defenses with the V8 Sandbox, hardware-assisted in-process isolation via Memory Protection Keys (deployed September 2025), and an improved DyPrIs that accounts for long-lived and I/O-heavy workloads. A full academic paper detailing the research, covering work from 2024 and early 2025, has been published alongside the writeup. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### How fast can a remote Spectre attack leak data from Cloudflare Workers in production? Researchers demonstrated a reliable remote Spectre attack against Cloudflare Workers achieving up to 12 bit/s leakage with more than 99% accuracy in production. The attack combined a speculative type-confusion gadget, tree-based PLRU cache amplification, and a remote WebSocket timer, and was used to exfiltrate a JWT token bitwise from a co-located victim Worker. _daily.dev surfaces deep security research like this for teams hardening multi-tenant runtimes against side-channel leaks._ ### What defenses has Cloudflare added to Workers to mitigate Spectre attacks after this research? Cloudflare deployed hardware-assisted in-process isolation using Memory Protection Keys (MPK) in September 2025, integrated the V8 Sandbox to remove raw 64-bit pointers from typed-array backing stores, and improved its Dynamic Process Isolation (DyPrIs) system to detect long-lived Durable Object and WebSocket-heavy workloads that previously evaded post-execution isolation checks. _Track how platforms like Cloudflare respond to Spectre-class threats by following security engineering updates on daily.dev._ ### Why didn't Cloudflare's Dynamic Process Isolation (DyPrIs) detect this Spectre attack sooner? DyPrIs only isolated scripts after an invocation finished, but the attack used Durable Object WebSocket keep-alive messages to hold a single invocation open for hours, letting the leak complete before isolation triggered. It also normalized branch mispredictions by iTLB accesses, and the attack's heavy WebSocket I/O inflated iTLB activity enough to push the detection ratio below threshold. _Engineers designing runtime anomaly detection can learn from these evasion patterns via security writeups on daily.dev._ ## Similar posts on daily.dev - [When Agentic Glue Melts: Exploiting Cloudflare Code Mode and Workers](https://daily.dev/posts/when-agentic-glue-melts-exploiting-cloudflare-code-mode-and-workers-jojdr6tpv) · Check Point Research · 7 upvotes · 7 comments - [Safe in the sandbox: security hardening for Cloudflare Workers](https://daily.dev/posts/safe-in-the-sandbox-security-hardening-for-cloudflare-workers-kzwtyy6nw) · Cloudflare · 2 upvotes · 0 comments --- Tags: [#cloudflare](https://daily.dev/tags/cloudflare), [#appsec](https://daily.dev/tags/appsec), [#v8](https://daily.dev/tags/v8) [View this post on daily.dev](https://daily.dev/posts/a-revisit-of-remote-spectre-attacks-on-cloudflare-workers-abo8s6gpu)