Spectre Flaw Leaks JWT Tokens on Cloudflare Workers via Remote Timers

Researchers found that Cloudflare Workers' Dynamic Process Isolation (DyPrIs) defense, deployed to counter Spectre risks from its language-level (V8 isolate) rather than process-level tenant isolation, was insufficient in production. Using microarchitectural amplification techniques, they showed that restricted and coarsened timers could still be turned into precise timing primitives.

Leveraging these primitives alongside remote timing, the team built an end-to-end remote Spectre attack that exfiltrated a JWT token from a co-located victim worker. The attack improved on prior work by orders of magnitude, jumping from 2 bit/min to 12 bit/s at 99.16% accuracy, posing an immediate risk to customer data on a platform handling millions of requests per second.

In coordinated response, Cloudflare hardened the V8 Sandbox to limit transient access to 64-bit pointers, improved DyPrIs detection, and rolled out hardware-assisted MPK-based in-process isolation to confine each tenant's heap under a dedicated memory-protection key. The findings underscore the limits of language-level isolation in multi-tenant edge computing platforms.