Spectre Attack Against Cloudflare Workers Leaks JWTs From Co-Located Tenants at 12 Bits/Second

Security researchers have disclosed a remote Spectre-class side-channel attack targeting Cloudflare Workers, Cloudflare's multi-tenant serverless platform. According to the report, an attacker-controlled Worker was able to leak a JSON Web Token (JWT) from a co-located victim Worker running in Cloudflare's production environment, achieving a leak rate of up to 12 bits per second — reportedly 360 times faster than a comparable Spectre attack against the platform demonstrated in 2021.

The researchers ran an end-to-end experiment using an attacker Worker and a victim Worker they controlled themselves, rather than targeting a real third-party tenant, to demonstrate that speculative-execution leakage across co-located isolates in a shared V8-based runtime remains practically exploitable years after the original disclosure and subsequent mitigations.

Why It Matters for Defenders

Cloudflare Workers, like other multi-tenant edge/serverless platforms built on process-level isolation (V8 isolates rather than full VMs), rely heavily on the underlying JavaScript engine and CPU-level mitigations to prevent one tenant from reading another's memory. A demonstrated 360x improvement in leak rate is significant because it substantially lowers the time required to exfiltrate meaningful secrets — such as authentication tokens, API keys, or session material — from a neighboring tenant sharing the same physical hardware.

This is relevant to any organization running workloads on Cloudflare Workers or similar multi-tenant edge compute platforms, particularly where sensitive tokens or secrets are processed in-memory alongside untrusted or third-party code paths.

What Defenders Should Watch For

• Treat JWTs and other bearer tokens processed inside multi-tenant edge/serverless functions as subject to potential cross-tenant memory disclosure risk, not just network-level exposure.
• Minimize the lifetime and scope of tokens handled inside edge compute functions — short-lived, narrowly-scoped tokens reduce the value of anything leaked via a slow side channel.
• Review whether sensitive secret material needs to reside in Worker memory at all, versus being fetched just-in-time from a secrets store or proxied through a more isolated backend.
• Monitor vendor advisories from Cloudflare for any platform-level mitigations or isolation changes in response to this research, and track whether proof-of-concept code is released publicly.
• This is a side-channel/hardware-adjacent issue rather than a traditional exploitable vulnerability with a patch — expect mitigation to come primarily from the platform provider (Cloudflare) rather than from application-level fixes.

Developing Story

This is net-new intelligence and details are still emerging; there is no associated CVE or confirmed real-world exploitation at this time, and the demonstration described was conducted by researchers against their own test Workers rather than a live third-party target. We will monitor for updates, including any Cloudflare response or mitigation guidance. Read the original report at The Hacker News.