Spectre Attack on Cloudflare Workers Leaks JWT at 12 Bits/s — 360 Times Faster Than 2021
Cybersecurity researchers have disclosed details of a remote Spectre attack against Cloudflare Workers that successfully extracted a JSON Web Token (JWT) from a co-located Worker in the production environment at a rate of up to 12 bits per second. This is 360 times faster than the rate of a similar attack demonstrated in 2021.
The end-to-end experiment involved an attacker Worker and a victim Worker, both controlled by the researchers. The JWT was intentionally placed in the victim's memory. The research paper explicitly states that no customer data was accessed during the experiment.
Cloudflare confirmed that the attack has already been mitigated in production. The company improved its Dynamic Process Isolation (DyPrIs) mechanism, integrated the V8 Sandbox, and deployed Memory Protection Keys (MPK)-based in-process isolation. Cloudflare added that it found no indicators of active exploitation of this vulnerability over the past three years.
"We demonstrate that the production implementation of DyPrIs was insufficient," the researchers stated in the paper.
Cloudflare Workers runs code from multiple tenants in separate V8 isolates within the same operating system process. This architecture relies on language-level isolation rather than strict process isolation to reduce startup latency. However, a memory read within a shared Worker process can lead to cross-tenant leakage, according to Cloudflare.
The attack requires the attacker and victim Workers to be co-located in separate V8 isolates within the same Worker process. The attacker controls valid code in its own isolate. Native code execution is outside the threat model, and the attack does not depend on a V8 software exploit or sandbox escape.
Cloudflare noted that Workers restrict local timing sources, which made the attack more challenging but not impossible. The researchers were able to bypass these restrictions using a technique that exploits the shared memory between isolates. The company has since implemented additional countermeasures to prevent similar attacks in the future.
