Researchers Leak Cloudflare Workers JWTs at 12 Bits/Second as Spectre Attack Bypasses DyPrIs
Updated
Updated · The Hacker News · Aug 19
Researchers Leak Cloudflare Workers JWTs at 12 Bits/Second as Spectre Attack Bypasses DyPrIs
3 articles · Updated · The Hacker News · Aug 19
Summary
A remote Spectre attack on Cloudflare Workers extracted a JSON Web Token from a co-located Worker at up to 12 bits per second with 99.16% accuracy, using only attacker-controlled code in a separate V8 isolate.
The researchers said DyPrIs failed because a long-lived Durable Object could keep the victim running before post-invocation isolation, while WebSocket-heavy I/O suppressed the branch-misprediction signal the detector relied on.
Cloudflare said no customer data was accessed, found no signs of active exploitation over the past three years, and has already mitigated the issue in production with improved DyPrIs, the V8 Sandbox, and MPK-based isolation.
The tests ran on AMD EPYC Zen 2 and Zen 3 servers under light nighttime load; higher utilization slowed leakage but did not eliminate it, underscoring that shared-process multi-tenant isolation can still permit cross-tenant memory reads.
The result is 360 times faster than a 2021 remote Spectre demonstration against Workers, challenging earlier claims that DyPrIs could match strict process isolation for the attacks evaluated then.