In a nutshell: A newly demonstrated Spectre side-channel attack extracts JWTs from a co-located Cloudflare Worker at up to 12 bits per second, 360 times faster than a 2021 attack technique.
Security researchers have demonstrated a practically feasible Spectre attack against Cloudflare Workers that reads out a JSON Web Token from a co-located worker in the production environment at up to 12 bits per second. This corresponds to 360 times the data rate of a comparable attack from 2021.
For their end-to-end experiment, the researchers set up a self-controlled attacker worker and a likewise self-controlled victim worker, both co-located within Cloudflare’s production Workers environment. Using a Spectre-based side-channel technique, they succeeded in reading secret data from the victim worker’s memory — specifically a JSON Web Token (JWT), which is commonly used for authentication and session management. The achieved data rate of up to 12 bits per second exceeds an earlier attack technique against the same platform, presented in 2021, by a factor of 360.
For security officers at organizations using multi-tenant serverless platforms such as Cloudflare Workers, the finding is relevant because it shows that the isolating effect of V8 isolates against microarchitectural side-channel attacks is not complete. JWTs frequently contain session or authentication data; extraction by a foreign, co-located tenant could, in the worst case, lead to session hijacking or identity abuse, provided sufficient time and access to the target environment exist.
The published details suggest that this is a proof of concept under controlled conditions, in which both the attacker and victim workers were operated by the researchers themselves. Exploitation in a real-world scenario requires an attacker to be able to deliberately place a worker in close physical proximity to a target worker, which depends on the platform’s scheduling logic. Security teams processing sensitive secrets such as JWTs or session tokens in serverless environments should assess to what extent additional isolation mechanisms, short token lifetimes, and in-memory encryption could serve as compensating measures until the provider delivers any countermeasures.
Source: thehackernews.com · Published August 19, 2026
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