OnionFlation attacks succeed by inflating puzzle difficulty without causing detectable congestion at the targeted service, meaning the attack leaves no noticeable traffic-volume signature at the victim — standard congestion-based anomaly detection cannot identify the attack in progress.
From 2025-lee-onions-got-puzzled — Onions Got Puzzled: On the Challenges of Mitigating Denial-of-Service Problems in Tor Onion Services
· Abstract
· 2025
· USENIX Security 2025
Implications
Detecting puzzle-inflation attacks requires monitoring client-observed puzzle difficulty (or introduction-circuit failure rates) rather than server-side throughput metrics — instrument the client path, not only the server.
Censors or adversaries seeking to deny access to onion services without generating detectable attack traffic now have a documented low-cost technique; circumvention tools should assume this attack class is available to well-resourced adversaries.