Invariant
U5
Category
Universal
Priority
Stretch
Violation class
Gas Abuse
What it proves
A payment’s cheap, structural checks — signature validity, declared amount/receiver/asset match — can all pass at/verify time even though the underlying transfer can never actually succeed
(e.g. the client’s real balance can’t cover it). The failure only shows up when the facilitator
tries to actually settle. If resubmitting the same doomed payload gets progressively more
expensive for the facilitator to reject, a client (or attacker) can force it to spend unbounded
work rejecting a payment that was never going to succeed.
Source: USENIX Security ‘26 facilitator study — Gas Abuse violation class.
How it works
Ward builds a payload requesting an amount comfortably beyond any realistic TestNet USDC balance — large enough that/verify’s declared-terms checks pass cheaply, but the actual transfer can never
succeed — then resubmits that same doomed payload to /settle five times, timing each attempt.
Pass condition
Two conditions must both hold:neverSucceeded— the unaffordable payload must never succeed on any resubmission. If it ever does, that means/verify’s cheap checks and/settle’s real-balance check disagree in a dangerous direction.boundedCost— no resubmission’s latency may exceed 10x the first attempt’s latency. A latency blowup beyond that factor is treated as evidence of unbounded internal amplification (e.g. a naive retry-with-backoff loop triggered per resubmission) rather than a flat, bounded rejection cost.