Security audit: gas-budget waive/cap escalation paths

[abhicris]

Problem

Once issue #78 (block-builder-level budget enforcement) lands, the gas budget becomes a chain-side invariant. Any waiver path — committee 2-of-N quorum lifting the cap for emergency drain, operator-set overrides, epoch resets — is a potential escalation vector. A buggy waiver flow lets an attacker silently inflate cap or trigger an unintended drain, undoing the protective intent of the budget.

We need an explicit security audit of every code path that can raise, reset, or bypass an agent's gas budget.

Proposed Approach

1. Enumerate all paths that mutate AgentBudget state:
   • release events that increment *_spent
   • x402_middleware.settle events that increment *_spent
   • waiveEpoch(agentId, newCap) (committee path)
   • resetEpoch() (time-based reset)
   • setCap(agentId, hourly, daily) (initial provisioning)
2. For each, identify the trust assumption (which key signs, which contract calls, what state transitions are allowed).
3. Threat model:
   • Malicious committee member tries to silently raise cap during a release window
   • Reentrancy via release -> external call -> waiveEpoch -> release chain
   • Replay of an old waiver signature against a new epoch
   • Time-based reset triggered ahead of schedule via chain re-org
4. Doc: docs/security/gas-budget-threat-model.md covers all enumerated paths + the mitigation per path.
5. Third-party review on the waiver flow specifically (target: existing Switchboard auditor, or one of the Lux ecosystem auditors).

Acceptance Criteria

• Threat model doc published
• Reentrancy guard verified on all waiver-relevant entry points
• Replay protection: waiver signatures bound to (agentId, epoch, nonce) tuple
• Third-party audit signed off on the waiver flow before mainnet
• No silent-escalation path identified in the published model

References

• Issue Per-agent epoch gas budget enforcement at block-builder level (runtime hint) #78 — Per-agent epoch gas budget enforcement at block-builder level (parent)
• PR feat: gas budget tracker with configurable hour/day limits (closes #5) #14 — gas budget tracker (client-side baseline)
• internal: see stress-test 2026-06-01 section 1.2 on governance capture concerns