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CVE-2026-82750 – Gas-Cost Inflation / Unauthorized EIP-7702 Account Delegation – ZenHive mpp 0.2.0 t

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Summary

CVE-2026-82750 is an improper input validation vulnerability in ZenHive’s mpp library (an Elixir implementation of the Machine Payments Protocol, used as HTTP 402 payment middleware for AI-agent and machine-to-machine commerce). The flaw allows an unauthenticated remote client to dramatically inflate the gas costs charged to a transaction fee-payer/sponsor and to force the sponsor to fund EIP-7702 account delegations chosen by the client. The issue has been assigned a CVSS score of 8.3 (High).

Technical details

  • Root cause: The function MPP.Methods.Tempo.FeePayerPolicy.measure/3 (in lib/mpp/methods/tempo/fee_payer_policy.ex) validates several fields of a client-signed Tempo payment envelope — gas limits, fee caps, validity window, and access list — but never inspects or bounds the aa_authorization_list (EIP-7702 authorization/delegation list) field.
  • Trigger conditions: A client submits a payment request in which the fee-payer/sponsor covers gas costs (sponsored/Tempo payment flow). The client attaches multiple signed EIP-7702 delegation entries, typically from disposable/throwaway authority keys, inside the unvalidated aa_authorization_list field.
  • Attack vector: Network-based; no authentication is required to submit the crafted envelope to a server acting as fee-payer.
  • Impact: Each unvalidated delegation entry is charged as intrinsic gas before the payment call executes. Testing by the reporter showed that with seven delegation entries, sponsored gas cost rose from roughly 46,575 gas to roughly 1,884,087 gas — an approximate 40x cost inflation — while still staying within the default gas limit ceiling. Because EIP-7702 delegations are persistent set-code operations, an attacker can also use this flaw to permanently upgrade its own accounts to delegated smart-contract code, all funded at the sponsor’s expense. This can destroy the economics of sponsored/low-value transactions and enable unauthorized persistent account modifications.

Affected software

  • Vendor/Project: ZenHive
  • Package: mpp (Elixir/Erlang, Hex package)
  • Vulnerable versions: 0.2.0 up to (but not including) 0.16.1
  • Vulnerable commit range: d29d54e507918db00a5b65d90136b73166c017d7 up to (but not including) the patch commit 0482572b47e1ffe1537ab80ab613d47b92833c2d
  • Affected files: lib/mpp/methods/tempo/fee_payer_policy.ex, lib/mpp/methods/tempo.ex

Severity

  • CVSS Score: 8.3 (High)
  • CVSS v4.0 Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N
  • CWE: CWE-1284 (Improper Validation of Specified Quantity in Input) / CWE-20 (Improper Input Validation)

Mitigation and recommended actions

  • Immediate: Upgrade the mpp package to version 0.16.1 or later, which validates/bounds the aa_authorization_list (EIP-7702 authorization list) field in fee-payer sponsorship logic.
  • If immediate patching is not possible:
    • Disable or restrict fee-payer/sponsorship functionality for Tempo payment flows until the patch can be applied.
    • Enforce strict server-side limits on the number and gas cost of EIP-7702 authorization/delegation entries accepted in incoming payment envelopes, independent of the library’s built-in checks.
    • Monitor sponsored transaction gas consumption for anomalous spikes that may indicate exploitation attempts.
    • Restrict which clients are permitted to use sponsored/fee-payer payment paths, reducing exposure to unauthenticated remote clients.

References

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