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CVE-2026-73541 – Sponsor Wallet Drain via Unbounded Concurrent Fee Sponsorship – ZenHive mpp 0.2.0 t

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Summary

CVE-2026-73541 is a resource-allocation vulnerability (CWE-770) in ZenHive’s mpp package — an Elixir implementation of the Machine Payments Protocol (MPP), an HTTP 402 payment middleware used for AI-agent and machine-to-machine commerce. The flaw resides in the Tempo fee-sponsorship feature, which validates each sponsored transaction’s fee individually but never tracks or bounds the sponsor’s aggregate exposure across concurrent requests. An unauthenticated remote attacker can exploit this to drain a fee-payer (sponsor) wallet by submitting many simultaneous sponsored transactions, resulting in high impact to availability and integrity of the sponsoring service.

Technical details

  • Root cause: MPP.Methods.Tempo.FeePayerPolicy and the MPP.Methods.Tempo module enforce a per-transaction ceiling (max_total_fee) but apply no throttling or aggregate accounting across concurrently in-flight sponsored requests. As the vendor advisory states, committed sponsor exposure is effectively "N times max_total_fee, bounded by nothing in the library."
  • Trigger conditions: The target server must have Tempo fee sponsorship enabled (method_config set with "fee_payer" => true). This is not the library default.
  • Compounding factor: The default 900-second transaction validity window keeps co-signed sponsored transactions broadcastable for an extended period, widening the window in which concurrently issued transactions can all still be settled against the sponsor wallet.
  • Attack vector: Network — no authentication or user interaction is required. An attacker simply issues many simultaneous sponsored-payment requests to a vulnerable, sponsorship-enabled endpoint.
  • Impact: Exhaustion/drain of the fee-payer (sponsor) wallet’s funds, causing denial of the payment-sponsorship service and integrity loss of the intended spending limits; confidentiality is not affected.

Affected software

  • Package: mpp (ZenHive), Hex package pkg:hex/mpp, GitHub pkg:github/zenhive/mpp
  • Affected versions: 0.2.0 through versions prior to 0.12.0
  • Precondition: Only deployments with Tempo fee sponsorship explicitly enabled ("fee_payer" => true) are exploitable.

Severity

  • CVSS v4.0 Base Score: 8.3 (High)
  • Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N
  • Attack Vector: Network | Attack Complexity: Low | Attack Requirements: Present | Privileges Required: None | User Interaction: None | Confidentiality: None | Integrity: High | Availability: High

Mitigation and recommended actions

  • Immediate: Upgrade mpp to version 0.12.0 or later, which introduces atomic aggregate fee-budget reservations enforced across concurrent requests with explicit worst-case-fee and reservation-count ceilings.
  • If unable to patch immediately:
    • Disable Tempo fee sponsorship entirely by setting "fee_payer" => false (this is the library’s default configuration).
    • If sponsorship must remain enabled, reduce max_total_fee and max_validity_window_seconds in the method configuration to minimize per-request and aggregate exposure until the upgrade can be applied.

References

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