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CVE-2026-82751 – Fee-Payer Gas Cost Inflation & Unauthorized Key Provisioning – ZenHive mpp 0.2.0 to

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Summary

CVE-2026-82751 is an improper input validation vulnerability (CWE-1284) in ZenHive’s mpp library — an Elixir implementation of the Machine Payments Protocol (MPP) used as HTTP 402 payment middleware for AI agents and machine-to-machine commerce. When a server operates as a Tempo fee-payer sponsor, an unauthenticated remote attacker can attach a signed key_authorization field to a payment envelope that the server’s fee policy fails to validate, inflating sponsored gas costs by roughly 39x (from ~46,587 gas to ~1,808,700 gas) and provisioning free, attacker-controlled access keys at the sponsor’s expense. The flaw carries a CVSS score of 8.3 (High) and affects only deployments that have explicitly enabled fee-payer sponsorship.

Technical details

  • Root cause: MPP.Methods.Tempo.FeePayerPolicy.measure/3 validates gas limits (≤2,000,000) and max fees (≤100 Gwei) on sponsored Tempo transactions, but does not validate the optional key_authorization field carried in the payment envelope.
  • Trigger conditions: The target server must have Tempo fee-payer sponsorship enabled ("fee_payer" => true); this is not the default configuration.
  • Attack vector: Network-based, unauthenticated. An attacker submits a payment request with a fully-signed key authorization attached, which passes existing gas/fee policy bounds while still enabling additional, unvalidated on-chain operations.
  • Impact: The sponsoring server pays substantially inflated gas costs (integrity impact: high) and the attacker is issued new access keys with token spending limits, which can be reused for future transactions. Confidentiality is not affected; availability impact is low (increased cost/resource drain rather than outright denial of service).

Affected software

  • ZenHive mpp (Hex package / GitHub repository), versions 0.2.0 up to and including 0.16.0
  • Fixed in version 0.16.1
  • Only affects deployments with Tempo fee-payer sponsorship explicitly enabled

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:L/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade mpp to version 0.16.1 or later, which validates the key_authorization field in the fee-payer policy.
  • If unable to patch immediately:
    • Disable Tempo fee-payer sponsorship ("fee_payer" => false), which is the library’s default setting and fully prevents exploitation.
    • If sponsorship must remain enabled, reduce the max_gas limit in the fee-payer policy configuration (e.g., to approximately 50,000 gas for legitimate transfer use cases) to constrain the attacker’s ability to inflate sponsored costs.

References

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