ZenHive mpp EUVD-2026-45158

The server must be deployed as a Tempo gas fee payer (fee_payer: true) so that MPP.Tempo.Transaction.cosign_fee_payer/3 co-signs client-supplied envelopes - this is the AT:P attack requirement and the single decisive precondition; instances without fee-payer/sponsorship enabled are not affected. Given that mode, no authentication is required (PR:N) and no user interaction is needed (UI:N): a remote client simply submits one 0x76 AASigned envelope carrying oversized max_fee_per_gas / max_priority_fee_per_gas fields. Limiting factors are the non-default deployment (must be an active gas sponsor), the affected version window (0.2.0 through 0.5.x, fixed in 0.6.0), and that the fee-payer wallet must hold a spendable balance for loss to occur.

This is a genuine high-priority risk for anyone running mpp in fee-payer mode, though its blast radius is bounded by that configuration. The CVSS 4.0 vector (AV:N/AC:L/AT:P/PR:N/UI:N, VI:H/VA:H) correctly captures network-reachable, unauthenticated, low-complexity exploitation with an attack requirement (AT:P) - the server must be deployed as a fee payer - and impact that is integrity/availability (funds spent, sponsorship service exhausted) rather than confidentiality. Note the input Tags label this 'Information Disclosure,' which is inaccurate: nothing is disclosed; the impact is financial loss and denial of the sponsorship service (VC:N in the vector confirms this). No EPSS score and no KEV/POC signals were provided, so exploitation probability cannot be quantified numerically; the low-complexity, unauthenticated, single-request nature means weaponization would be trivial if any actor targeted an exposed fee-payer deployment. The main limiting factor is prevalence and configuration - only fee_payer: true instances are exploitable, and mpp is a niche library - so treat this as a critical fix for the small population of affected operators rather than a broad internet-wide event.

An attacker discovers an mpp server running in fee-payer mode, crafts a 0x76 AASigned envelope embedding astronomically high max_fee_per_gas / max_priority_fee_per_gas values, signs it as the client, and submits a single payment request. The server co-signs and broadcasts it, the on-chain effective_gas_price bills the inflated rate against the server's wallet, draining it in one transaction and halting gas sponsorship for all legitimate users. No exploit code is publicly identified, but the low-complexity, unauthenticated network vector makes this trivial to reproduce.