CVE-2026-59694 Unbounded access list in mpp Tempo fee-payer inflates gas cost per payment
An unauthenticated attacker can inflate gas costs for fee-payers by submitting transactions with excessive access list entries, draining sponsor funds.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-59694?
CVE-2026-59694 is a high-severity denial of service vulnerability in ZenHive mpp, published on 17 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Improper Validation of Specified Quantity in Input in ZenHive mpp allows an unauthenticated remote client to inflate the fee-payer's gas cost per payment by a large multiplier, degrading the sponsor's operating margin.
When the mpp Elixir library is configured as fee payer (fee_payer: true), MPP.Tempo.Transaction.cosign_fee_payer/3 re-signs the client-supplied base fields of the 0x76 AASigned envelope verbatim, including the EIP-2930 access list, without validating its length or contents. EIP-2930 access list entries incur intrinsic gas (~2,400 gas per address, plus 1,900 gas per storage key) charged before any opcode executes, regardless of whether the listed addresses are ever touched. A malicious client submits a valid transferWithMemo call alongside a large number of fabricated access-list entries. The server co-signs and broadcasts the transaction. The intended transfer executes normally, but the fee-payer wallet pays a large multiple of the expected gas cost with no corresponding on-chain work.
At the maintainer's default of 137 access-list entries (fitting within Bandit's 10,000-byte per-header-field limit) and 100 Gwei max_fee_per_gas, per-payment gas cost rises from ~51,287 to ~380,087 gas, a 7.4x multiplier. Sustained abuse destroys the sponsor's operating margin on low-cost payments and, over time, drains the fee-payer wallet.
This issue affects mpp: from 0.2.0 before 0.6.0.
Weakness: CWE-1284
Which versions are affected by CVE-2026-59694?
3 open source packages ship an affected version, including mpp.
| Package | Affected | Fixed in |
|---|---|---|
mpp Hex | From 0.2.0 | 0.6.0 |
mpp Hex | From 0.2.0 | 0.6.0 |
mpp Hex | From 0.2.0 | 0.6.0 |
How do I fix CVE-2026-59694?
Upgrade mpp to 0.6.0 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-59694 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-59694 exists.
Sources
Also known as EEF-CVE-2026-59694, GHSA-qpxh-ff8m-c62v, GHSA-r4hx-mhrp-xf73.