# 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.

- **Severity:** High 8.3 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 17 Jul 2026
- **Updated:** 18 Jul 2026
- **Also known as:** EEF-CVE-2026-59694, GHSA-qpxh-ff8m-c62v, GHSA-r4hx-mhrp-xf73

## 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 (Improper Validation of Specified Quantity in Input)

## Which versions are affected by CVE-2026-59694?

3 open source packages ship an affected version, including mpp.


| Package | Ecosystem | 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.

## Is CVE-2026-59694 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-59694 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-59694 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-59694 exists.

Affected packages and fixed versions:
- mpp (Hex): affected from 0.2.0; fixed in 0.6.0
- mpp (Hex): affected from 0.2.0; fixed in 0.6.0
- mpp (Hex): affected from 0.2.0; fixed in 0.6.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-59694/ (Markdown: https://insights.sbomify.com/CVE-2026-59694/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-59694
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-59694
- OSV: https://osv.dev/vulnerability/CVE-2026-59694

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-59694/
Summaries are generated and may contain mistakes; the official description above is the record.