EEF-CVE-2026-59694 CVE-2026-59694 GHSA-qpxh-ff8m-c62v

Unbounded access list in mpp Tempo fee-payer inflates gas cost per payment

Medium Hex

Severity: 0.0 / 10; Medium · CVSS 4.0
Exploit chance (EPSS): 0.52%; Chance of exploitation in the next 30 days. Higher than 44.44% of scored vulnerabilities.
Known exploited: No; Not in CISA's catalog
Weakness: CWE-1284 Improper Validation of Specified Quantity in Input
Fix: 0.6.0; Upgrade mpp to this version or later

Description

Summary

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.

Configurations

The vulnerable code path is only reachable when the MPP.Methods.Tempo method is configured with fee_payer: true (server-side gas sponsorship). Default deployments with fee_payer: false are not affected, since the client pays for gas out of its own wallet.

Affected versions

Hex

Package	Vulnerable	Fixed in
mpp	`>= 0.2.0, < 0.6.0`	`0.6.0`

How to fix

Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.

mpp Hex → `0.6.0`

mix

{:mpp, "~> 0.6.0"} # then: mix deps.update mpp

Severity breakdown

CVSS 4.0 0.0

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

Attack vector: Network
Attack complexity: Low
Attack requirements: Present
Privileges required: None
User interaction: None
Confidentiality (vulnerable system): None
Integrity (vulnerable system): High
Availability (vulnerable system): Low
Confidentiality (subsequent systems): None
Integrity (subsequent systems): None
Availability (subsequent systems): None

Weakness

CWE-1284: Improper Validation of Specified Quantity in Input

The product receives input that is expected to specify a quantity (such as size or length), but it does not validate or incorrectly validates that the quantity has the required properties.

How to prevent it

Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

References

Advisories

github.com/ZenHive/mpp/security/advisories/GHSA-qpxh-ff8m-c62v

Fixes and patches

github.com/ZenHive/mpp/commit/5d6338e2334084c5f2a78cfcca474830733ed7e8

Packages

Timeline

Jul 17, 2026 Published
Oct 24, 2026 Last updated by the source
Nov 1, 2026 Latest EPSS score

Advisory	Severity	Why it's related
EEF-CVE-2026-87119 mpp Tempo subscription key authorization is not bound to the issuing challenge, allowing a captured activation credential to be replayed	Not rated yet	Also affects mpp
EEF-CVE-2026-89420 Session voucher adding no new funds is accepted without a charge in mpp, serving paid resources for free	Not rated yet	Also affects mpp
EEF-CVE-2026-88255 mpp Tempo keys its pre-broadcast dedup reserve on the caller-supplied transaction encoding, so a re-encoded signed transaction reserves a second slot	Not rated yet	Also affects mpp
EEF-CVE-2026-89186 mpp writes Payment-Receipt and Cache-Control before the wrapped application runs, letting a consumer's own Cache-Control expose paid responses to shared caches	Not rated yet	Also affects mpp
EEF-CVE-2026-82750 Unbounded EIP-7702 authorization list in mpp Tempo fee-payer sponsorship inflates gas cost and sponsors account delegation	Not rated yet	Also affects mpp
EEF-CVE-2026-82751 Unbounded key authorization in mpp Tempo fee-payer sponsorship inflates gas cost and sponsors access-key provisioning	Not rated yet	Also affects mpp
EEF-CVE-2026-67581 On-chain transfer proof is not single-use in mpp EVM payment method, enabling cross-challenge replay	High	Also affects mpp
EEF-CVE-2026-73541 Tempo fee sponsorship in mpp bounds each transaction but not aggregate exposure, allowing concurrent sponsor-wallet drain	High	Also affects mpp

Credits: E.FU, Jonatan Männchen / EEF, Kian Kai Ang

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