---
id: CVE-2026-89420
title: >-
  Improper Validation of Specified Quantity in Input in ZenHive mpp allows a
  client holding an open payment channel to obtain paid resources without being
  charged.


  MPP.Session.Actions.accept_voucher/3 in lib/mpp/session/actions.ex treats …
summary: >-
  Improper Validation of Specified Quantity in Input in ZenHive mpp allows a
  client holding an open payment channel to obtain paid resources without being
  charged.


  MPP.Session.Actions.accept_voucher/3 in lib/mpp/session/actions.ex treats …
severity: high
cvss: 7.1
cvssVector: 'CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N'
cwe:
  - CWE-1284
vendor: ZenHive
product: mpp
affected:
  - mpp >= 0.14.0 < 0.16.2
  - >-
    zenhive/mpp >= 82df569c898be1137189e3648e1edb4af6363651 <
    7270edc1dcfb58250cc5ee812876609206564165
published: '2026-09-22'
updated: '2026-09-22'
sourceUpdated: '2026-09-22T19:09:32.273'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-89420'
references:
  - url: 'https://cna.erlef.org/cves/CVE-2026-89420.html'
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/ZenHive/mpp/commit/7270edc1dcfb58250cc5ee812876609206564165
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/ZenHive/mpp/commit/82df569c898be1137189e3648e1edb4af6363651
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: 'https://github.com/ZenHive/mpp/security/advisories/GHSA-8c63-r789-xrrf'
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: 'https://osv.dev/vulnerability/EEF-CVE-2026-89420'
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: 'https://github.com/ZenHive/mpp/security/advisories/GHSA-8c63-r789-xrrf'
    label: 134c704f-9b21-4f2e-91b3-4a467353bcc0
tags:
  - nvd
  - cve.org
  - exploit-available
exploitAvailable: true
ssvc:
  exploitation: poc
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-09-22T12:03:58.968313Z'
cvssSource: cna
ingestedAt: '2026-09-22T12:03:01.593Z'
epss: 0.00466
epssPercentile: 0.39561
---

## Overview

Improper Validation of Specified Quantity in Input in ZenHive mpp allows a client holding an open payment channel to obtain paid resources without being charged.

MPP.Session.Actions.accept_voucher/3 in lib/mpp/session/actions.ex treats a voucher whose cumulativeAmount equals the channel's already-accepted cumulative amount as an idempotent success, returning the channel unchanged without calling maybe_spend/2. The credential verifies, the protected resource is served, and spent and units stay where they were. Because the server issues a fresh challenge per request and the credential replay store keys on challenge id and payload, the same signed voucher can be re-presented under every new challenge, so one paid voucher yields an unbounded number of paid units. The path is reachable from any method built on MPP.Session.Method through the Plug, MCP, JSON-RPC and WebSocket transports.

This issue affects mpp: from 0.14.0 before 0.16.2.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.