CVE-2026-89420 (GCVE-0-2026-89420)
Vulnerability from cvelistv5 – Published: 2026-09-22 11:16 – Updated: 2026-09-22 12:04
VLAI
EPSS
VEX
Title
Session voucher adding no new funds is accepted without a charge in mpp, serving paid resources for free
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 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.
Severity
SSVC
Exploitation: poc
Automatable: no
Technical Impact: partial
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2026-09-22 12:03 UTC
CWE
- CWE-1284 - Improper Validation of Specified Quantity in Input
Assigner
References
5 references
| URL | Tags |
|---|---|
| https://github.com/ZenHive/mpp/security/advisorie… | relatedvendor-advisory |
| https://cna.erlef.org/cves/CVE-2026-89420.html | related |
| https://osv.dev/vulnerability/EEF-CVE-2026-89420 | related |
| https://github.com/ZenHive/mpp/commit/82df569c898… | related |
| https://github.com/ZenHive/mpp/commit/7270edc1dcf… | patch |
Impacted products
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"versionStartIncluding": "0.14.0",
"vulnerable": true
}
],
"negate": false,
"operator": "OR"
}
],
"operator": "AND"
}
],
"credits": [
{
"lang": "en",
"type": "finder",
"value": "E.FU"
},
{
"lang": "en",
"type": "remediation developer",
"value": "E.FU"
},
{
"lang": "en",
"type": "coordinator",
"value": "Jonatan M\u00e4nnchen / EEF"
}
],
"dateAssigned": "2026-09-15T15:23:05.000Z",
"descriptions": [
{
"lang": "en",
"supportingMedia": [
{
"base64": false,
"type": "text/html",
"value": "\u003cp\u003eImproper Validation of Specified Quantity in Input in ZenHive mpp allows a client holding an open payment channel to obtain paid resources without being charged.\u003c/p\u003e\n\u003cp\u003e\u003ccode\u003eMPP.Session.Actions.accept_voucher/3\u003c/code\u003e in \u003ccode\u003elib/mpp/session/actions.ex\u003c/code\u003e treats a voucher whose \u003ccode\u003ecumulativeAmount\u003c/code\u003e equals the channel\u0027s already-accepted cumulative amount as an idempotent success, returning the channel unchanged without calling \u003ccode\u003emaybe_spend/2\u003c/code\u003e. The credential verifies, the protected resource is served, and \u003ccode\u003espent\u003c/code\u003e and \u003ccode\u003eunits\u003c/code\u003e 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 \u003ccode\u003eMPP.Session.Method\u003c/code\u003e through the Plug, MCP, JSON-RPC and WebSocket transports.\u003c/p\u003e\n\u003cp\u003eThis issue affects mpp: from 0.14.0 before 0.16.2.\u003c/p\u003e"
},
{
"base64": false,
"type": "text/markdown",
"value": "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.\n\n`MPP.Session.Actions.accept_voucher/3` in `lib/mpp/session/actions.ex` treats a voucher whose `cumulativeAmount` equals the channel\u0027s 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.\n\nThis issue affects mpp: from 0.14.0 before 0.16.2."
}
],
"value": "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.\n\nMPP.Session.Actions.accept_voucher/3 in lib/mpp/session/actions.ex treats a voucher whose cumulativeAmount equals the channel\u0027s 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.\n\nThis issue affects mpp: from 0.14.0 before 0.16.2."
}
],
"impacts": [
{
"capecId": "CAPEC-212",
"descriptions": [
{
"lang": "en",
"supportingMedia": [
{
"base64": false,
"type": "text/html",
"value": "\u003cp\u003eA client that has opened and funded a payment channel once can re-present the same signed voucher indefinitely and receive paid resources without any further charge. Operators running mpp as paid-API middleware lose revenue in proportion to how much the client consumes, with nothing in the channel state recording the usage.\u003c/p\u003e"
},
{
"base64": false,
"type": "text/markdown",
"value": "A client that has opened and funded a payment channel once can re-present the same signed voucher indefinitely and receive paid resources without any further charge. Operators running mpp as paid-API middleware lose revenue in proportion to how much the client consumes, with nothing in the channel state recording the usage."
}
],
"value": "A client that has opened and funded a payment channel once can re-present the same signed voucher indefinitely and receive paid resources without any further charge. Operators running mpp as paid-API middleware lose revenue in proportion to how much the client consumes, with nothing in the channel state recording the usage."
}
]
}
],
"metrics": [
{
"cvssV4_0": {
"Automatable": "NOT_DEFINED",
"Recovery": "NOT_DEFINED",
"Safety": "NOT_DEFINED",
"attackComplexity": "LOW",
"attackRequirements": "NONE",
"attackVector": "NETWORK",
"baseScore": 7.1,
"baseSeverity": "HIGH",
"privilegesRequired": "LOW",
"providerUrgency": "NOT_DEFINED",
"subAvailabilityImpact": "NONE",
"subConfidentialityImpact": "NONE",
"subIntegrityImpact": "NONE",
"userInteraction": "NONE",
"valueDensity": "NOT_DEFINED",
"vectorString": "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",
"version": "4.0",
"vulnAvailabilityImpact": "NONE",
"vulnConfidentialityImpact": "NONE",
"vulnIntegrityImpact": "HIGH",
"vulnerabilityResponseEffort": "NOT_DEFINED"
},
"format": "CVSS",
"scenarios": [
{
"lang": "en",
"value": "GENERAL"
}
]
}
],
"problemTypes": [
{
"descriptions": [
{
"cweId": "CWE-1284",
"description": "CWE-1284 Improper Validation of Specified Quantity in Input",
"lang": "en",
"type": "CWE"
}
]
}
],
"providerMetadata": {
"dateUpdated": "2026-09-22T11:16:29.573Z",
"orgId": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
"shortName": "EEF"
},
"references": [
{
"name": "GitHub Advisory",
"tags": [
"related",
"vendor-advisory"
],
"url": "https://github.com/ZenHive/mpp/security/advisories/GHSA-8c63-r789-xrrf"
},
{
"name": "EEF CNA record for CVE-2026-89420",
"tags": [
"related"
],
"url": "https://cna.erlef.org/cves/CVE-2026-89420.html"
},
{
"name": "OSV record EEF-CVE-2026-89420",
"tags": [
"related"
],
"url": "https://osv.dev/vulnerability/EEF-CVE-2026-89420"
},
{
"name": "Introducing commit 82df569 in ZenHive/mpp",
"tags": [
"related"
],
"url": "https://github.com/ZenHive/mpp/commit/82df569c898be1137189e3648e1edb4af6363651"
},
{
"name": "Fix commit 7270edc in ZenHive/mpp",
"tags": [
"patch"
],
"url": "https://github.com/ZenHive/mpp/commit/7270edc1dcfb58250cc5ee812876609206564165"
}
],
"source": {
"discovery": "INTERNAL"
},
"title": "Session voucher adding no new funds is accepted without a charge in mpp, serving paid resources for free"
}
},
"cveMetadata": {
"assignerOrgId": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
"assignerShortName": "EEF",
"cveId": "CVE-2026-89420",
"datePublished": "2026-09-22T11:16:29.573Z",
"dateReserved": "2026-09-11T19:00:02.531Z",
"dateUpdated": "2026-09-22T12:04:43.291Z",
"state": "PUBLISHED"
},
"dataType": "CVE_RECORD",
"dataVersion": "5.2"
}
}
}
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Experimental. This forecast is provided for visualization only and may change without notice. Do not use it for operational decisions.
Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the
vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base
model, served locally by ML-Gateway.
They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
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Related by attack behaviour
Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.
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