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CVE-2026-89420— Session voucher adding no new funds is accepted without a charge in mpp, serving paid resources for free

Quick assessment

Affected
ZenHive mpp
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

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 cumulati

CVSS 7.1 · High

Affected Version Matrix 2

VendorProduct Version RangeStatus
ZenHive mpp 0.14.0< 0.16.2 affected
82df569c898be1137189e3648e1edb4af6363651< 7270edc1dcfb58250cc5ee812876609206564165 affected
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I. Basic Information for CVE-2026-89420

Vulnerability Information

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Vulnerability Title
Session voucher adding no new funds is accepted without a charge in mpp, serving paid resources for free
Source: CVE Program / CVE List V5
Vulnerability Description
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.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
CWE-1284
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
ZenHive mpp 0.14.0 ~ 0.16.2 cpe:2.3:a:ZenHive:mpp:*:*:*:*:*:*:*:*
ZenHive mpp 82df569c898be1137189e3648e1edb4af6363651 ~ 7270edc1dcfb58250cc5ee812876609206564165 cpe:2.3:a:ZenHive:mpp:*:*:*:*:*:*:*:*

II. Public POCs for CVE-2026-89420

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III. Intelligence Information for CVE-2026-89420

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Patches & Fixes for CVE-2026-89420 (1)

Other References for CVE-2026-89420 (3)

Other References for CVE-2026-89420 (1)

IV. Related Vulnerabilities

Same product: mpp
Same vendor: ZenHive
Same weakness: CWE-1284

V. Comments for CVE-2026-89420

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