CVE-2026-89420 PUBLISHED

Session voucher adding no new funds is accepted without a charge in mpp, serving paid resources for free

Assigner: EEF
Reserved: 11.09.2026 Published: 22.09.2026 Updated: 22.09.2026

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.

Metrics

CVSS 4.0

CVSS Vector: 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
CVSS Score: 7.1

Exploitability Metrics		Vulnerable System Impact Metrics		Subsequent System Impact Metrics
Attack Vector	Network	Confidentiality	None	Confidentiality	None
Attack Complexity	Low	Integrity	High	Integrity	None
Attack Requirements	None	Availability	None	Availability	None
Privileges Required	Low
User Interaction	None

CVSS 4.0

Product Status

Vendor	ZenHive
Product	mpp
Versions	Default: unaffected affected from 0.14.0 to 0.16.2 (excl.)

Vendor	ZenHive
Product	mpp
Versions	Default: unaffected affected from 82df569c898be1137189e3648e1edb4af6363651 to 7270edc1dcfb58250cc5ee812876609206564165 (excl.)

Credits

E.FU finder
E.FU remediation developer
Jonatan Männchen / EEF coordinator

References

Problem Types

CWE-1284 Improper Validation of Specified Quantity in Input CWE

Impacts

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.