CVE-2026-89420: Session voucher adding no new funds is accepted without a charge in mpp, serving paid resources for free
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.acceptvoucher/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 maybespend/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.
Affected Software
Event History
Frequently Asked Questions
Which deployments are exposed?
Deployments using mpp versions from 0.14.0 before 0.16.2 are affected if they serve paid resources through a method built on MPP.Session.Method. The vulnerable path is reachable through Plug, MCP, JSON-RPC, and WebSocket transports.
What does an attacker need to exploit this issue?
The attacker needs to hold an open payment channel and possess a valid signed voucher whose cumulative amount matches the amount already accepted for that channel. They can present that voucher against fresh server challenges to receive additional paid resources without further charges.
Is a replayed voucher limited to a single additional use?
No. A fresh challenge is issued for each request, while replay tracking is keyed on the challenge ID and payload. This allows the same signed voucher to be reused under successive challenges for an unbounded number of paid units.
How can we determine whether exploitation may have occurred?
Look for repeated successful paid-resource requests associated with a channel where the accepted voucher cumulative amount, spent amount, and units do not increase. Reuse of the same signed voucher across different challenge IDs is also indicative of this behavior.