CVE-2026-89420 Session voucher adding no new funds is accepted without a charge in mpp, serving paid resources for free
Attackers can access paid resources for free by replaying session vouchers, bypassing payment validation in the ZenHive mpp library.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-89420?
CVE-2026-89420 is a high-severity vulnerability in ZenHive mpp, published on 22 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official 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.
Weakness: CWE-1284
Which versions are affected by CVE-2026-89420?
1 open source package ship an affected version, including mpp.
| Package | Affected | Fixed in |
|---|---|---|
mpp Hex | From 0.14.0 | 0.16.2 |
How do I fix CVE-2026-89420?
Upgrade mpp to 0.16.2 or later.
Is CVE-2026-89420 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-89420 exists.
Sources
Also known as EEF-CVE-2026-89420, GHSA-8c63-r789-xrrf, GHSA-96m7-5wfv-978m.