CVE-2026-89420

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.

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Weakness Type (CWE)

CWE-1284 CWE-1284

References

Other References

https://cna.erlef.org/cves/CVE-2026-89420.html https://github.com/ZenHive/mpp/commit/7270edc1dcfb58250cc5ee812876609206564165 https://github.com/ZenHive/mpp/commit/82df569c898be1137189e3648e1edb4af6363651 https://github.com/ZenHive/mpp/security/advisories/GHSA-8c63-r789-xrrf https://osv.dev/vulnerability/EEF-CVE-2026-89420 https://github.com/ZenHive/mpp/security/advisories/GHSA-8c63-r789-xrrf

Frequently Asked Questions

What is CVE-2026-89420? +

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.

How do I check if I'm vulnerable to CVE-2026-89420? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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