The race is only reachable when MPP.Methods.Tempo is configured with a dedup store (\"store\" in method_config). The default is nil, which runs the library statelessly and provides no application-level replay protection at all, a documented property rather than part of this issue.
Time-of-check Time-of-use (TOCTOU) Race Condition in ZenHive mpp allows an unauthenticated remote client to redeem one confirmed on-chain payment for multiple paid-resource accesses.
\nThe type=\"hash\" credential path in MPP.Methods.Tempo.verify/2 guards against replay with a non-atomic check-then-mark sequence: check_hash_unused/2 reads the dedup store, an eth_getTransactionReceipt round trip verifies the payment on chain, and only then does mark_hash_used/2 write the mark. Concurrent requests carrying the same settled payment hash all pass the read before any of them writes, so each is issued a receipt. The store's atomic check_and_mark/2 primitive is available and used by the type=\"transaction\" path, but the hash path calls plain get and put even when the configured store implements it. Exploitation requires a dedup store to be configured; the default nil store is stateless and documented as offering no replay protection at all.
This issue affects mpp: from 0.2.0 before 0.6.1.
"},{"base64":false,"type":"text/markdown","value":"Time-of-check Time-of-use (TOCTOU) Race Condition in ZenHive mpp allows an unauthenticated remote client to redeem one confirmed on-chain payment for multiple paid-resource accesses.\n\nThe `type=\"hash\"` credential path in `MPP.Methods.Tempo.verify/2` guards against replay with a non-atomic check-then-mark sequence: `check_hash_unused/2` reads the dedup store, an `eth_getTransactionReceipt` round trip verifies the payment on chain, and only then does `mark_hash_used/2` write the mark. Concurrent requests carrying the same settled payment hash all pass the read before any of them writes, so each is issued a receipt. The store's atomic `check_and_mark/2` primitive is available and used by the `type=\"transaction\"` path, but the hash path calls plain `get` and `put` even when the configured store implements it. Exploitation requires a dedup store to be configured; the default `nil` store is stateless and documented as offering no replay protection at all.\n\nThis issue affects mpp: from 0.2.0 before 0.6.1."}],"value":"Time-of-check Time-of-use (TOCTOU) Race Condition in ZenHive mpp allows an unauthenticated remote client to redeem one confirmed on-chain payment for multiple paid-resource accesses.\n\nThe type=\"hash\" credential path in MPP.Methods.Tempo.verify/2 guards against replay with a non-atomic check-then-mark sequence: check_hash_unused/2 reads the dedup store, an eth_getTransactionReceipt round trip verifies the payment on chain, and only then does mark_hash_used/2 write the mark. Concurrent requests carrying the same settled payment hash all pass the read before any of them writes, so each is issued a receipt. The store's atomic check_and_mark/2 primitive is available and used by the type=\"transaction\" path, but the hash path calls plain get and put even when the configured store implements it. Exploitation requires a dedup store to be configured; the default nil store is stateless and documented as offering no replay protection at all.\n\nThis issue affects mpp: from 0.2.0 before 0.6.1."}],"impacts":[{"capecId":"CAPEC-29","descriptions":[{"lang":"en","value":"CAPEC-29 Leveraging Time-of-Check and Time-of-Use (TOCTOU) Race Conditions"}]}],"metrics":[{"cvssV4_0":{"Automatable":"NOT_DEFINED","Recovery":"NOT_DEFINED","Safety":"NOT_DEFINED","attackComplexity":"LOW","attackRequirements":"PRESENT","attackVector":"NETWORK","baseScore":6.3,"baseSeverity":"MEDIUM","privilegesRequired":"NONE","providerUrgency":"NOT_DEFINED","subAvailabilityImpact":"NONE","subConfidentialityImpact":"NONE","subIntegrityImpact":"NONE","userInteraction":"NONE","valueDensity":"NOT_DEFINED","vectorString":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N","version":"4.0","vulnAvailabilityImpact":"NONE","vulnConfidentialityImpact":"NONE","vulnIntegrityImpact":"LOW","vulnerabilityResponseEffort":"NOT_DEFINED"},"format":"CVSS","scenarios":[{"lang":"en","value":"GENERAL"}]}],"problemTypes":[{"descriptions":[{"cweId":"CWE-367","description":"CWE-367 Time-of-check Time-of-use (TOCTOU) Race Condition","lang":"en","type":"CWE"}]}],"providerMetadata":{"dateUpdated":"2026-08-19T17:19:43.696Z","orgId":"6b3ad84c-e1a6-4bf7-a703-f496b71e49db","shortName":"EEF"},"references":[{"tags":["vendor-advisory","related"],"url":"https://github.com/ZenHive/mpp/security/advisories/GHSA-w8j7-7qc3-5f24"},{"tags":["related"],"url":"https://cna.erlef.org/cves/CVE-2026-73829.html"},{"tags":["related"],"url":"https://osv.dev/vulnerability/EEF-CVE-2026-73829"},{"tags":["patch"],"url":"https://github.com/ZenHive/mpp/commit/46c5b0e1311da7d92190dc7d9ea89027a1d365e9"}],"source":{"discovery":"EXTERNAL"},"title":"Non-atomic hash-credential dedup in mpp Tempo allows replay of a confirmed payment under a concurrent race"}},"cveMetadata":{"assignerOrgId":"6b3ad84c-e1a6-4bf7-a703-f496b71e49db","assignerShortName":"EEF","cveId":"CVE-2026-73829","datePublished":"2026-08-19T17:19:43.696Z","dateReserved":"2026-08-17T11:15:02.320Z","dateUpdated":"2026-08-19T17:19:43.696Z","state":"PUBLISHED"},"dataType":"CVE_RECORD","dataVersion":"5.2"}