# CVE-2026-73829: ZenHive mpp TOCTOU race condition in payment verification

Source: https://junglewise.ai/threats/cve-2026-73829-zenhive-mpp-toctou-race-condition-in-payment-verification  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-19. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-73829 · Severity: low · CVSS 3.7 · Published 2026-08-19

ZenHive mpp is a payment processing library that handles on-chain cryptocurrency payments for resource access. A race condition in the payment verification logic allows an attacker to redeem a single confirmed blockchain payment multiple times by submitting concurrent requests, resulting in free access to paid resources. This vulnerability only affects systems configured with a deduplication store; the default configuration is unaffected.

Technologies: [ZenHive Machine Payments Protocol](https://junglewise.ai/threats/technologies/machine-payments-protocol), [ZenHive Mpp](https://junglewise.ai/threats/technologies/mpp). Vendors: [ZenHive](https://junglewise.ai/threats/vendors/zenhive).

## Quick answers

**Is CVE-2026-73829 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-73829 being exploited in the wild.

**How severe is CVE-2026-73829?** Low severity, with a CVSS score of 3.7, published 19 August 2026.

**Which products and versions are affected?** ZenHive Mpp 0.2.0 before 0.6.1.

**Is there a fix for CVE-2026-73829?** A patch was published on 0.6.1. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-73829 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-73829?** Check whether you run ZenHive Machine Payments Protocol or ZenHive Mpp at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The vulnerability is a Time-of-Check Time-of-Use (TOCTOU) race condition in the hash-based credential verification path (MPP.Methods.Tempo.verify/2). The vulnerable code checks whether a payment hash has been used (check_hash_unused/2), verifies the payment on-chain via eth_getTransactionReceipt, and only then marks it as used (mark_hash_used/2). This sequence is non-atomic: concurrent requests can all pass the initial check before any performs the mark operation, allowing each to receive a valid receipt for the same payment. The underlying dedup store provides an atomic check_and_mark/2 primitive that is used by the transaction-based path but not the hash path. Exploitation requires a dedup store to be explicitly configured; the default nil store does not deduplicate and is unaffected. Attack vector is network-based and requires no authentication. Patched in version 0.6.1.

## Affected products

- ZenHive mpp 0.2.0 before 0.6.1

## Timeline

- 2026-08-19: disclosed
- 0.6.1: patched

## References

- [https://repo.hex.pm/](https://repo.hex.pm/)
- [https://github.com/](https://github.com/)

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