CVE-2026-88255: mpp Tempo keys its pre-broadcast dedup reserve on the caller-supplied transaction encoding, so a re-encoded signed transaction reserves a second slot
Improper Validation of Unsafe Equivalence in Input in ZenHive mpp allows an unauthenticated remote client to pass the Tempo duplicate-submission gate twice with one signed transaction.
MPP.Methods.Tempo reserves the pre-broadcast dedup slot on the caller-supplied hex in reservehashatomic/2, keyed through storekey/1 on tx.raw rather than on a canonical form of the transaction. The deserializer stores the caller's hex verbatim and accepts both recovery-id encodings, so one signed transaction submitted once with v=27 and once with v=0 yields two distinct reserve keys, and both pass the reserve and reach the broadcast path. The plug-level credential replay store is deliberately carved out for tempo in lib/mpp/replay.ex, leaving this reserve as the only gate, and the post-broadcast mark writes the canonical hash key that the raw-keyed reserve never reads.
What the duplicate submission yields depends on the node: a nonce-reuse rejection fails closed, while a node that answers with the canonical hash for an already-known transaction returns a second valid Payment-Receipt for a single on-chain payment.
This issue affects mpp: from 0.2.0 before 0.16.2.
Affected Software
Event History
Frequently Asked Questions
Which deployments are affected?
ZenHive mpp versions from 0.2.0 up to, but not including, 0.16.2 are affected. The affected path is MPP.Methods.Tempo.
Does an attacker need credentials or a second signed transaction?
No credentials are required. A remote client can submit the same signed transaction twice using alternate accepted recovery-ID encodings, such as v=27 and v=0.
Is the plug-level credential replay store a mitigating control for this path?
No. Tempo is deliberately excluded from the plug-level credential replay store, leaving the pre-broadcast reserve as its only duplicate-submission gate.
What is the practical result of a successful duplicate submission?
The result depends on the node's handling of an already-known transaction. Nodes that reject nonce reuse fail closed, while nodes that return the canonical hash can issue a second valid Payment-Receipt for the same on-chain payment.