GHSA-pf4j-pf3w-95f9 nimiq-transaction: UpdateValidator transactions allows voting key change without proof-of-knowledge

• crates.io nimiq-transaction: <= 0.2.0 (no fix yet)

Impact The staking contract accepts `UpdateValidator` transactions that set `new_voting_key=Some(...)` while omitting `new_proof_of_knowledge`. this skips the proof-of-knowledge requirement that is needed to prevent BLS rogue-key attacks when public keys are aggregated. Because tendermint macro block justification verification aggregates validator voting keys and verifies a single aggregated BLS signature against that aggregate public key, a rogue-key voting key in the validator set can allow an attacker to forge a quorum-looking justification while only producing a single signature. While the impact is critical, the exploitability is low: The voting keys are fixed for the epoch, so the attacker would need to know the next epoch validator set (chosen through VRF), which is unlikely. Patches The patch for this vulnerability is included as part of v1.3.0. Workarounds No known workarounds.