Sequential Validator Front-Running: Deterministic Ordering Exploits in Proof-of-Stake Networks

Proof-of-Stake (PoS) blockchain networks replace energy-intensive mining with validator-based block production and transaction ordering. While this design improves efficiency and scalability, it introduces a largely unexamined structural vulnerability: deterministic or predictable validator sequencing. This paper defines Sequential Validator Front-Running as a validator-level exploit whereby participants leverage foreknowledge of execution slots to extract economic advantage, manipulate outcomes, or preempt competing transactions. Unlike classical Miner/Maximal Extractable Value (MEV), this exploit arises from consensus scheduling itself rather than transaction visibility alone. We analyze the structural causes of this vulnerability, demonstrate why conventional mitigations such as governance controls, slashing, and post-hoc monitoring are insufficient, and propose a logic-layer sequencing constraint framework enforced at block construction time. The proposed approach neutralizes sequencing abuse while preserving throughput, finality, and protocol compliance.