Layer 2 rollups offer promising solutions to address Ethereum's scalability issues. However, the centralized nature of the sequencer in these rollups makes them vulnerable to denial of service attacks, in which adversaries overwhelm the sequencer with invalid transactions that cannot be included in blocks, thereby exhausting its computational resources for transaction processing. To mitigate such threat, layer 2 rollups implement the legality check mechanism to filter out invalid transactions before they reach the sequencer.

Circom is a popular programming language for writing arithmetic circuits that can be used to generate zero-knowledge proofs (ZKPs) like zk-SNARKS. ZKPs have received tremendous attention in protocols like zkRollups. The Circom circuits are compiled to Rank-1 Constraint Systems (R1CS) circuits, based on which zk-SNARK proofs are generated. However, one major challenge associated with R1CS circuits is the problem of under-constrained circuits, which are susceptible to allowing incorrect computations to pass verification due to insufficient constraints, potentially leading to security vulnerabilities. In this paper, we propose a novel framework CONSCS to automatically verify Circom circuits. Our contributions are threefold: 1) we propose novel circuit inference rules to help reduce the size of circuits and to extract more comprehensive information than existing works; 2) we introduce the novel Binary Property Graph (BPG) as a highly efficient reasoning engine, outperforming all existing tools in effectiveness and efficiency; 3) we leverage fine-grained domain-specific information to guide the SMT solving to address non-linear constraints, increasing the success rate of SMT queries of existing works from 2.68% to 48.84%. We conduct experiments to show that CONSCS enhances the solved rate of existing works from around 50-60% to above 80%.