Cross-Chain Balance Inconsistency Exploit Generator

Overview

The cross-chain balance inconsistency exploit generator validates findings from the cross_chain_balance, bridge_balance, and related detectors by simulating an attack where 1000 tokens are burned on the source chain but 2000 tokens are minted on the destination chain — a 2x supply inflation. The generator classifies which specific atomicity gap caused the inconsistency (replay, non-atomic operations, weak verification, or finality exploit) and generates a targeted proof of concept.

Cross-chain bridge balance inconsistencies have produced some of the largest losses in DeFi history. The Poly Network lost $611M when attackers bypassed its cross-chain proof verification. The Wormhole Bridge lost $326M from a signature verification bypass. The Nomad Bridge lost $190M when an initialization bug treated all messages as pre-verified. In each case, the fundamental invariant — totalSupply(destination) == totalLocked(source) — was violated.

Note: Exploit generation in Sigvex is for vulnerability validation purposes only.

Attack Scenario

Replay attack (missing proof deduplication):

1. A user burns 1000 tokens on the source chain.
2. A relayer submits the burn proof to the destination chain, which mints 1000 tokens.
3. The destination bridge does not track which proofs have been used.
4. The attacker re-submits the same burn proof 10 times.
5. The destination mints 10,000 tokens total from a single 1000-token burn.

Non-atomic operations (mint before confirmed burn):

1. A bridge emits a BurnRequested event before actually burning tokens.
2. A relayer sees the event and mints on the destination.
3. The source chain burn transaction runs out of gas or reverts after the event was emitted.
4. Tokens were minted on the destination but never burned on the source.
5. The user holds tokens on both chains simultaneously.

Weak proof verification:

1. The destination bridge requires proof.length > 0 as its only validation.
2. An attacker submits bytes("fake") as the proof.
3. The bridge mints tokens without any cryptographic verification of a source chain burn.
4. Repeated 100 times, the attacker inflates supply by 100,000 tokens from zero burns.

Finality exploit (chain reorganization):

1. A user burns tokens on the source chain.
2. The destination bridge uses only 1 confirmation block before minting.
3. The source chain reorganizes — the burn transaction is removed from the canonical chain.
4. The destination’s mint remains because it confirmed before the reorg propagated.
5. The user holds tokens on both chains.

Exploit Mechanics

The generator simulates the balance inconsistency mathematically:

source_burned  = 1000 tokens (1000 * 1e18 units)
dest_minted    = 2000 tokens (2000 * 1e18 units)
supply_inflation = 1000 tokens

It classifies the vulnerability type from the finding description:

Description contains	Vulnerability type	Severity
replay	replay_attack	CRITICAL: Unlimited minting via replay
atomic, atomicity	non_atomic_bridge	CRITICAL: Double-spend tokens across chains
verify, proof	weak_verification	CRITICAL: Unlimited inflation via forged proofs
finality	finality_exploit	HIGH: Finality delays enable double-spend
(other)	balance_inconsistency	HIGH: Supply inconsistency across chains

Estimated gas: 80,000.

// VULNERABLE: No replay protection
contract VulnerableBridgeReplay {
    mapping(address => uint256) public balances;

    function mintFromBridge(address to, uint256 amount, bytes32 burnProof) external {
        require(burnProof != bytes32(0), "Invalid proof");
        // VULNERABILITY: burnProof can be submitted multiple times!
        balances[to] += amount;
    }
}

// Exploit: Submit same proof 10 times → 10x minting
contract ReplayExploit {
    function attack(bytes32 legitimateBurnProof) external {
        for (uint i = 0; i < 10; i++) {
            bridge.mintFromBridge(msg.sender, 1000 ether, legitimateBurnProof);
        }
        // Burned 1000 tokens, minted 10,000 → 9000 inflation
    }
}

// SECURE: Proof deduplication + deep finality + merkle verification
contract SecureBridge {
    mapping(bytes32 => bool) public usedProofs;
    uint256 public constant MIN_CONFIRMATIONS = 64; // Deep finality

    function mintFromBridge(
        address to, uint256 amount, bytes32 burnTxHash, uint256 sourceBlockNumber,
        bytes32[] memory merkleProof, bytes memory validatorSignatures
    ) external {
        bytes32 proofHash = keccak256(abi.encodePacked(burnTxHash, to, amount, sourceBlockNumber));

        // 1. Replay protection
        require(!usedProofs[proofHash], "Proof already used");

        // 2. Finality check (64 blocks for Ethereum mainnet)
        require(block.number >= sourceBlockNumber + MIN_CONFIRMATIONS, "Insufficient confirmations");

        // 3. Cryptographic proof verification
        require(verifyMerkleProof(merkleProof, burnTxHash), "Invalid merkle proof");

        // 4. Validator signature verification
        require(verifyValidatorSignatures(validatorSignatures, proofHash), "Invalid signatures");

        usedProofs[proofHash] = true;
        balances[to] += amount;
    }
}

Remediation

• Detector: Cross-Chain Balance Detector
• Remediation Guide: Cross-Chain Balance Remediation

Address each attack vector with a dedicated mitigation:

1. Replay protection: Track all processed proof hashes in a mapping. Use bytes32 proofHash = keccak256(abi.encodePacked(burnTxHash, to, amount, sourceBlockNumber)) as the deduplication key.

2. Atomic operations: Use lock-and-mint rather than burn-and-mint where possible. Lock tokens on the source (recoverable if mint fails) rather than burning them irrecoverably before the mint is confirmed.

3. Deep finality: Require a minimum of 64 block confirmations on Ethereum mainnet before processing a mint. For probabilistic finality chains, use a conservative estimate of the orphan risk.

4. Cryptographic proof verification: Implement Merkle proof verification of the source chain burn transaction. Require signatures from at least 2/3 of an independent validator set.

5. Balance reconciliation monitoring: Continuously monitor totalSupply(destination) == totalLocked(source). Emit alerts and trigger circuit breakers if the invariant is violated.

6. Transfer limits: Enforce per-transaction and per-period maximum transfer amounts. Large transfers require longer timelocks.

References

• SWC-104: Unchecked Call Return Value
• a major cross-chain messaging protocol Security Documentation
• CWE-345: Insufficient Verification of Data Authenticity