blockchain-security

Blockchain Security

Quick Start

1. Download and decompile contracts (source or bytecode)
2. Map storage layout and identify privileged operations
3. Check for delegatecall, CREATE address prediction, reentrancy, access control
4. Deploy exploit contracts via web3.py or cast/forge
5. Verify win condition (isSolved/flag endpoint)

HTB Blockchain Challenge Pattern

# Get connection info
curl http://$HOST:$PORT/connection_info  # -> PrivateKey, Address, TargetAddress, setupAddress
# RPC endpoint
RPC_URL="http://$HOST:$PORT/rpc"
# Win condition: Setup.isSolved() must return true

Key Attack Vectors

1. Delegatecall Storage Manipulation

When contract A does delegatecall to contract B, B's code runs with A's storage.

• Deploy exploit contract that mirrors A's storage layout
• Exploit contract writes to A's storage slots via delegatecall
• Critical: Storage layout must match exactly (same slot ordering)
• See reference/delegatecall-attacks.md

2. CREATE Address Prediction (Nonce Manipulation)

Contract addresses from CREATE are deterministic: keccak256(rlp([sender, nonce]))[12:]

• Brute-force nonce to find which nonce produces target address
• Send dummy transactions (self-transfers) to increment nonce
• Deploy exploit contract at the exact nonce that hits target address
• See reference/create-address-prediction.md

3. Storage Layout & Slot Computation

• Mappings: keccak256(h(key) || uint256(slot_number))
  • Value types: h(k) = abi.encode(k) (left-padded to 32 bytes)
  • String/bytes: h(k) = keccak256(k)
• Read private variables via eth_getStorageAt
• See reference/storage-layout.md

4. Empty Array / Zero-Length Input Bypass

When a function loops over a user-supplied array to validate items (signatures, approvals, votes), passing an empty array skips the loop entirely. If there's no minimum-length check, validation is bypassed.

• Check: for (uint i = 0; i < arr.length; i++) with no require(arr.length >= N)
• Exploit: Call the function with [] to skip all validation