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Blockchain Security
Quick Start
- Download and decompile contracts (source or bytecode)
- Map storage layout and identify privileged operations
- Check for delegatecall, CREATE address prediction, reentrancy, access control
- Deploy exploit contracts via web3.py or cast/forge
- Verify win condition (isSolved/flag endpoint)
Blockchain CTF Challenge Pattern
bash
# 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 trueKey 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 norequire(arr.length >= N) - Exploit: Call the function with
[]to skip all validation
5. ECDSA Signature Malleability
Raw ecrecover accepts both (v, r, s) and (v', r, N-s) (where N = secp256k1 order, v flipped 27↔28). If a contract deduplicates signatures by hash of raw bytes, the malleable form has a different hash but recovers to the same signer.
- Check:
ecrecoverused withouts <= N/2enforcement (OpenZeppelin's ECDSA.sol enforces this) - Exploit: Take a known valid signature, compute
new_s = N - s, flipv, submit as "new" signature
6. Common Vulnerability Classes
| Vulnerability | Check |
|---|---|
| Reentrancy | External calls before state updates |
| Access control | Missing onlyOwner / msg.sender checks |
| Integer overflow | Solidity < 0.8.0 without SafeMath |
| Delegatecall injection | User-controlled delegatecall target |
| tx.origin auth | tx.origin instead of msg.sender |
| Selfdestruct | Force-send ETH, reset contract nonce |
| Weak randomness | blockhash/timestamp as entropy source |
| Empty array bypass | Loop validation with no min-length check |
| Signature malleability | Raw ecrecover without s-normalization |
Tools
python
# web3.py essentials
from web3 import Web3
w3 = Web3(Web3.HTTPProvider(RPC_URL))
acct = w3.eth.account.from_key(PRIVATE_KEY)
# Read private storage
w3.eth.get_storage_at(contract_addr, slot)
# Deploy contract
from solcx import compile_source, install_solc
install_solc("0.8.13")
compiled = compile_source(source, output_values=["abi", "bin"], solc_version="0.8.13")
# Send raw bytecode deployment
tx = {'data': bytecode, 'gas': 3000000, 'gasPrice': w3.eth.gas_price, 'nonce': nonce, 'chainId': chain_id}
signed = acct.sign_transaction(tx)
w3.eth.send_raw_transaction(signed.raw_transaction)Reference
Critical Rules
- Always read storage before attacking (private vars are readable on-chain)
- Mirror exact storage layout when exploiting delegatecall
- For CREATE nonce brute-force, check nonces 0-100000+ systematically
- CTF instances are often ephemeral -- script the full attack for speed
