Table of Contents

• Overview
• Reconnaissance
• Exploitation
  • Reading contract storage
  • Decoding the storage
• Flag
• Answers
• Tools Used
• What Didn't Work
• Lessons Learned

PassCode — TryHackMe CTF Writeup #

Platform: TryHackMe (Hackfinity Battle CTF)
Category: Blockchain / Web3
Difficulty: Easy
Date: 2026-03-23
Author: t0nt0n
Reading time: ~4 min

Overview #

PassCode is a smart contract challenge. A deployed Ethereum contract holds a passcode that must be submitted to flip isSolved() to true. The trick: the passcode is stored in a private storage variable — which is NOT actually private on a public blockchain.

Reconnaissance #

The challenge API provides everything needed to get started:

1RPC_URL=http://10.129.160.37:8545
2API_URL=http://10.129.160.37
3
4curl -s ${API_URL}/challenge | jq .

1{
2  "name": "blockchain",
3  "player_wallet": {
4    "address": "0x738fB6Ba7A06E89F58df5d3da7c36C3Fb3e58ead",
5    "private_key": "0x8954bc7b973945979af2c8ab8440fea0ca0a161d33dd3338a6e41f88ac61e80d",
6    "balance": "1.0 ETH"
7  },
8  "contract_address": "0xf22cB0Ca047e88AC996c17683Cee290518093574"
9}

Exploitation #

Reading contract storage #

All contract storage is publicly readable via eth_getStorageAt, regardless of whether variables are declared private in Solidity. Reading slots 0–3:

1for i in 0 1 2 3; do
2  echo -n "Slot $i: "
3  curl -s -X POST http://10.129.160.37:8545 \
4    -H "Content-Type: application/json" \
5    -d "{\"jsonrpc\":\"2.0\",\"method\":\"eth_getStorageAt\",
6        \"params\":[\"0xf22cB0Ca047e88AC996c17683Cee290518093574\",
7                   \"0x$(printf '%064x' $i)\",\"latest\"],\"id\":1}" \
8    | jq -r '.result'
9done

Slot 0: 0x54484d7b776562335f6834636b316e675f636f64657d0000000000000000002c
Slot 1: 0x0000000000000000000000000000000000000000000000000000000000000000
Slot 2: 0x000000000000000000000000000000000000000000000000000000000000014d
Slot 3: 0x54686520636f646520697320333333000000000000000000000000000000001e

Decoding the storage #

 1# Slot 0 — Solidity short string encoding (last byte = length)
 2bytes.fromhex('54484d7b776562335f6834636b316e675f636f64657d').decode()
 3# → THM{w_redacted_e}
 4
 5# Slot 2 — uint256 passcode
 60x14d  # → 333
 7
 8# Slot 3 — hint string
 9bytes.fromhex('54686520636f646520697320333333').decode()
10# → "The code is 333"

The flag is directly readable from slot 0. Slot 3 confirms the passcode is 333 (also stored as 0x14d in slot 2).

Flag #

Answers #

Q — What is the flag?

Tools Used #

• curl — JSON-RPC calls to the Ethereum node (eth_getStorageAt, eth_getCode)
• python3 — hex decoding of storage slot values
• jq — JSON parsing

What Didn't Work #

• eth_sendTransaction without signing — the node rejects unsigned transactions with unknown account. Would need to sign with the private key via web3.py or cast (Foundry) to actually call the contract function. Not needed here since the flag was already in storage.

Lessons Learned #

• In Solidity, private only prevents other contracts from reading a variable — it does NOT hide it from anyone who can query the blockchain. All storage is public.
• Slot layout follows declaration order: first string → slot 0, first bool → slot 1, first uint256 → slot 2, etc.
• Short strings (≤ 31 bytes) are packed inline in their slot: the last byte encodes length * 2, the string data starts at the left of the slot.
• Always read raw storage before attempting contract interaction — the answer may already be there.

⬛⚪⬛
⬛⬛⚪  ☠ user
⚪⚪⚪  rm -rf /ignorance && echo 42 > /dev/brain