Token to Wonderland

hackthebox

Task: Buy Golden Key from Shop contract with only 100 SVC tokens when price is 25,000,000. Solution: Exploit integer underflow in Solidity 0.7.x ERC20 token where transfer() skips balance check, causing balance to wrap to 2^256-1, then approve and purchase.

blockchain integer_underflow solidity erc20 bytecode_analysis pre_0.8_solidity

dual_transfer_implementationerc20_balance_manipulationevm_bytecode_reverseinteger_underflow

$ ls tags/ techniques/

blockchain integer_underflow solidity erc20 bytecode_analysis pre_0.8_solidity

dual_transfer_implementationerc20_balance_manipulationevm_bytecode_reverseinteger_underflow

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Token to Wonderland — HackTheBox

Description

A group found a map to a treasure guarded by 3 keys. They need to acquire the "Golden Key" from a dwarf's shop, but they don't have enough "SilverCoins" (ERC20 tokens) to buy it. The challenge provides a private Ethereum chain with three contracts: Setup, SilverCoins (ERC20), and Shop.

Goal: Buy the Golden Key (item 2) from the Shop contract, having only 100 SVC tokens while the price is 25,000,000.

Analysis

Infrastructure

Private Ethereum chain (chain ID 31337)
Web interface: /rpc, /flag, /connection_info, /docs
Player receives 5000 ETH and 100 SVC (SilverCoins) tokens

Contracts

Setup (0x4e07...):

isSolved(address) — checks items[2].owner == player_address on Shop
TARGET() — Shop contract address

SilverCoins (0x045d...) — ERC20 token:

Compiled with Solidity 0.7.06 (no built-in overflow/underflow protection!)
Total supply: 1e15
Player receives 100 tokens, the rest goes to Setup

Shop (0xf39e...) — shop with 3 items:

Item	Name	Price	Owner
0	Diamond Necklace	1,000,000	Shop
1	Ancient Stone	70,000	Shop
2	Golden Key	25,000,000	Shop

buyItem Function

1. Loads item from storage
2. Checks item.owner == address(this) (shop must own it)
3. Calls token.transferFrom(msg.sender, address(this), item.price)
4. Checks return value: require(success, "Payment failed!")
5. Sets items[id].owner = msg.sender

buyItem uses transferFrom(), which correctly checks the balance. This means we need to have enough tokens.

Vulnerability: Dual _transfer Implementation

When analyzing the ERC20 token bytecode, two different internal _transfer functions were discovered:

_transfer for transferFrom() (offset 0x63b): Correctly checks require(balances[from] >= amount).

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