PP Farming 2 — sekai2026

Description

I fixed the issue. I think...

English summary: the attachment was an AES-encrypted ZIP. The prompt said to use the previous PP Farming flag as the password; that previous flag was only a password/context value, not the final flag for this challenge. After decryption, the goal was to drain an Ethereum PerformancePointATM instance so that isSolved() returned true.

Analysis

The original PP Farming bug was a straightforward reentrancy issue, but PP Farming 2 adds a noReentrancy modifier to withdrawPP(). The successful exploit is therefore not reentrancy.

The decrypted source contains two relevant contracts:

• PerformancePointHelper storage layout:
  • slot 0: id_number
  • slot 1: atm
  • slot 2: helping
• PerformancePointATM storage layout:
  • slot 0: scores mapping
  • slot 1: performancePointHelper
  • slot 2: locked

PerformancePointATM.withdrawPP() reads the caller's score, enters noReentrancy, and then executes:

performancePointHelper.delegatecall(
    abi.encodeWithSignature("processWithdrawal(address,uint256)", msg.sender, score)
);
scores[msg.sender] = 0;

The fallback function also proxies arbitrary calls to performancePointHelper using delegatecall. It blocks only the selector for processWithdrawal(address,uint256), but it does not block other helper functions such as setATM(address).

That creates a storage-collision primitive. Calling setATM(address) on the ATM reaches the fallback, which delegatecalls the helper. In the helper, setATM writes helper slot 1 (atm). Under delegatecall, slot 1 belongs to the ATM, so this overwrites PerformancePointATM.performancePointHelper. An attacker can replace the helper implementation with an arbitrary contract.

Once the helper address is replaced, withdrawPP() delegatecalls attacker-controlled code. During a delegatecall, address(this) is still the ATM, so attacker code can send the ATM's entire balance.

Solution

The malicious helper ignores the requested withdrawal amount and transfers address(this).balance to the recipient:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

...