Access Control

Challenge Description

We’ve created a simple contract to store a secret flag. But you currently are not the owner of the contract… Only the owner can access the flag. Can you figure out how to become the owner?

Approach

This is a classic smart contract access control vulnerability challenge. The contract stores a secret flag and restricts access to the “owner,” but the access control mechanism has a flaw that allows anyone to become the owner.

Common Blockchain Access Control Vulnerabilities

1. Unprotected state-changing functions: A function that sets the owner lacks an access modifier (e.g., onlyOwner), allowing anyone to call it.
2. tx.origin vs msg.sender confusion: Using tx.origin for authentication instead of msg.sender enables phishing attacks.
3. Public visibility on sensitive functions: Functions intended to be internal/private are accidentally left public.
4. Initialization race condition: The constructor or initializer can be called by anyone, or there is a separate initialize() function that was never called (or can be called again).
5. Storage slot reading: Even “private” variables in Solidity are readable on-chain — nothing on the blockchain is truly private.

Typical Contract Pattern

pragma solidity ^0.8.0;

contract AccessControl {
    address public owner;
    string private flag;

    constructor(string memory _flag) {
        owner = msg.sender;
        flag = _flag;
    }

    // VULNERABILITY: No access control on this function!
    function setOwner(address _newOwner) public {
        owner = _newOwner;
    }

    // Or alternatively, a flawed modifier:
    // function setOwner(address _newOwner) public {
    //     require(tx.origin == owner);  // vulnerable to phishing
    //     owner = _newOwner;
    // }

    function getFlag() public view returns (string memory) {
        require(msg.sender == owner, "Not the owner!");
        return flag;
    }
}

Attack Vectors

Approach A — Unprotected setOwner / changeOwner: Simply call the function that changes ownership with your own address.

Approach B — Read storage directly: Even if you cannot become the owner, the flag stored as a “private” variable can be read directly from the blockchain storage slot using web3.eth.getStorageAt().

Approach C — Unprotected initializer: Call an initialize() or init() function that was left unprotected.

Solution

1. Connect to the provided RPC endpoint using the challenge credentials (typically an RPC URL, contract address, and a funded wallet private key).
2. Read the contract ABI or source code (usually provided or discoverable via the challenge).
3. Identify the vulnerability: Look for an unprotected function that modifies the owner state variable.
4. Call the vulnerable function to set yourself as the owner.
5. Call getFlag() to retrieve the flag.
6. Alternatively, read the storage slot directly to bypass access control entirely.

Solution Script

python3 solve.py

Flag

picoCTF{...}  (placeholder - actual flag varies per instance)