Timestamp Dependence

block.timestamp is set by the miner who proposes a block. Ethereum allows miners to adjust timestamps within a tolerance of approximately 15 seconds (in PoW; validators in PoS have similar flexibility within a slot). Any contract that uses block.timestamp as randomness or as a fine-grained trigger is potentially vulnerable.

How it works

If a contract pays out a reward to whoever calls it at exactly the right millisecond, a miner can mine a block with a crafted timestamp to ensure they win. More broadly, anything that derives "randomness" from block.timestamp, block.number, or blockhash is predictable to miners.

Vulnerable pattern

// VULNERABLE — using timestamp for randomness
function random() internal view returns (uint) {
    return uint(keccak256(abi.encode(block.timestamp, msg.sender))) % 100;
}

// Or: a lottery that can be gamed by a miner
function isWinner() public view returns (bool) {
    return block.timestamp % 15 == 0;  // miner picks timestamp to win
}

Safe pattern

// SAFE — use Chainlink VRF for verifiable randomness
import "@chainlink/contracts/src/v0.8/vrf/VRFConsumerBaseV2.sol";

// Request random number from Chainlink VRF
function rollDice() public returns (uint256 requestId) {
    requestId = COORDINATOR.requestRandomWords(
        keyHash, subscriptionId, requestConfirmations, callbackGasLimit, numWords
    );
}

// Callback with proven-random value
function fulfillRandomWords(uint256 requestId, uint256[] memory randomWords) internal override {
    result = randomWords[0] % 100;
}

When it's OK to use block.timestamp

Using block.timestamp is fine for coarse-grained time checks where 15-second manipulation doesn't matter — for example, checking that a lock period of 30 days has elapsed. It's only dangerous when the outcome depends on timestamp precision within seconds.

How to prevent it

• Use Chainlink VRF for any on-chain randomness.
• Use commit-reveal schemes if you can't use VRF.
• Only use block.timestamp for durations that are large compared to the ~15-second manipulation window.