Aptos Pizza Drop
Cyfrin
Beginner FriendlyGameFi
100 EXP
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Submission Details
Severity: medium
Valid

Insecure Randomness via Timestamp

sageceo

Root + Impact

The contract uses timestamp::now_microseconds() as the randomness source. Attackers can manipulate or predict the reward outcome, repeatedly calling the function to maximize payouts.

Description

  • Normal behavior: The function get_random_slice() is supposed to assign users a random reward.

  • Issue: The randomness is derived from timestamp::now_microseconds(), which is predictable and can be influenced by validators. This makes the outcome guessable and manipulable.

let time = timestamp::now_microseconds(); //@> predictable value
let random_val = time % 401;

Risk

Likelihood:

  • Reason 1 Every time the function runs, the timestamp is predictable and partially controllable by block producers.

  • Reason 2

Impact:

  • Impact 1: The Entire reward distribution mechanism can be exploited, draining more APT than intended.

  • Impact 2

Proof of Concept

The attacker can keep calling get_random_slice until the reward is "high"

// Attacker repeatedly calls the function to bias the reward
script {
use 0xYourModule::pizza_drop;
​
fun main(attacker: signer) {
// Keep calling get_random_slice until reward is "high"
pizza_drop::get_random_slice(@attacker);
}
}
​

Recommended Mitigation

Use a verifiable randomness source (e.g., VRF or an oracle-based randomness beacon) instead of timestamps

- let time = timestamp::now_microseconds();
- let random_val = time % 401;
+ let random_val = vrf::get_random_u64(seed); // Example secure VRF API
Updates

Appeal created

bube Lead Judge 12 days ago
Submission Judgement Published
Validated
Assigned finding tags:

Predictable randomness

The `get_random_slice` function should only be called by the owner via the `register_pizza_lover` function. Also, the `owner` is trusted and will not choose a specific time for a new user to register. Therefore, I disagree with the claim of most reports in this group that an attacker can manipulate the random number of pizza slices. But I agree with the root cause of the reports in this group, that the random distribution is not completely random.

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