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

Randomness Predictability Vulnerability

py430896

Randomness Predictability Vulnerability

Description

The vulnerability stems from using timestamp::now_microseconds() as the source of randomness. This approach is fundamentally flawed because:

  1. Timestamps are predictable and can be anticipated by attackers.

  2. Miners/validators can influence or control block timestamps.

  3. The same timestamp will always produce the same "random" value.

#[randomness]
entry fun get_random_slice(user_addr: address) acquires ModuleData, State {
let state = borrow_global_mut<State>(get_resource_address());
@> let time = timestamp::now_microseconds();
let random_val = time % 401;
let random_amount = 100 + random_val; // 100-500 APT (in Octas: 10^8 smallest unit)
table::add(&mut state.users_claimed_amount, user_addr, random_amount);
}
​

Risk

Likelihood:
High

Impact:

  • Impact

  1. Users can potentially manipulate the system to receive maximum airdrop amounts (500 APT instead of random 100-500 APT)

  2. Fairness Impact: Undermines the fairness of the airdrop distribution mechanism

  3. Compromises the integrity of the entire airdrop system

Proof of Concept

An attacker could:

  1. Monitor blockchain timestamps

  2. Time their transaction submission to coincide with favorable timestamp values

  3. Potentially receive maximum airdrop amounts consistently

#[test]
fun test_secure_randomness() {
// With Aptos randomness, attacker cannot predict outcomes
let results = vector[];
for (i in 0..1000) {
let random_amount = get_secure_random_amount();
results.push_back(random_amount);
}
// Verify proper distribution (should be roughly uniform)
let distribution = analyze_distribution(results);
assert!(distribution.is_uniform, "Randomness should be properly distributed");
assert!(no_pattern_detected(results), "No predictable patterns should exist");
}
​

Recommended Mitigation

Use Aptos' built-in randomness API which provides cryptographically secure, unpredictable randomness:

+# use aptos_framework::random;
+
#[randomness]
entry fun get_random_slice(user_addr: address) acquires ModuleData, State {
let state = borrow_global_mut<State>(get_resource_address());
- let time = timestamp::now_microseconds();
- let random_val = time % 401;
+ let random_val = random::random_u64() % 401; // Secure on-chain randomness
let random_amount = 100 + random_val; // 100-500 APT (in Octas: 10^8 smallest unit)
table::add(&mut state.users_claimed_amount, user_addr, random_amount);
}
​
Updates

Appeal created

bube Lead Judge about 1 month 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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