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

[L-1] Random APT amount for the slice can be predicted based on the time of the registration

emerjux

Root + Impact

Description

  • The function to get the random amount of APT for the slide is based on time mod on 401, that can easily guessed by an attacker

  • When knowing which occurrences on time gives the maximum

#[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:

  • If the attacker knows when in time the owner will register the users, can manipulate the request to maximize the chances to get the highest reward

  • If the owner is aware and wants to rig the system

Impact:

  • It defies the main purpose of randomnes by giving the exact amount on rewards the user will get based on the time of registration

Proof of Concept

A quick query with AI will list all the iterations of the epoch in milliseconds of the day where you can get the 500 APT

The first 4 ocurrencies for 29 of August that will get the 400

  1. Timestamp: 1756425600000110

    • Time (UTC): 2025-08-29 00:00:00.000110

  2. Timestamp: 1756425600000511

    • Time (UTC): 2025-08-29 00:00:00.000511

  3. Timestamp: 1756425600000912

    • Time (UTC): 2025-08-29 00:00:00.000912

  4. Timestamp: 1756425600001313

    • Time (UTC): 2025-08-29 00:00:00.001313

Recommended Mitigation

Use the random generator from aptos framework for randomize the number between 0 and 400

use aptos_framework::randomness::{u64_range};
​
#[randomness]
entry fun get_random_slice(user_addr: address) acquires ModuleData, State {
let state = borrow_global_mut<State>(get_resource_address());
// Securely generate a random value in the desired range (0 to 400)
let random_val = u64_range(0, 400);
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 9 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.

emerjux Submitter
8 days ago
bube Lead Judge
8 days ago
bube Lead Judge 6 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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