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

`timestamp::now_microseconds()` is predictable and manipulable by block producers

strapontin

Root + Impact

Description

  • Normal behavior: During registration, the contract assigns the user a random slice size and stores it in users_claimed_amount. Later, the user claims exactly that assigned amount from the resource account balance.

  • Issue: get_random_slice derives randomness from timestamp::now_microseconds() and reduces it with % 401. Block producers can influence timestamps within consensus bounds, making outcomes predictable and biasable. The #[randomness] attribute is present but no aptos_framework::randomness API is used.

// sources/pizza_drop.move
#[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:

  • Validators routinely set/adjust block timestamps within allowed skew during registration transactions; the monotonic, low-entropy input produces predictable residues in % 401.

Impact:

  • Fairness is undermined: attackers bias for larger slices.

Recommended Mitigation

#[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 = randomness::u64_range(100, 501); // 100-500 inclusive
- 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 11 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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