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

Weak Randomness Implementation

0xiamwai

Root + Impact

Description

The contract uses timestamp-based randomness to determine slice amounts, which is predictable and can be manipulated by validators or sophisticated attackers. The randomness is generated using timestamp::now_microseconds() % 401 + 100.

#[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(); // @> Predictable randomness source
let random_val = time % 401; // @> Simple modulo operation
let random_amount = 100 + random_val; // @> Results in 100-500 range
table::add(&mut state.users_claimed_amount, user_addr, random_amount);
}

Risk

Likelihood:

  • Validators can manipulate transaction timing to influence randomness outcomes

  • Attackers can predict slice amounts by observing timestamp patterns

  • MEV (Maximum Extractable Value) opportunities exist for sophisticated actors

Impact:

  • Unfair distribution of rewards favoring certain participants

  • Loss of randomness guarantees breaks the intended game mechanics

  • Potential for systematic exploitation reducing available funds

Proof of Concept

// Attacker can predict the outcome by timing their registration
// If current timestamp is X, the slice will be (X % 401) + 100
// Validators can delay/accelerate block production to manipulate this

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 = 100 + random_val;
- table::add(&mut state.users_claimed_amount, user_addr, random_amount);
- }
​
+ #[randomness]
+ entry fun get_random_slice(user_addr: address) acquires ModuleData, State {
+ let state = borrow_global_mut<State>(get_resource_address());
+ // Use Aptos randomness API for secure randomness
+ use aptos_framework::randomness;
+ let random_bytes = randomness::bytes(32);
+ let random_val = randomness::u64_range(100, 501); // 100-500 inclusive
+ table::add(&mut state.users_claimed_amount, user_addr, random_val);
+ }
Updates

Appeal created

bube Lead Judge 10 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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