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

Predictable Randomness `pizza_drop::get_random_slice`

parmakhanm786

Root + Impact

Timestamp-Based Randomness is Manipulable + Medium-High Impact

Description

  • The contract generates random amounts using block timestamp modulo operation

  • Block timestamps are predictable and can be influenced by miners/validators, making the randomness gameable

let time = timestamp::now_microseconds(); // @> Predictable timestamp source
let random_val = time % 401; // @> Easily calculable result
let random_amount = 100 + random_val; // @> Manipulable final amount

Risk

Likelihood:

  • Miners/validators can manipulate block timestamps within small ranges

  • Attackers can time transactions to land in favorable timestamp windows

Impact:

  • Unfair airdrop distribution favoring miners and sophisticated attackers

  • Loss of trust in the airdrop's fairness and randomness guarantees

  • Potential legal and reputational damage due to biased outcomes

Proof of Concept

// Miner can predict and manipulate:
// Current timestamp: 1641024000000000
// timestamp % 401 = 1641024000000000 % 401 = 287
// Result: 100 + 287 = 387 APT
​
// Miner adjusts timestamp slightly to get better result:
// Modified timestamp: 1641024000000400
// 1641024000000400 % 401 = 0
// Result: 100 + 0 = 100 APT (worse) - try again
​
// Next attempt: 1641024000000350
// 1641024000000350 % 401 = 350
// Result: 100 + 350 = 450 APT (optimal)

Recommended Mitigation

- let time = timestamp::now_microseconds();
- let random_val = time % 401;
- let random_amount = 100 + random_val;
+ use aptos_framework::randomness;
+
+ let random_seed = randomness::randomness();
+ let random_val = (random_seed % 401);
+ let random_amount = 100 + 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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