Snowman Merkle Airdrop
AI First Flight #10
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Submission Details
Severity: low
Valid

`Snow::buySnow` resets the shared earn cooldown on every purchase, blocking all free earns for another week

furkanumut

Root + Impact

Description

Buying Snow and earning Snow are documented as independent acquisition paths ("earned for free once a week, or bought at anytime"). However, buySnow unconditionally overwrites the shared s_earnTimer, so every purchase pushes everyone's next free-earn window out by another week — and each purchaser irrationally forfeits their own earn eligibility too.

// File: src/Snow.sol — Lines 79–90
function buySnow(uint256 amount) external payable canFarmSnow {
if (msg.value == (s_buyFee * amount)) {
_mint(msg.sender, amount);
} else {
i_weth.safeTransferFrom(msg.sender, address(this), (s_buyFee * amount));
_mint(msg.sender, amount);
}
@> s_earnTimer = block.timestamp; // unrelated state stomped on every buy
emit SnowBought(msg.sender, amount);
}

The timer is the single global gate for earnSnow (reported separately). Writing it from the purchase path means routine buying activity — which the protocol wants to encourage — acts as a standing suppression of the free faucet: each buy restarts the one-week lockout for all users. There is no documented reason for a purchase to affect earn eligibility at all; the write serves no accounting purpose (nothing ever reads the timer in relation to buying). The harm also cuts the other way: a user who buys Snow and then tries to collect their weekly free earn is reverted for a week, so honest users are punished for using both documented acquisition paths.

Risk

Likelihood:

  • Occurs on every buySnow call, paid or not — ordinary purchase activity continuously resets the cooldown.

  • A motivated griefer can time purchases to each weekly window to keep the faucet closed (and, as reported separately, a zero-amount buy does this for free).

Impact:

  • Free distribution is suppressed for the entire user base for one week per purchase — the more the protocol is used, the less the faucet works.

  • Buyers individually lose their own earn eligibility for a week after buying, contradicting the documented buy/earn independence.

Proof of Concept

function test_PoC_PaidBuyResetsTimer() public {
vm.warp(block.timestamp + 1 weeks); // earn window open for everyone
​
vm.deal(bob, 5 ether);
vm.prank(bob);
snow.buySnow{value: 5 ether}(1); // ordinary paid purchase, succeeds
assertEq(snow.balanceOf(bob), 2); // bob had 1 from setup
​
vm.prank(alice);
vm.expectRevert(Snow.S__Timer.selector);
snow.earnSnow(); // alice's free earn blocked by bob's unrelated buy
}

Run: add the test to test/TestSnowmanAirdrop.t.sol, then forge test --match-test test_PoC_PaidBuyResetsTimer -vv

Output (actual run):

[PASS] test_PoC_PaidBuyResetsTimer() (gas: 61476)
Logs:
FC-0004: paid buySnow reset the global earn cooldown

Recommended Mitigation

Remove the timer write from the purchase path.

} else {
i_weth.safeTransferFrom(msg.sender, address(this), (s_buyFee * amount));
_mint(msg.sender, amount);
}
- s_earnTimer = block.timestamp;
emit SnowBought(msg.sender, amount);
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 19 hours ago
Submission Judgement Published
Validated
Assigned finding tags:

[L-02] Global Timer Reset in Snow::buySnow Denies Free Claims for All Users

## Description: The `Snow::buySnow` function contains a critical flaw where it resets a global timer `(s_earnTimer)` to the current block timestamp on every invocation. This timer controls eligibility for free token claims via `Snow::earnSnow()`, which requires 1 week to pass since the last timer reset. As a result: Any token purchase `(via buySnow)` blocks all free claims for all users for 7 days Malicious actors can permanently suppress free claims with micro-transactions Contradicts protocol documentation promising **"free weekly claims per user"** ## Impact: * **Complete Denial-of-Service:** Free claim mechanism becomes unusable * **Broken Protocol Incentives:** Undermines core user acquisition strategy * **Economic Damage:** Eliminates promised free distribution channel * **Reputation Harm:** Users perceive protocol as dishonest ```solidity function buySnow(uint256 amount) external payable canFarmSnow { if (msg.value == (s_buyFee * amount)) { _mint(msg.sender, amount); } else { i_weth.safeTransferFrom(msg.sender, address(this), (s_buyFee * amount)); _mint(msg.sender, amount); } @> s_earnTimer = block.timestamp; emit SnowBought(msg.sender, amount); } ``` ## Risk **Likelihood**: • Triggered by normal protocol usage (any purchase) • Requires only one transaction every 7 days to maintain blockage • Incentivized attack (low-cost disruption) **Impact**: • Permanent suppression of core protocol feature • Loss of user trust and adoption • Violates documented tokenomics ## Proof of Concept **Attack Scenario:** Permanent Free Claim Suppression * Attacker calls **buySnow(1)** with minimum payment * **s\_earnTimer** sets to current timestamp (T0) * All **earnSnow()** calls revert for **next 7 days** * On day 6, attacker repeats **buySnow(1)** * New timer reset (T1 = T0+6 days) * Free claims blocked until **T1+7 days (total 13 days)** * Repeat step **4 every 6 days → permanent blockage** **Test Case:** ```solidity // Day 0: Deploy contract snow = new Snow(...); // s_earnTimer = 0 // UserA claims successfully snow.earnSnow(); // Success (first claim always allowed) // Day 1: UserB buys 1 token snow.buySnow(1); // Resets global timer to day 1 // Day 2: UserA attempts claim snow.earnSnow(); // Reverts! Requires day 1+7 = day 8 // Day 7: UserC buys 1 token (day 7 < day 1+7) snow.buySnow(1); // Resets timer to day 7 // Day 8: UserA retries snow.earnSnow(); // Still reverts! Now requires day 7+7 = day 14 ``` ## Recommended Mitigation **Step 1:** Remove Global Timer Reset from `buySnow` ```diff function buySnow(uint256 amount) external payable canFarmSnow { // ... existing payment logic ... - s_earnTimer = block.timestamp; emit SnowBought(msg.sender, amount); } ``` **Step 2:** Implement Per-User Timer in `earnSnow` ```solidity // Add new state variable mapping(address => uint256) private s_lastClaimTime; function earnSnow() external canFarmSnow { // Check per-user timer instead of global if (s_lastClaimTime[msg.sender] != 0 && block.timestamp < s_lastClaimTime[msg.sender] + 1 weeks ) { revert S__Timer(); } _mint(msg.sender, 1); s_lastClaimTime[msg.sender] = block.timestamp; // Update user-specific timer emit SnowEarned(msg.sender, 1); // Add missing event } ``` **Step 3:** Initialize First Claim (Constructor) ```solidity constructor(...) { // Initialize with current timestamp to prevent immediate claims s_lastClaimTime[address(0)] = block.timestamp; } ```

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