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

Global `s_earnTimer` state variable in `Snow.sol` causes DoS on `earnSnow` for all users

sergiocarvajal30

Shared s_earnTimer variable in Snow.sol allows any buyer to reset the global timer and block other users from farming tokens.

Description

  • The Snow::earnSnow function allows users to farm Snow tokens for free once per week.

  • However, s_earnTimer is declared as a single global state variable instead of being mapped per user. Whenever any user calls buySnow or earnSnow, s_earnTimer is updated to block.timestamp globally.

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);
}
​
function earnSnow() external canFarmSnow {
@> if (s_earnTimer != 0 && block.timestamp < (s_earnTimer + 1 weeks)) {
revert S__Timer();
}
_mint(msg.sender, 1);
@> s_earnTimer = block.timestamp;
emit SnowEarned(msg.sender, 1);
}
​

Risk

Likelihood:

  • Any purchase of Snow tokens automatically resets s_earnTimer for the entire protocol.

  • An attacker can make small buySnow transactions periodically to maintain s_earnTimer continuously updated.

Impact:

  • Legitimate users are prevented from invoking earnSnow(), causing a protocol-wide Denial of Service on token farming.


Proof of Concept

PoC Explanation: The test test_PoC_Snow_GlobalTimer_DoS demonstrates how a global shared state variable creates a Denial of Service (DoS) vulnerability affecting all protocol users in Snow.sol:
Shared Timer Manipulation: An attacker calls buySnow, which executes s_earnTimer = block.timestamp, updating the single global s_earnTimer storage variable.
Time Advancement: Time is advanced by 1 day (vm.warp(block.timestamp + 1 days)), which is well within the 1-week cooldown window required by earnSnow().
Victim Blocked: A legitimate user (victim) calls earnSnow(). The function evaluates s_earnTimer != 0 && block.timestamp < (s_earnTimer + 1 weeks). Because s_earnTimer was set globally by the attacker, the check evaluates to true, and the victim's call reverts with S__Timer().
Vulnerability Validation: The test confirms that an attacker can repeatedly invoke buySnow or earnSnow to permanently block all other users from farming Snow tokens.

function test_PoC_Snow_GlobalTimer_DoS() public {
address attacker = makeAddr("attacker");
address victim = makeAddr("victim");
​
vm.deal(attacker, 10 ether);
​
vm.prank(attacker);
snow.buySnow{value: 1 ether}(1);
​
vm.warp(block.timestamp + 1 days);
​
vm.prank(victim);
vm.expectRevert(Snow.S__Timer.selector);
snow.earnSnow();
}
​

Recommended Mitigation

Mitigation Explanation: To eliminate the protocol-wide Denial of Service and isolate farming timers per account:
Replace State Variable with Mapping: Replace the single global uint256 private s_earnTimer variable with a mapping mapping(address => uint256) private s_earnTimers to track timestamps on a per-user basis.
Update User-Specific Timestamps: Modify buySnow and earnSnow to read and update s_earnTimers[msg.sender] instead of s_earnTimer.
Isolate Farming Logic: This ensures each user manages their own independent 1-week cooldown, preventing any user's actions from impacting another user's ability to farm tokens.

- uint256 private s_earnTimer;
+ mapping(address => uint256) private s_earnTimers;
​
function buySnow(uint256 amount) external payable canFarmSnow {
...
- s_earnTimer = block.timestamp;
+ s_earnTimers[msg.sender] = block.timestamp;
emit SnowBought(msg.sender, amount);
}
​
function earnSnow() external canFarmSnow {
- if (s_earnTimer != 0 && block.timestamp < (s_earnTimer + 1 weeks)) {
+ if (s_earnTimers[msg.sender] != 0 && block.timestamp < (s_earnTimers[msg.sender] + 1 weeks)) {
revert S__Timer();
}
_mint(msg.sender, 1);
- s_earnTimer = block.timestamp;
+ s_earnTimers[msg.sender] = block.timestamp;
emit SnowEarned(msg.sender, 1);
}
​
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 2 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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