#41699 [SC-Insight] Silent Transfer Failures in Native Token Handling

Submitted on Mar 17th 2025 at 16:38:27 UTC by @ZeroXGondar for Audit Comp | Yeet

• Report ID: #41699

• Report Type: Smart Contract

• Report severity: Insight

• Target: https://github.com/immunefi-team/audit-comp-yeet/blob/main/src/contracts/Zapper.sol

• Impacts:

Description

Summary

In Zapper::_sendNativeToken when transferring native tokens, the contract uses unsafe transfer method instead of the recommended low level call method.

This is problematic since transfer method only passes 2300 gas, which can cause the transaction to be reverted if the interacting contract uses more gas in its receive/fallback function.

Details

The _sendNativeToken() function uses the deprecated transfer() method to send native BERA tokens. This approach has multiple security flaws:

• No verification of transfer success

• Silent failures possible when transfers fail

• Limited to 2300 gas, insufficient for contracts with complex receive/fallback functions

Note that this function can be accessed in many flows, making the issue more serious. It can be accessed by directly calling Zapper::zapOutNative function, or, in more complex scenarios, it can be accessed via StakeV2::claimRewardsInNative function which in turn calls Zapper::zapOutNative that calls _sendNativeToken

Impact

• Users may believe their funds were successfully transferred when they weren't

• Protocol accounting may become inconsistent with actual token balances

• Lack of error handling prevents recovery mechanisms from being triggered

• Potential permanent loss of funds due to silent failures

Faulty block

function _sendNativeToken(address receiver, uint256 amount) internal {
    if (amount > 0) {
        wbera.withdraw(amount);
        payable(receiver).transfer(amount);
    }
}

Recommended Fix

Replace transfer() with call() and add success verification:

function _sendNativeToken(address receiver, uint256 amount) internal {
    if (amount > 0) {
        wbera.withdraw(amount);
        (bool success, ) = payable(receiver).call{value: amount}("");
        require(success, "Native token transfer failed");
    }
}

Proof of Concept

POC

1. A contract with a gas-intensive receive function (consuming >2300 gas) is deployed on the network.

2. The vulnerable flow is triggered in one of two ways:

   • Direct call to Zapper::zapOutNative with the gas-intensive contract as the receiver

   • Indirect call through StakeV2::claimRewardsInNative which ultimately calls _sendNativeToken

3. When _sendNativeToken executes:

   • It successfully withdraws tokens from wbera with wbera.withdraw(amount)

   • It attempts to transfer native tokens using payable(receiver).transfer(amount)

   • The transfer silently fails because the receiver's receive function exceeds the 2300 gas limit

   • No error is thrown or checked since the contract doesn't verify the transfer's success

4. The transaction completes without reverting, making it appear successful to users and the protocol.