xDonations A-1

Security Audit

February 10, 2023

Version 1.0.0

Presented by 0xMacro

Table of Contents


This document includes the results of the security audit for xDonations's smart contract code as found in the section titled ‘Source Code’. The security audit was performed by the Macro security team on February 10, 2023.

The purpose of this audit is to review the source code of certain xDonations Solidity contracts, and provide feedback on the design, architecture, and quality of the source code with an emphasis on validating the correctness and security of the software in its entirety.

Disclaimer: While Macro’s review is comprehensive and has surfaced some changes that should be made to the source code, this audit should not solely be relied upon for security, as no single audit is guaranteed to catch all possible bugs.

Overall Assessment

The following is an aggregation of issues found by the Macro Audit team:

Severity Count Acknowledged Won't Do Addressed
High 2 - - 2
Low 1 1 - -
Code Quality 1 - - 1
Informational 1 - - -
Gas Optimization 1 - - 1

xDonations was quick to respond to these issues.


Our understanding of the specification was based on the following sources:

Source Code

The following source code was reviewed during the audit:

Specifically, we audited the following contracts within this repository:

Contract SHA256


Note: This document contains an audit solely of the Solidity contracts listed above. Specifically, the audit pertains only to the contracts themselves, and does not pertain to any other programs or scripts, including deployment scripts.

Issue Descriptions and Recommendations

Click on an issue to jump to it, or scroll down to see them all.

Security Level Reference

We quantify issues in three parts:

  1. The high/medium/low/spec-breaking impact of the issue:
    • How bad things can get (for a vulnerability)
    • The significance of an improvement (for a code quality issue)
    • The amount of gas saved (for a gas optimization)
  2. The high/medium/low likelihood of the issue:
    • How likely is the issue to occur (for a vulnerability)
  3. The overall critical/high/medium/low severity of the issue.

This third part – the severity level – is a summary of how much consideration the client should give to fixing the issue. We assign severity according to the table of guidelines below:

Severity Description

We recommend the client must fix the issue, no matter what, because not fixing would mean significant funds/assets WILL be lost.


We recommend the client must address the issue, no matter what, because not fixing would be very bad, or some funds/assets will be lost, or the code’s behavior is against the provided spec.


We recommend the client to seriously consider fixing the issue, as the implications of not fixing the issue are severe enough to impact the project significantly, albiet not in an existential manner.


The risk is small, unlikely, or may not relevant to the project in a meaningful way.

Whether or not the project wants to develop a fix is up to the goals and needs of the project.

Code Quality

The issue identified does not pose any obvious risk, but fixing could improve overall code quality, on-chain composability, developer ergonomics, or even certain aspects of protocol design.


Warnings and things to keep in mind when operating the protocol. No immediate action required.

Gas Optimizations

The presented optimization suggestion would save an amount of gas significant enough, in our opinion, to be worth the development cost of implementing it.

Issue Details


Slippage calculation is incorrect

Sandwich Attack

amountOutMInium is calculated incorrectly for uniswap swap inside _sweep.

uint256 amountInNormalized = normalizeDecimals(IERC20Metadata(fromAsset).decimals(), donationAssetDecimals, amountIn);

// Set up uniswap swap params.
ISwapRouter.ExactInputSingleParams memory params =
      tokenIn: fromAsset,
      tokenOut: donationAsset,
      fee: poolFee,
      recipient: address(this),
      deadline: block.timestamp,
      amountIn: amountIn,
      amountOutMinimum: amountInNormalized * (10_000 - uniswapSlippage) / 10_000, // @audit 
      sqrtPriceLimitX96: 0

It's being derived directly from amountIn using decimals and slippage, instead of a current market price from uniswap pool.

Hence it would either overvalue the amount expected or undervalue it.


In the case of overvalued, it would revert only, while in the case of undervalued, it would allow a sandwich attack.

Remediations to consider

Consider calculating amountOutMinium off-chain using any price oracle or uniswap quoter and making amountOutMinium a user input.


Default slippage is set to a high number

Sandwich Attack
function sweep (
        address fromAsset,
        uint24 poolFee,
        uint256 amountIn
    ) external payable onlySweeper {
        _sweep (
            1000, // 1% default max slippage
            100 // 1% default max slippage

sweep() sets slippage for swap to 1_000, and the comment states that slippage is intended to be set to 1%.

amountOutMinimum: amountInNormalized * (10_000 - uniswapSlippage) / 10_000,

However, the calculation is performed using 10_000 as a precision basis, and this causes slippage to be calculated as 10%, which is a huge slippage and will make swaps vulnerable to sandwich attacks.

Remediations to consider

Use 100 as the default slippage.


The bridge has unlimited approvals for donationAsset

Trust Model

The bridge is given unlimited token transfer approvals. Although the bridge is a trusted contract, it is recommended to give only required approvals to external parties since any potential bugs or vulnerabilities in the bridge may cause a loss of assets in the donation contract.

if (!approvedDonationAsset) {
    approvedDonationAsset = true;
    // use max approval for assset
    TransferHelper.safeApprove(donationAsset, address(connext), MAX_INT);

Remediations to consider

Remove MAX_INT approval and approve only the required amounts.


Incorrect comment giving the wrong impression to the reader

Quality Impact
address public immutable donationAsset; // should be USDC

The readers of the contracts can assume that donations could be made through USDC only, consider removing this comment.


Token approval could be moved to the constructor to eliminate redundant SLOADs

Gas Savings
// Approve connext to bridge donationAsset.
if (!approvedDonationAsset) {
    approvedDonationAsset = true;
    // use max approval for assset
    TransferHelper.safeApprove(donationAsset, address(connext), MAX_INT);

Every _sweep() reads the storage variable approvedDonationAsset , to verify if there is a need to do a token approval. Instead, approval for donationAsset could be moved to the constructor to avoid these redundant reads.


_sweep() does not check for max slippage

Sandwich Attack

sweep() is a function that can be called with any slippage, and _sweep() only checks for the MIN_SLIPPAGE. Since this function expects parameters with a precision basis, it is prone to human errors(see H-2) and a wrong slippage passed here may lose funds to sandwich attacks.

Adding an upper limit for slippages, similar to MIN_SLIPPAGE, would resolve this issue; but that may block the sweeper from swapping tokens with low liquidity and is not recommended if such tokens are expected to be handled. Calling sweep() through a client app is recommended to minimize human errors.


Macro makes no warranties, either express, implied, statutory, or otherwise, with respect to the services or deliverables provided in this report, and Macro specifically disclaims all implied warranties of merchantability, fitness for a particular purpose, noninfringement and those arising from a course of dealing, usage or trade with respect thereto, and all such warranties are hereby excluded to the fullest extent permitted by law.

Macro will not be liable for any lost profits, business, contracts, revenue, goodwill, production, anticipated savings, loss of data, or costs of procurement of substitute goods or services or for any claim or demand by any other party. In no event will Macro be liable for consequential, incidental, special, indirect, or exemplary damages arising out of this agreement or any work statement, however caused and (to the fullest extent permitted by law) under any theory of liability (including negligence), even if Macro has been advised of the possibility of such damages.

The scope of this report and review is limited to a review of only the code presented by the xDonations team and only the source code Macro notes as being within the scope of Macro’s review within this report. This report does not include an audit of the deployment scripts used to deploy the Solidity contracts in the repository corresponding to this audit. Specifically, for the avoidance of doubt, this report does not constitute investment advice, is not intended to be relied upon as investment advice, is not an endorsement of this project or team, and it is not a guarantee as to the absolute security of the project. In this report you may through hypertext or other computer links, gain access to websites operated by persons other than Macro. Such hyperlinks are provided for your reference and convenience only, and are the exclusive responsibility of such websites’ owners. You agree that Macro is not responsible for the content or operation of such websites, and that Macro shall have no liability to your or any other person or entity for the use of third party websites. Macro assumes no responsibility for the use of third party software and shall have no liability whatsoever to any person or entity for the accuracy or completeness of any outcome generated by such software.