My First Audit Portfolio: Lessons from Monetrix V1

Introduction Security auditing in DeFi isn't just about reading code; it's about understanding architectural intent. Recently, I decided to build my personal audit portfolio by diving into the Monetrix V1 codebase. This journey taught me that sometimes what looks like a critical bug is actually a design feature, and sometimes a simple order of operations can lead to user fund loss.

In this post, I want to share two key findings from my analysis.

Case 1: The "Surplus Bug" — When Architecture Meets Misunderstanding During my audit, I encountered a report suggesting a bug in the distributableSurplus calculation. The claim was that the protocol failed to decrement the surplus variable after minting yield.

The Initial Hypothesis: The contract was caching the surplus and failing to update it after mint().

The Audit Reality: After deep-diving into the MonetrixAccountant.sol logic, I realized the protocol employs a dynamic state calculation:

Solidity function surplus() public view returns (int256) { return totalBackingSigned() — int256(usdm.totalSupply()); } Because the system relies on totalSupply() as the source of truth, the surplus updates automatically whenever tokens are minted. Lesson: Always check if the state is cached or computed dynamically before flagging it as a state-inconsistency bug.

Case 2: Withdrawal Security — The CEI Pattern I audited the withdrawal flow in MonetrixVault.sol, specifically the claimRedeem function.

The Vulnerability: The function executes usdm.burn() before interacting with the RedeemEscrow contract.

Solidity usdm.burn(amount); IRedeemEscrow(redeemEscrow).payOut(msg.sender, amount); The Risk: If the payOut external call fails (due to unexpected conditions), the user's tokens are burned, but they never receive the underlying USDC. This is a clear violation of the Checks-Effects-Interactions (CEI) pattern.

Recommendation: The protocol should prioritize the external interaction (or ensure atomicity) to prevent permanent loss of user funds.

Final Thoughts Building a portfolio isn't just about finding "Critical" bugs — it's about demonstrating your ability to reason about complex systems. You can check my full analysis and PoCs in my Monetrix-audit GitHub repository.

Have you encountered similar architecture vs. bug discussions? Let's discuss in the comments!

https://github.com/rdin777/Monetrix-audit

#defi, #security, #smartcontracts, #solidity