concord (fixed-term peer-to-peer lending protocol) - Splyce

1. Introduction

Splyce engaged Halborn to perform a security assessment of Concord, its fixed-term peer-to-peer lending protocol built on Stellar/Soroban, beginning on June 16th, 2026 and ending on July 1st, 2026. The assessment was scoped to the code and components provided by the Splyce team. Commit hashes and the full list of reviewed components are available in the Scope section of this report.

Concord is a fixed-term, over-collateralized lending market. Borrowers post collateral and publish borrow requests describing the principal, term, and rate they are willing to pay; lenders fund a per-request ERC-4626-style lending vault and receive tradable share tokens representing their claim on the loan. When a request is matched, the borrower receives principal and the collateral is escrowed; at maturity the loan is repaid, or, if the borrower defaults or the position becomes unhealthy, the collateral is liquidated and distributed back to lenders. An optional SEP-0040 oracle enables price-based liquidation.

The review focused on the core on-chain components of the protocol: the offer_book request registry and whitelists, the settlement_engine that is the sole mover of collateral and the authority for loan and vault state transitions, the per-request lending_vault (share token plus vault accounting) and its lending_vault_factory, the loan_position and collateral_vault records and escrow, the optional request_router convenience wrapper, and the shared common crate covering oracle math, share-token metadata, and event helpers.

2. Assessment Summary

Halborn allocated time for this engagement and assigned full-time security engineers with strong backgrounds in smart contract security, decentralized finance, and Stellar/Soroban development.

The goals for this assessment were to:

• Confirm that the lending lifecycle — request creation, matching, repayment, default, and liquidation — behaves the way it is meant to across every state transition.

• Verify that collateral and lender funds are custodied, accounted for, and released safely throughout the life of each loan and vault.

• Review the access-control model and the trust boundaries between the offer book, settlement engine, vaults, and their factory.

• Check the correctness of the vault share accounting, interest-tier and fee math, and the oracle-based valuation and liquidation logic.

• Identify anything that could affect the safety of the protocol or the assets it holds.

In summary, Halborn identified a number of areas where the security of the system can be further strengthened, ranging from a high-severity issue down to several low-severity observations. Each one is described in detail later in this report, together with clear and practical recommendations. The Splyce team was responsive and engaged throughout the engagement. All issues identified during the audit have since been addressed, further enhancing the robustness of an already well-designed solution.