Regnum: a deposit prices utilization on stale debt and refunds 160 raUSD

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 66504-h-03-incorrect-utilization-rate-in-deposit-and-withdraw-oper. Standalone Foundry PoC and full write-up: 66504-h-03-incorrect-utilization-rate-in-deposit-and-withdraw-oper_exp in the evm-hack-registry mirror.

Vulnerability classes: vuln/logic/fee-calculation · vuln/logic/state-update

Key info#

TL;DR#

updateInterestRatesAndLiquidity adds the new deposit to totalLiquidity and then computes utilization from reserve.totalUsage. That field is only written on borrow, repay, and liquidation. Accrued debt lives in normalizedDebt. After a 1000 raUSD deposit the pool has 2000 liquidity, stale usage 400, and real debt 800. The rate is 20 percent instead of 40 percent. settle charges 160 raUSD and refunds the other 160 from a 320 raUSD escrow. The control, which rates on normalized debt, refunds nothing.

Background#

Pashov Audit Group's Regnum Aurum review dated 2025-06-26, finding H-03. The report shows totalUsage assigned only inside _borrow, _repay, and finalizeLiquidation. Deposit and withdraw call updateInterestRatesAndLiquidity without refreshing usage first, so the borrow rate and the protocol fee are computed on a stale, lower utilization.

The vulnerable code#

if (liquidityAdded > 0) {
    reserve.totalLiquidity += liquidityAdded;
}
uint256 totalDebt = reserve.totalUsage;
// @> VULN: utilization is calculated from stale totalUsage, not the interest-accrued debt
uint256 utilizationRate = calculateUtilizationRate(reserve.totalLiquidity, totalDebt);
currentUsageRate = utilizationRate;

calculateUtilizationRate is usage * 1e18 / liquidity. Here that is 400 ether * 1e18 / 2000 ether = 0.2 ether.

Root cause#

Interest accrual updates the normalized debt and does not write totalUsage. The next deposit still divides the old usage by the new, larger liquidity. The charged interest is normalizedDebt * currentUsageRate / 1e18, so the stale rate is applied to the real debt and the borrower pays too little.

Preconditions#

• totalLiquidity is 1000 ether, totalUsage is 400 ether, and normalizedDebt is 800 ether.
• The borrower prepaid 320 ether, which is one period of interest at the true 40 percent rate.
• A supplier deposits 1000 ether. That deposit is a normal user action, not an admin call.

Attack walkthrough#

1. The supplier deposits 1000 raUSD. Liquidity becomes 2000. totalUsage stays 400.
2. Utilization is stored as 0.2 ether.
3. settle charges 800 ether * 0.2 = 160 ether.
4. The escrow was 320 ether. The difference, 160 ether, is transferred to the borrower.
5. The control rates on the 800 ether of normalized debt, stores 0.4 ether, charges 320 ether, and refunds 0.

Diagrams#

Impact#

The borrower receives 160 ether of raUSD that was reserved as interest for lenders. The true charge on 800 ether of debt at 40 percent is 320 ether. The protocol and the suppliers are short that 160 ether for the period. The control leaves the borrower's balance at 0.

Remediation#

Set reserve.totalUsage from the normalized debt before calculateUtilizationRate. Deposit, withdraw, borrow, repay, and liquidation should all read the same accrued usage.

How to reproduce#

cd 66504-h-03-incorrect-utilization-rate-in-deposit-and-withdraw-oper_exp
forge test -vv

test_exploit_stale_usage_refunds_160_ether asserts a 0.2 rate, 160 interest, and a 160 refund. test_control_accrued_debt_charges_the_full_320 asserts a 0.4 rate and a 0 refund. Captured output is output.txt.

Sources#

• AuditVault finding: https://github.com/Auditware/AuditVault/blob/main/findings/66504-h-03-incorrect-utilization-rate-in-deposit-and-withdraw-oper.md
• Report: https://github.com/pashov/audits/blob/master/team/md/RegnumAurum-security-review_2025-06-26.md

Sources & further analysis#

Reproductions & code

• Standalone PoC + full trace: 66504-h-03-incorrect-utilization-rate-in-deposit-and-withdraw-oper_exp (evm-hack-registry mirror).
• AuditVault finding: 66504-h-03-incorrect-utilization-rate-in-deposit-and-withdraw-oper.

Alerts & third-party analyses

• Web3Sec X hacked database: search.
• Rekt leaderboard: search.
• Solodit incident search: search.

These dashboards index community alerts tweets, post-mortems, and independent write-ups. Reach them through the protocol name above to cross-check this reproduction against other analyses.