LEND: cross-chain repayment wipes the borrower's same-chain debt

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 782. Standalone Foundry PoC and full write-up: 58385-lend-cross-chain-repayment-updates-same-chain-borrow-balances_exp in the evm-hack-registry mirror.

Vulnerability classes: vuln/theft · vuln/logic

Reproduction: a faithful minimal reproduction of the vulnerable finding — repayBorrowInternal and the LendStorage accounting helpers it touches are reproduced verbatim (the vulnerable line marked @>) with faithful minimal doubles; local deploy, no fork.

Root cause#

In Lend-V2/src/LayerZero/CoreRouter.sol, repayBorrowInternal runs its // Update same-chain borrow balances block unconditionally — there is no if (_isSameChain) guard. When a cross-chain debt is repaid (_isSameChain == false), borrowedAmount is the cross-chain balance, yet the block still calls removeBorrowBalance(borrower, _lToken), deleting the borrower's unrelated same-chain borrowBalance record for the same lToken. The vulnerable lines, reproduced verbatim:

SOLIDITY

// Update same-chain borrow balances
if (repayAmountFinal == borrowedAmount) {
@>  lendStorage.removeBorrowBalance(borrower, _lToken);
    lendStorage.removeUserBorrowedAsset(borrower, _lToken);
} else {
    lendStorage.updateBorrowBalance(
        borrower, _lToken,
        borrowedAmount - repayAmountFinal,
        LTokenInterface(_lToken).borrowIndex()
    );
}

The same-chain loan and the cross-chain loan live in separate storage slots, but this block only ever addresses the same-chain borrowBalance mapping. So paying off the cross-chain leg silently erases the same-chain leg, and the same-chain lenders who funded it lose their money.

Why it's exploitable here#

Following the finding's vulnerability path with concrete numbers:

Alice owes a 100e18 same-chain loan (recorded in borrowBalance[Alice][lToken]) and a separate 30e18 cross-chain loan of the same lToken (recorded in the cross-chain records). The two debts occupy independent storage slots.
Alice (or a liquidator) calls repayBorrow(..., _isSameChain = false) to repay exactly the 30e18 cross-chain debt. Because _isSameChain is false, borrowedAmount = borrowWithInterest() = 30e18.
LErc20Interface(_lToken).repayBorrow(30e18) correctly reduces the on-chain borrows for the cross-chain loan — that part is fine.
Then, because repayAmountFinal (30e18) == borrowedAmount (30e18), the unconditional block calls removeBorrowBalance(Alice, lToken), deleting the untouched 100e18 same-chain record.
Alice paid 30e18 and had her 100e18 same-chain debt erased for free — a direct 100e18 loss to the same-chain lenders.

Attack path#

flowchart TD S0["Router approves the market"] S1["Market accrues interest first"] S2["Cross-chain debt repaid on-chain"] S3["Reset stale allowance"] S4["Cross-chain amount validated"] H["Same-chain 100e18 debt wiped — lenders lose funds"] S0 --> S1 S1 --> S2 S2 --> S3 S3 --> S4 S4 --> H

Marked-line walkthrough (Playground)#

The EVM Playground pins each step to the exact executed source line in 0xbd4fd5a3…:

L49 — Router approves the market: The router's safeApprove grants the lToken market an allowance so it can pull the repay underlying from the caller.
L54 — Market accrues interest first: repayBorrowInternal first calls accrueInterest on the lToken so the debt is measured against the current borrow index.
L58 — Cross-chain debt repaid on-chain: The router calls repayBorrow on the market, transferring 30e18 and correctly reducing the on-chain borrows for the cross-chain loan.
L253 — Reset stale allowance: Inside _approveToken, any existing nonzero allowance is first reset to zero before the router re-approves the repay amount.
L280 — Cross-chain amount validated: With _isSameChain false, borrowedAmount is the 30e18 cross-chain debt from borrowWithInterest, so this nonzero check passes.
L296 — Same-chain debt wiped unconditionally: Root cause: with no if(_isSameChain) guard, this cross-chain repay calls removeBorrowBalance and deletes Alice's untouched 100e18 same-chain debt for free.

PoC#

Registry (Foundry, local deploy — verbatim vulnerable source + harm-asserting test):

BASH

cd 58385-lend-cross-chain-repayment-updates-same-chain-borrow-balances_exp && forge test -vvv

The browser Playground replays the same synthetic opcode-for-opcode and measures the harm: repay only the 30e18 cross-chain debt and watch Alice's untouched 100e18 same-chain debt get wiped, so the same-chain lenders lose 100e18. Both gates are green (registry forge test PASS + Playground _verify-poc VERDICT: PASS).

Sources & further analysis#

Reproductions & code

Standalone PoC + full trace: 58385-lend-cross-chain-repayment-updates-same-chain-borrow-balances_exp (evm-hack-registry mirror).
AuditVault finding: 782.
Upstream DeFiHackLabs PoC directory: src/test.

Alerts & third-party analyses

DeFiHackLabs incident explorer: search "LEND: cross-chain repayment wipes the borrower's same-chain debt".
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.

Source-level debugger — step opcodes and Solidity in sync