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Reproduced Exploit

LEND H-3: both cross-chain mappings populated bricks `borrowWithInterest` (liquidation evasion)

May 2025•Other•untagged•3 min read

Chain

Other

Category

untagged

Date

May 2025

Source

AuditVault

PoC & write-upPoC for your local evm-hack-analyzerAuditVault PoCs

EVM Playground

Source-level debugger — step opcodes and Solidity in sync

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The attack is replayed in an in-browser EVM preloaded with the exact dumped fork state. The execution tree shows every call; step by Solidity line or by opcode across all depths — source, Stack, Memory, Storage, Balances (native / ERC-20 / NFT), Transient storage and Return value stay in sync. Click a tree node, opcode, or source line to jump. No backend, no live RPC.

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Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 58372-h-3-user-can-evade-liquidation-and-bridge-funds-by-exploitin. Standalone Foundry PoC and full write-up: 58372-h-3-user-can-evade-liquidation-and-bridge-funds-by-exploit_exp in the evm-hack-registry mirror.

Vulnerability classes: cross-chain · broken-invariant · liquidation-DoS · bad-debt

Reproduction: a faithful minimal reproduction of LendStorage.borrowWithInterest (Sherlock 2025-05-lend-audit-contest, LendStorage.sol L478-486 @ 713372a1) — the reverting both-populated invariant is reproduced verbatim (marked @>). The lToken index and the liquidation gate are faithful minimal doubles. Local deploy, no fork.

Root cause#

borrowWithInterest assumes that, for a given user and asset on a chain, only ONE of crossChainBorrows / crossChainCollaterals is ever populated, and enforces it with a hard require:

SOLIDITY
Borrow[] memory borrows = crossChainBorrows[borrower][_token];
Borrow[] memory collaterals = crossChainCollaterals[borrower][_token];

require(borrows.length == 0 || collaterals.length == 0, "Invariant violated: both mappings populated"); // @>

That invariant is not actually enforced elsewhere. A user can populate both mappings for the same asset on the same chain by opening cross-chain borrows in both directions:

  1. Supply (nearly all) collateral on Chain A, borrow on Chain B → crossChainBorrows populated on A.
  2. Supply a dust amount on Chain B, borrow a dust amount on Chain A → crossChainCollaterals populated on A.

Now both arrays are non-empty on Chain A, so the require reverts on every call.

Impact#

  • Position becomes uncloseable and non-liquidatable. Every function that reads the borrow balance — crucially the liquidation path — calls borrowWithInterest, which now reverts. The underwater position can never be liquidated.
  • Collateral bridged out; protocol eats bad debt. The user withdraws/bridges out nearly all their collateral while the outstanding borrow can never be repaid or seized — a direct solvency threat.
  • In the PoC, the liquidation attempt is bricked and the attacker retains the full 1,000-token collateral it should have lost.

Attack walkthrough#

flowchart TD A["Supply collateral on A, borrow on B<br/>→ crossChainBorrows[A] populated"] --> C["Both mappings populated on Chain A"] B["Supply dust on B, borrow dust on A<br/>→ crossChainCollaterals[A] populated"] --> C C --> D["borrowWithInterest: require(borrows==0 || collaterals==0)"] D -->|"both non-empty → false"| E["REVERT"] E --> F["liquidation read reverts → position non-liquidatable"] F --> G["attacker bridges out / retains collateral; protocol eats bad debt"]

PoC#

Registry (Foundry, local deploy — exploit path + a redesigned-tracking control):

BASH
cd 58372-h-3-user-can-evade-liquidation-and-bridge-funds-by-exploit_exp
forge test -vv

Expected: test_attacker_bricksLiquidationViaBothMappings PASS (borrowWithInterest reverts with "Invariant violated: both mappings populated"; attacker retains the full 1,000-token collateral) and test_control_fixedDoesNotRevert PASS (the redesigned read returns the real debt without reverting even with both mappings populated). The browser EVM Playground is served at /hacks/58372-h-3-user-can-evade-liquidation-and-bridge-funds-by-exploit/.

Remediation#

Redesign the cross-chain borrow/collateral tracking so both mappings cannot be populated for the same user and asset on the same chain (or compute the borrow balance without a reverting invariant that a user can trigger at will).

References#

Sources & further analysis#

Reproductions & code

Alerts & third-party analyses

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