Connext contest - 0xsanson's results

Handle

0xsanson

Vulnerability details

Impact

User and router can collude to exploit the contract and steal funds (any erc20 tokens) in the following way. They can set the callTo to an exploit contract that can call the receivingAssetId.transferFrom function. The logic in the fulfill function approves the token to callTo, without "dis-approving" in the case of sending transaction directly (in the case of error in the try-catch block). After both user and router have fulfilled, they can call the exploiter contract that steals an amount of funds from TransactionManager in the receiving chain.

Proof of Concept

https://github.com/code-423n4/2021-07-connext/blob/main/contracts/TransactionManager.sol#L388

(I can provide exploiter contract and js test file if necessary)

Tools Used

Manual Analysis

Recommended Mitigation Steps

If the tokens are transferred directly, the tokens must be dis-approved.