Beanstalk: The Finale

Summary

Context:

• LibTractor.sol#L28-L31

• LibTractor.sol#L46

• LibTractor.sol#L139-L154

Tractor is a new feature of Beanstalk that allow users to authorize third parties to execute operation on their behalf. TractorFacet.sol relies on LibTractor.sol to generate the authorizations by implementing Typed Structured Data Hash according to EIP-712.

Users need to sign a blueprint that represent the set of instructions allowed to be performed by the third party, the problem is that the BLUEPRINT_TYPE_HASH does not uses the same types declared in the struct Blueprint, resulting in the generation of an incorrect typed structured data hash for signatures.

Vulnerability Details

Blueprint is defined as follows:

According to EIP-712 the Typehash is calculated as follows: typeHash = keccak256(encodeType(typeOf(s))).

Where encodeType is the type of a struct that is encoded as: name ‖ "(" ‖ member₁ ‖ "," ‖ member₂ ‖ "," ‖ … ‖ memberₙ ")".

The Typehash is calculated like this:

As you can see the third member is of type bytes when in fact is of type bytes32[].

Impact

Generating an incorrect typed structured data hash can case issues to tools or wallets that are EIP-712 compliant to integrate with Beanstalk.

Tools Used

Manual review.

Recommendations

Modify BLUEPRINT_TYPE_HASH to generate the hash with the correct types so it points to the correct data structure.