Sandwitch attack over `harvest` function

[code423n4]

Lines of code

https://github.com/Badger-Finance/vested-aura/blob/d504684e4f9b56660a9e6c6dfb839dcebac3c174/contracts/MyStrategy.sol#L249 https://dev.balancer.fi/resources/swaps/single-swap#swap-function

Vulnerability details

Impact

Due to a bug with the slippage configuration it is possible to make a call to the harvest method do a swap with 0 BALETH_BPT tokens.

Proof of Concept

According to the balancer documentation the limit argument represents:

limit: The meaning of limit depends on the value of singleSwap.kind GIVEN_IN: The minimum amount of tokens we would accept to receive from the swap.

So if an attacker detects the harvest transaction in the memory pool, and configures two transactions to perform a sadwitch attack, the contract will perform a swap with an expected minimum of 0 tokens.

Reference:

• https://dev.balancer.fi/resources/swaps/single-swap#swap-function

Affected source code:

• MyStrategy.sol#L249

Recommended Mitigation Steps

• Use a percentage for the slippage

[GalloDaSballo]

Dup of #6

[0x1f8b]

Dup of https://github.com/code-423n4/2022-06-badger-findings/issues/6

Is not related to #6 @GalloDaSballo

[KenzoAgada]

Duplicate of #155