PancakeHunny Flash Loan Minting Attack Analysis

The Exploit

On Oct 20, 9 AM UTC, an attacker exploited PancakeHunny by manipulating the price of WBNB/TUSD on PCS using flash loans, minted ~12M of Hunny tokens in 15 transactions, and dumped.

As a result, the hacker has taken out 2.3M (642k of stable coin + 435.31 ETH) and crashed the price of the HUNNY token from $0.3 to $0.1.

The first exploit transaction happened at 09:22:01 AM:

The Root Cause

Using an inappropriate fixed path for swap makes it vulnerable to pricing manipulation using flashloans.

The harvest() function of the VaultStrategyAlpacaRabbit contract is configured to use a fixed path to swap the underlying reward token to the staking token for reinvesting.

For the Alpaca TUSD vault, the path will be [ALPACA, WBNB, TUSD].

However, since the WBNB/TUSD pool got only a few hundreds of dollars worth of liquidity, the price of WBNB/TUSD can be easily manipulated using flashloans.

$0.01 of ALPACA for $200k of TUSD

That’s the first step of the attack:

The attacker borrowed 2.7M worth of TUSD and swap to WBNB through the WBNB/TUSD pool, inflating the price of WBNB against TUSD to an extreme value.

Then the attacker then triggered harvest, swapped out hundreds of thousands of TUSD with merely a few cents of underlying rewards, resulting in the profit of the whole vault increasing by a lot.

The attacker then called getReward() to mint HUNNY tokens with the artificially created profits by paying a 30% performance fee.

When the minted HUNNY tokens are worthing more than the 30% performance fee, it enables a profitable economic attack.

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