What happened
On 3 September 2020 a trader using the pseudonym Amplify turned about 0.5 ETH (roughly $200) into 747 ETH, then about $250,000-$290,000, through a bug in Soft Yearn (SYFI), a rebasing copy of yearn.finance's YFI. At SYFI's first rebase the trader's roughly 2 SYFI grew to about 15,551 SYFI. SYFI's rebase did not call sync() on its Uniswap pool afterwards, so the pool still priced SYFI at the old rate, and the trader sold into it for 747 ETH, emptying the pool's liquidity.
The project collapsed afterwards. The trader said they had not known about the bug and had simply pressed a button. They said they gave about $10,000 to a Gitcoin grant and put most of the rest into YFI vaults.
How it happened
- The trader bought about 2.01 SYFI for 0.5 ETH on Uniswap (tx
0xed33e727dd5b2f8e5164f6e15dabc1923652f2e933645378a87c45bf33c4e59a). - SYFI's first scheduled rebase increased holder balances, and the trader's balance rose to about 15,551 SYFI.
- The rebase did not call
sync()on the SYFI/ETH Uniswap pair. YAM-style rebasers do this in anafterRebasestep, so the pair's stored reserves still reflected the pre-rebase price. - The trader sold the SYFI into the stale pool for about 747 ETH, draining nearly all of its ETH.
Protocol details
Evidence
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