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code-423n4 / 2021-11-streaming-findings

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Token allowances vulnerable to front-running #193

Closed code423n4 closed 2 years ago

code423n4 commented 2 years ago

Handle

toastedsteaksandwich

Vulnerability details

Impact

The LockeERC20.approve() function is vulnerable to front-running, as described in the following scenario:

1) Alice approves Bob to transfer 5 tokens 2) Alice decides to reduce this allowance to 3 tokens 3) Bob notices the allowance reduction tx in the mempool and frontruns it to spend the initial allowance of 5 tokens 4) Bob is granted a new allowance of 3 tokens, and spends it for a total of 8 tokens spent.

Impacted code

https://github.com/code-423n4/2021-11-streaming/blob/main/Streaming/src/LockeERC20.sol#L86-L92

Recommended Mitigation Steps

The approve() function should be split into two functions, increaseAllowance() and decreaseAllowance(), that increase or decrease the allowance respectively.

Extra reading

The original issue is documented here https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729

brockelmore commented 2 years ago

duplicate #74

0xean commented 2 years ago

duplicate of #55