This document is a security review of SushiSwap.
Timeline: 8/31/20 - 9/2/20
Auditors:
- Ed Zulkoski, Senior Research Engineer
- Shunsuke Tokoshima, Blockchain Researcher
- Joseph Xu, Technical R&D Advisor
Commit: b3672d93
- For
GovernorAlpha.sol,Timelock.sol, only diffs were considered as compared tocompound-financecommit 857c223. - For
uniswapv2/*, we only considered the diff as specified inuniswapv2/README.md, i.e.,git diff 4c4bf551417e3df09a25aa0dbb6941cccbbac11a ..
Documentation: SushiSwap Medium Article
** 10 ** were reported including:
- 2 medium risk issues;
- 3 low risk issues;
- 5 informational.
No critical or high risk issues were found. Although we provide some recommendations for fixes below, since the contracts are already deployed, these are primarily for informational purposes. No found issues were critical enough to suggest re-deployment of the existing contracts, however caution should be used when using them.
On L106 it is noted: "// XXX DO NOT add the same LP token more than once. Rewards will be messed up if you do."
If a token is mistakenly added more than once, it would reset the rewards variables associated with the token (e.g., accSushiPerShare).
This could be prevented by creating a mapping from addresses to booleans, such that LP tokens get mapped to true once they've been added. The function could then have a require-statement preventing the same LP token from being added twice.
Migrator can be set to any contract, potentially allowing the theft of funds, particularly if the owner's private key is compromised. The migrator can be set at any time (and multiple times) by the owner. In the worst case, this could be set to a contract that transfers all underlying LP tokens to an arbitrary address.
Note: since the owner is a timelock contract, users would have 2 days to mass exit the pool before the change could occur.
According to the provided docs, "...10% of every SUSHI distribution is set aside for the development & future iterations, including security audit."
However, according to L195-196, devaddr is receiving 1/11 = 9% of the total sushi distribution.
The function _delegate() invokes _moveDelegates() with the delegator's full balance instead of remaining undelegated balance. This can cause users to lose delegation ability if additional SUSHI tokens are acquired without minting (i.e., via transfers).
Consider the folowing scenario:
- Alice has 10 SUSHI, which are delegated to Bob.
- Alice acquires 1 additional SUSHI from a transfer.
- If Alice attempts to re-delegate her 11 SUSHI tokens to Carol, it will fail due to the SafeMath check on L201; effectively, the function will attempt to undelegate 11 tokens from Bob instead of 10, and revert.
In general, if Alice's balance is ever more than the number of tokens minted toward her account (due to transfers), she will not be able to delegate. This can be mitigated by Alice by simply transferring the excess tokens out of her account, however this scenario may not be clear to end-users from a UX-perspective.
It is not clear if this functionality is as intended. If so, no changes are needed, but user documentation should exist describing the scenario above. If the scenario above is undesirable, _moveDelegates() should be invoked in _transfer() as well. Note however that with this approach, votes can be more easily "bought" by acquiring SUSHI tokens on exchanges.
This may have implications on calling functions add() and set().
The values for user.amount and user.rewardDebt are not zeroed until after the external call to pool.lpToken.safeTransfer(). Although the LP tokens are assumed trusted as of now, if a compromised token is added, reentrancy may allow the theft of tokens.
In general we recommend following the checks-effects-interactions pattern. Zero out the user's state variables prior to the external safeTransfer() call (ensuring the correct amount is still passed in safeTransfer() using a local variable).
When migrate() is called in Migrator.sol, it calls mint() in the new UniswapV2Pair, which checks the migrator address from the new UniswapV2Factory. The migrator address in the new UniswapV2Factory needs to be set properly to ensure proper liquidity provision.
The feeToSetter role in UniswapV2Factory has a high level of privilege, including changing the migrator address, which might affect the liquidity of newly migrated pools.
The owner of MasterChef.sol may add arbitrary tokens to the system, as well as change rewards allocation. As noted above, they may also update the Migrator contract.
The privileged power associated with these addresses should be made clear to end users in the documentation.
In the constructors for SushiMaker.sol, SushiBar.sol, and Migrator.sol, the address parameters are not checked to be non-zero. Similarly, the function MasterChef.setMigrator() does not have input validation.
In general we recommend ensuring that address arguments are non-zero in constructor and setter functions in order to mitigate deployment issues.
Some contracts import contracts from @openzeppelin/contracts.
The version of @openzeppelin/contracts is not locked in package.json.
Unlocked dependency versions may result in compiling the project using versions of dependencies that are different from the ones that were tested or audited.
In package.json, lock dependency versions, e.g., "@openzeppelin/contracts": "3.1.0".
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