June 17, 2021

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Breaking Aave Upgradeability

Breaking Aave Upgradeability

On December 3rd, Aave deployed version 2 of their codebase. While we were not hired to look at the code, we briefly reviewed it the following day. We quickly discovered a vulnerability that affected versions 1 and 2 of the live contracts and reported the issue. Within an hour of sending our analysis to Aave, their team mitigated the vulnerability in the deployed contracts. If exploited, the issue would have broken Aave, and impacted funds in external DeFi contracts.

Five different security firms reviewed the Aave codebase, including some that used formal verification; however, this bug went unnoticed. This post describes the issue, how the bug escaped detection, and other lessons learned. We are also open-sourcing a new Slither detector that identifies this vulnerability to increase security in the larger Ethereum community.

The vulnerability

Aave uses the delegatecall proxy pattern that we have thoroughly discussed in past writeups on this blog. At a high-level, each component is split into two contracts: 1) A logic contract that holds the implementation and 2) a proxy that contains the data and uses delegatecall to interact with the logic contract. Users interact with the proxy while the code is executed on the logic contract. Here’s a simplified representation of the delegatecall proxy pattern:

Breaking Aave Upgradeability

In Aave, LendingPool (LendingPool.sol) is an upgradeable component that uses a delegatecall proxy.

The vulnerability we discovered relies on two features in these contracts:

  • Functions on the logic contract can be called directly, including initialization functions
  • The lending pool has its own delegatecall capabilities

Initializing upgradeable contracts

A limitation of this upgradeability pattern is that the proxy cannot rely on the logic contract’s constructor for initialization. Therefore, state variables and initial setup must be performed in public initialization functions that do not benefit from the constructor safeguards.

In LendingPool, the initialize function sets the provider address (_addressesProvider):


function initialize(ILendingPoolAddressesProvider provider) public initializer {
_addressesProvider = provider;
}

LendingPool.sol#L90-L92

The initializer modifier prevents initialize from being called multiple times. It requires that the following condition is true:

   require(
      initializing || isConstructor() || revision > lastInitializedRevision,
      'Contract instance has already been initialized'
    );

VersionedInitializable.sol#L32-L50

Here:

  1. initializing allows multiple calls to the modifier within the same transaction (hence multiple initialize functions)
  2. isConstructor() is what the proxy needs to execute the code
  3. revision > lastInitializedRevision allows calling the initialization functions again when the contract is upgraded

While this works as expected through the proxy, 3 also allows anyone to call initialize directly on the logic contract itself. Once the logic contract is deployed:

The bug: Anyone can set _addressesProvider on the LendingPool logic contract.

Arbitrary delegatecall

LendingPool.liquidationCall delegatecalls directly to the addresses returned by _addressProvider:

   address collateralManager = _addressesProvider.getLendingPoolCollateralManager();

    //solium-disable-next-line
    (bool success, bytes memory result) =
      collateralManager.delegatecall(
        abi.encodeWithSignature(
          'liquidationCall(address,address,address,uint256,bool)',
          collateralAsset,
          debtAsset,
          user,
          debtToCover,
          receiveAToken
        )
      );

LendingPool.sol#L424-L450

This lets anyone initiate the LendingPool logic contract, set a controlled addresses provider, and execute arbitrary code, including selfdestruct.

The exploit scenario: Anyone can destruct the lending pool logic contract. Here’s a simplified visual representation:

Lack-of-existence check

By itself, this issue is already severe since anyone can destruct the logic contract and prevent the proxy from executing the lending pool code (pour one out for Parity).

However, the severity of this issue is amplified by the use of OpenZeppelin for the proxy contract. Our 2018 blogpost highlighted that a delegatecall to a contract without code would return success without executing any code. Despite our initial warning, OpenZeppelin did not fix the fallback function in their proxy contract:

   function _delegate(address implementation) internal {
        //solium-disable-next-line
        assembly {
            // Copy msg.data. We take full control of memory in this inline assembly
            // block because it will not return to Solidity code. We overwrite the
            // Solidity scratch pad at memory position 0.
            calldatacopy(0, 0, calldatasize)

            // Call the implementation.
            // out and outsize are 0 because we don't know the size yet.
            let result := delegatecall(gas, implementation, 0, calldatasize, 0, 0)

Proxy.sol#L30-L54

If the proxy delegatecalls to a destroyed lending pool logic contract, the proxy will return success, while no code was executed.

This exploit would not be persistent since Aave can update the proxy to point to another logic contract. But in the timeframe where the issue could be exploited, any third-party contracts calling the lending pool would act as if some code was executed when it was not. This would break the underlying logic of many external contracts.

Affected contracts

  • All ATokens (Aave tokens): AToken.redeem calls pool.redeemUnderlying (AToken.sol#L255-L260). As the call does nothing, the users would burn their ATokens without receiving back their underlying tokens.
  • WETHGateway (WETHGateway.sol#L103-L111): Deposits would be stored in the gateway, allowing anyone to steal the deposited assets.
  • Any codebase based on Aave’s Credit Delegation v2 (MyV2CreditDelegation.sol)

If the issue we discovered were exploited, many contracts outside of Aave would have been affected in various ways. Determining a complete list is difficult, and we did not attempt to do so. This incident highlights the underlying risks of DeFi composability. Here are a few that we found:

Fixes and recommendations

Luckily, no one abused this issue before we reported it. Aave called the initialize functions on both versions of the lending pool and thus secured the contracts:

Long term, contract owners should:

  • Add a constructor in all logic contracts to disable the initialize function
  • Check for the existence of a contract in the delegatecall proxy fallback function
  • Carefully review delegatecall pitfalls and use slither-check-upgradeability

Formally verified contracts are not bulletproof

Aave’s codebase is “formally verified.” A trend in the blockchain space is to think that safety properties are the holy grail of security. Users might try to rank the safety of various contracts based on the presence or absence of such properties. We believe this is dangerous and can lead to a false sense of security.

The Aave formal verification report lists properties on the LendingPool view functions (e.g., they don’t have side effects) and the pool operations (e.g., the operations return true when successful and do not revert). For example, one of the verified properties is:

Yet this property can be broken if the logic contract is destroyed. So how could this have been verified? While we don’t have access to the theorem prover nor the setup used, likely, the proofs don’t account for upgradeability, or the prover does not support complex contract interactions.

This is common for code verification. You can prove behaviors in a targeted component with assumptions about the overall behavior. But proving properties in a multi-contract setup is challenging and time-consuming, so a tradeoff must be made.

Formal techniques are great, but users must be aware that they cover small areas and might miss attack vectors. On the other hand, automated tools and human review can help the developers reach higher overall confidence in the codebase with fewer resources. Understanding the benefits and limits of each solution is crucial for developers and users. The current issue is a good example. Slither can find this issue in a few seconds, an expert with training may quickly point it out, but it would take substantial effort to detect with safety properties.

Conclusion

Aave reacted positively and quickly fixed the bug once they were aware of the issue. Crisis averted. But other victims of recent hacks were not as fortunate. Before deploying code and exposing it to an adversarial environment, we recommend that developers:

We hope to prevent similar mistakes by sharing this post and the associated Slither detector for this issue. However, security is a never-ending process, and developers should contact us for security reviews before launching their projects.