Summary

Attackers can craft a malicious fallback handler with a clashing function selector to trick smart account owners into calling inbuilt functions.

Vulnerability Details

Nexus smart accounts allows the owner to install fallback handlers which provide extensibility to the account, allowing additional functionality not present in the implementation to be added and called through the fallback function.

ModuleManager.sol#L94-L109

Function dispatch in Solidity is done using function selectors. Selectors are represented by the first 4 bytes of the keccak hash of the function signature (name + argument types). It is possible (and not computationally difficult) to find different functions that have the same selector.

This means that a malicious actor can craft a fallback handler with a function signature carefully selected to match one of the functions present in the Nexus contract, and with an innocent looking implementation. While the fallback implementation may seem harmless, this function when called will actually trigger the function in the Nexus contract.

For example:

• BaseAccount.withdrawDepositTo() has the function signature of 0x4d44560d

• Attacker creates a fallback handler with a function which also has a signature of 0x4d44560d. This function looks harmless.

• Owner of a Nexus account installs the fallback handler using installModule().

• Owner attempts to call the function in the fallback handler:

  • When his transaction is sent through the entrypoint contract, it ends up calling BaseAccount.withdrawDepositTo() instead as it has the same function signature.

  • ETH is transferred out of the account to the to address.

In this example, through the malicious fallback handler, the attacker tricks the owner into transferring ETH out of his Nexus account.

This is similar to the exploit reported on proxies in this article, which caused the proposal of the transparent proxy pattern.

Impact

Owners of Nexus accounts can be tricked into calling sensitive functions in Nexus implementation, such as withdrawDepositTo().

Recommendations

Consider documenting that modules with clashing function selectors should not be installed or called.

Additionally, consider checking for these selectors in _installFallbackHandler() and reverting if the owner attempts to install them.