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Safe has adopted a modular and flexible approach to integrating the ERC-4337, allowing users to turn their Safe account into an ERC-4337 smart account. Safe ERC-4337 compatibility is provided via Safe Modules and the Fallback Handler. This means the functionality is not implemented directly in the Safe Smart Account, but in the Safe4337Module contract, which can be enabled in any Safe account at the Safe deployment time or afterward.

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Safe4337Module

This module is an extension to the Safe Smart Account that acts both as a Fallback Handler, meaning that the Safe Proxy contract will fallback to this contract when its functions are called in the proxy, and a Safe Module, having the right to execute Safe transactions once it’s enabled in a Safe account. It implements the ERC-4337 interface, including the functions to validate and execute the user operation, and it’s limited to the EntryPoint address.
This module must only be used with Safe v1.4.1 or newer.

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UserOperation validation

The Safe Proxy contract receives a call to the validateUserOp function from the EntryPoint and forwards it to the Safe4337Module. The module validates the UserOperation object by checking that the Safe owners signed the UserOperation hash and returns the result. It also executes a module transaction to pay the fees back. diagram validate UserOp

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UserOperation execution

After successful validation, the EntryPoint calls the executeUserOp function, forwarding it again to the module, which executes a module transaction with the target and data specified in the UserOperation object. diagram execute UserOp