Show a warning when changing the fallback handler to an untrusted contract

[mmv08]

What is the feature about

A fallback handler is a contract that gets CALLed by a Safe contract when no matching method was found in proxies/singleton contracts. This great feature allows us to extend Safe's functionality (e.g., ERC-4337), but it's dangerous when misused.

If set to a bad contract (e.g., a token address), anyone could call the safe, and the safe would forward the call to a token as if it'd be legitimately calling the contract through execTransaction.

The list of requirements

A transaction that sets the fallback handler to an untrusted address should display a warning similar to the delegate call.
Example transaction: https://app.safe.global/transactions/tx?safe=sep:0x12c0916493b9BEd2A9882d7722191660194eB47d&id=multisig_0x12c0916493b9BEd2A9882d7722191660194eB47d_0x68004c3de4755bca601f1b3e7b945c563a8b997f5a06dedc6063151989f8ee49

Designs/sketches

To be filled by the team

Links

safe-global/safe-smart-account#879

[mmv08]

a tiny node script if someone wants to create a "bad" tx:

import { ethers } from "ethers"
import { default as SafeApiKit } from "@safe-global/api-kit"
import Safe from "@safe-global/protocol-kit"

function build1193Provider(provider: ethers.InfuraProvider) {
  return {
    request: (args: any) => provider.send(args.method, args.params),
  }
}

const SAFE_ADDRESS = "0x12c0916493b9BEd2A9882d7722191660194eB47d"
const SENDER_ADDRESS = "0x02270bd144e70cE6963bA02F575776A16184E1E6"
const PRIVATE_KEY = ""
const provider = new ethers.InfuraProvider("sepolia", "")

// @ts-ignore
const protocolKit = await Safe.default.init({
  provider: build1193Provider(provider),
  signer: PRIVATE_KEY,
  safeAddress: SAFE_ADDRESS,
})

const changeFallbackHandlerTx = await protocolKit.createEnableFallbackHandlerTx(
  "0xB4F1737Af37711e9A5890D9510c9bB60e170CB0D"
)

// @ts-ignore
const apiKit = new SafeApiKit.default({
  chainId: 11155111n,
})

const safeTxHash = await protocolKit.getTransactionHash(changeFallbackHandlerTx)
const senderSignature = await protocolKit.signHash(safeTxHash)

await apiKit.proposeTransaction({
  safeAddress: SAFE_ADDRESS,
  safeTransactionData: changeFallbackHandlerTx.data,
  safeTxHash,
  senderAddress: SENDER_ADDRESS,
  senderSignature: senderSignature.data,
})