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Description

Bybit suffered from a security breach in Safe’s hosting infrastructure losing 1.4bn USD. The attackers, the lazarus group, gained access to the server and was able to modify the served Safe Wallet frontend. More on the attack: https://x.com/safe/status/1894768522720350673 The malicious code was specifically targeting Bybit’s next transaction. As of today, it is commonly agreed that the signers of the transaction were shown a fake UI, seeming to be the intended transaction to sign, but in fact it was a malicious transaction. Unfortunately, it is often very hard to verify the correctness of the message that is signed in software or hardware wallets. Attack steps:
  1. Attackers gained access to the hosting server by compromising a developer’s machine.
  2. A malicious frontend was uploaded to the server, which targeted Bybit’s next transaction.
  3. The frontend would show the intended transaction but in fact the message to be signed was malicious
  4. The malicious message was a delegate call which would change the implementation of the Safe’s proxycontract

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How assertions could have prevented this

In Safe’s proxy implementation, the first storage slot is the implementation address. 
/**
 * @title Singleton - Base for singleton contracts (should always be the first super contract)
 *        This contract is tightly coupled to our proxy contract (see `proxies/SafeProxy.sol`)
 * @author Richard Meissner - @rmeissner
 */
abstract contract Singleton {
    // singleton always has to be the first declared variable to ensure the same location as in the Proxy contract.
    // It should also always be ensured the address is stored alone (uses a full word)
    address private singleton;
}
The malicious transaction was executing a delegate call to a malicious contract, effectively changing the implementation address of Bybit’s proxy contract. A change of the implementation address allows an attacker to almost completely exchange the logic of the contract. Therefore it was possible to be able to drain all the assets of the Safe without the need of any additional approvals by the original owners. It should be highlighted that even if there is no logic to change the implemenation address in the implementation by safe, delegate calls can modify storage slots of the calling contract. This allows for full flexibility but opens up the possibility for attacks.

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Preventing attacks through assertions

It might be fair to assume that most users of safes would never actually want to change the implementation address of their safe, but they have no way to express their preferences. Assertions give users the ability to express additional invariants on the state of the contract. A change of the implementation address could be prevented by analyzing the state changes within a transaction. PhEvm precompiles used:
  • forkPreState allowing to fork the state of the contract before a transaction
  • load reading specific storage slots
  • getStateChangesAddress getting all state changes within a transaction for a given slot
// SPDX-License-Identifier: MIT
pragma solidity 0.8.28;

import {Assertion} from "../../lib/credible-std/src/Assertion.sol"; // Credible Layer precompiles
import {Safe} from "../../lib/safe-contracts/src/Safe.sol"; // Safe contract
import {PhEvm} from "../../lib/credible-std/src/PhEvm.sol";

contract SafeAssertion is Assertion {
    Safe safe;

    constructor(address payable safe_) {
        safe = Safe(safe_); // Define address of Safe contract
    }

    // Define the triggers for the assertions
    function triggers() public view override {
        triggerRecorder.registerStateChangeTrigger(this.implementationAddressChange.selector, 0x0);
    }

    function implementationAddressChange() external view {
        ph.forkPreState();
        address preImplementationAddress = address(uint160(uint256(ph.load(address(safe), bytes32(0x0)))));

        address[] memory addresses = getStateChangesAddress(address(safe), bytes32(0x0));
        for (uint256 i = 0; i < addresses.length; i++) {
            if (addresses[i] != preImplementationAddress) {
                revert("Implementation address has changed within transaction");
            }
        }
    }
}
To even enhance security properties, a complentary assertion could enforce transfers to a whitelist of addresses. In the below example, the assertion checks if the balance of the safe decreased while the balance of the whitelisted addresses increased with the respective amount. 
 function assertionSafeDrain() external {
        ph.forkPreState();
        uint256 preBalance = address(bybitSafeAddress).balance;
        uint256[] memory preWhitelistBalances = new uint256[](whitelistedAddresses.length);
        for (uint256 i = 0; i < whitelistedAddresses.length; i++) {
            preWhitelistBalances[i] = address(whitelistedAddresses[i]).balance;
        }

        ph.forkPostState();
        uint256 postBalance = address(bybitSafeAddress).balance;
        if (postBalance > preBalance) {
            return; // Balance increased, not a hack
        }
        uint256 diff = preBalance - postBalance;
        for (uint256 i = 0; i < whitelistedAddresses.length; i++) {
            uint256 postWhitelistBalance = address(whitelistedAddresses[i]).balance;
            if (postWhitelistBalance == preWhitelistBalances[i] + diff) {
                return; // Balance increased, not a hack
            }
        }
        // None of the whitelisted addresses have increased in balance, so it's a hack
        revert("Bybit safe address balance decreased");
    }
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