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Solidity Memory Corruption Exploits via BytesToTypes, mSlice, and handleSynPackage

Overview

This document provides a detailed breakdown of a class of origin-level memory corruption attacks in Solidity smart contracts. These attacks abuse unsafe memory manipulation via inline assembly, BytesToTypes, and mSlice-style functions, often in relayer functions such as handleSynPackage. The attack vectors result in fund redirection, logic compromise, or cross-chain exploitation — and can extend into kernel/OS compromise via malicious validator nodes.

Attack Architecture

Diagram: Attack Flow Overview

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flowchart TD
    A[User or Bot sends malicious tx] --> B[Malicious Validator Node]
    B --> C[handleSynPackage() invoked]
    C --> D[Unsafe call to BytesToTypes.bytesToAddress()]
    D --> E[Memory loaded from attacker-controlled offset]
    E --> F[Corrupt address or logic derived]
    F --> G[Funds drained or logic hijacked]
    G --> H[Malicious cross-chain relaying or validator syncing]

Vulnerable Function: handleSynPackage

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function handleSynPackage(uint8 channelId, bytes calldata msgBytes) external override returns (bytes memory responsePayload) {
    address recipient = BytesToTypes.bytesToAddress(32, msgBytes);
    uint256 amount = BytesToTypes.bytesToUint256(64, msgBytes);
    token.transfer(recipient, amount);
}

Vulnerability:

• No bounds checking on msgBytes
• Attacker can craft calldata that tricks inline assembly into reading out-of-bounds memory or misaligned memory

Exploit Modules

1. BytesToTypes.bytesToAddress

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function bytesToAddress(uint256 _offst, bytes memory _input) internal pure returns (address _output) {
    assembly {
        _output := mload(add(_input, _offst))
    }
}

Risk:

• Directly reads from memory using unchecked offsets

• Can be used to:

  • Read beyond input boundaries
  • Corrupt storage pointers
  • Return attacker-controlled address

2. mSlice(bytes, offset, length)

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function mSlice(bytes memory data, uint offset, uint length) internal pure returns (bytes memory result) {
    assembly {
        result := mload(0x40)
        mstore(result, length)
        let dest := add(result, 0x20)
        let src := add(add(data, 0x20), offset)
        for { let i := 0 } lt(i, length) { i := add(i, 0x20) } {
            mstore(add(dest, i), mload(add(src, i)))
        }
        mstore(0x40, add(dest, length))
    }
}

Risk:

• Performs raw memory copy without bounds validation

• Can:

  • Overread and overwrite memory
  • Collide with other variables on the heap

Example Exploit Payload

Attacker's Crafted msgBytes

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| Offset | Data                          |
|--------|-------------------------------|
| 0x20   | 0x000000000000...c0ffee...ab | <- Address payload injected here
| 0x40   | 0x000000000000...000000003e8 | <- Amount = 1000 (in Wei or token units)

• When bytesToAddress(32, msgBytes) is called:

  • It reads from the address at offset 0x20 (32 bytes in)
  • The attacker can place arbitrary 32 bytes there, which is mload-ed and truncated to 20 bytes

Mind Map: Memory Corruption Exploit Vectors

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mindmap
  root((Memory Exploits in Solidity))
    HandleSynPackage
      bytesToAddress
        Assembly mload
        Offset manipulation
      bytesToUint256
    mSlice
      Offset + Length unchecked
      Overwrite memory
    Cross-Chain Relays
      Malicious Validators
        Propagate corrupted payload
        Manipulate consensus

Exploit Implications

Defensive Strategy

🔒 Hardened Decoding

• Replace unsafe assembly decoding with abi.decode
• Validate msgBytes.length before any slicing or offset reads
• Add explicit length checks:

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require(msgBytes.length >= expectedLength, "Invalid msgBytes length");

🛡️ Validate Inputs

• Verify the structure and alignment of relayer data
• Use safeSlice() with bounds checking before passing to decoder

🔍 On-Chain Monitoring

• Detect malformed msgBytes patterns
• Flag suspicious validator nodes that relay corrupted packages

Next Steps

1. Simulate attack payload on a local testnet using Foundry or Hardhat
2. Patch vulnerable contracts with secure decoding logic
3. Integrate validator-level payload verification
4. Harden node infrastructure with syscall auditing (Linux) and SIM-based signature verification