> ## Documentation Index
> Fetch the complete documentation index at: https://cosmos-docs-sync-security-docs.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# EIP-155: Replay Protection

> EIP-155 is an Ethereum Improvement Proposal that introduced replay protection by including chain ID information in signed transaction data. This prevents a signed transaction from being valid on multiple networks, protecting users from replay attacks.

## Background

**Replay attacks** occur when a signed transaction valid on one network can be replayed on another network without the user's consent. This is particularly problematic for Ethereum-based networks because:

1. **Address Format**: Ethereum uses hexadecimal addresses (e.g., `0x123...`) that are identical across all EVM chains
2. **Transaction Format**: The same transaction structure and signing scheme across networks
3. **Network Forks**: When chains fork, the same addresses exist on both chains with potentially different balances

EIP-155 solves this by incorporating the chain ID into the transaction signing process, making signatures network-specific.

## How Cosmos EVM Handles Replay Protection

### Dual Protection System

Cosmos EVM implements replay protection at two levels:

<CardGroup cols={2}>
  <Card title="Cosmos Transactions" icon="shield-halved">
    **Inherent Protection**: Uses Bech32 addresses with chain-specific prefixes (e.g., `cosmos1...`, `osmo1...`). Addresses from one chain are invalid on others, providing automatic replay protection.
  </Card>

  <Card title="EVM Transactions" icon="ethereum">
    **EIP-155 Protection**: Requires chain ID in transaction signatures. The EVM module enforces this by default, rejecting transactions without proper chain ID.
  </Card>
</CardGroup>

### Chain ID Configuration

As documented in the [Chain ID](/evm/latest/documentation/concepts/chain-id) page, Cosmos EVM uses:

* **EVM Chain ID** (integer): Used for EIP-155 replay protection in Ethereum transactions
* **Cosmos Chain ID** (string): Used for Cosmos SDK transactions with Bech32 addresses

<Warning>
  The EVM Chain ID must be unique across all EVM-compatible chains to ensure replay protection. Check [chainlist.org](https://chainlist.org/) before selecting your chain ID.
</Warning>

## Configuring Replay Protection

By default, replay protection is **enabled** on all Cosmos EVM nodes. The system enforces EIP-155 signed transactions, rejecting any unprotected transactions.

### Allowing Unprotected Transactions (Not Recommended)

<Warning>
  Disabling replay protection is **strongly discouraged** as it exposes users to replay attacks. Only consider this for specific testing scenarios or migration purposes.
</Warning>

If absolutely necessary, unprotected transactions can be allowed through a two-step process:

#### Step 1: Global Module Parameter

The EVM module parameter must be changed via governance proposal or genesis configuration:

```go theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}}
// In x/vm/types/params.go
DefaultAllowUnprotectedTxs = false  // Default value

// To enable in genesis.json
{
  "vm": {
    "params": {
      "allow_unprotected_txs": true  // Must be set to true
    }
  }
}
```

#### Step 2: Node Configuration

Even when globally enabled, each node operator must individually opt-in by modifying their node configuration:

```toml theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}}
# In $HOME/.evmd/config/config.toml
[json-rpc]
# AllowUnprotectedTxs restricts unprotected (non EIP-155 signed) transactions
# to be submitted via the node's RPC when the global parameter is disabled.
allow-unprotected-txs = true  # false by default
```

<Note>
  Both conditions must be met for unprotected transactions to be accepted:

  1. Global parameter `allow_unprotected_txs` = true
  2. Node configuration `allow-unprotected-txs` = true
</Note>

## Transaction Types and Protection

### Protected Transactions (EIP-155)

All modern Ethereum transaction types include chain ID:

```javascript theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}}
// EIP-1559 Dynamic Fee Transaction
const tx = {
  type: 2,
  chainId: 9000,  // EVM Chain ID included
  nonce: 0,
  maxFeePerGas: ...,
  maxPriorityFeePerGas: ...,
  // ... other fields
}
```

### Legacy Unprotected Transactions

Pre-EIP-155 transactions without chain ID:

```javascript theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}}
// Legacy transaction without chain ID (rejected by default)
const tx = {
  nonce: 0,
  gasPrice: ...,
  gasLimit: ...,
  to: ...,
  value: ...,
  data: ...,
  // No chainId field
}
```

## Security Implications

### With Replay Protection Enabled (Default)

**Protected from:**

* Cross-chain replay attacks
* Accidental transaction execution on wrong networks
* Fork-related replay vulnerabilities

### With Replay Protection Disabled

**Vulnerable to:**

* Transactions being replayed on any EVM chain
* Loss of funds through replay attacks
* Unintended contract interactions on multiple chains

## Best Practices

1. **Always use EIP-155**: Ensure your wallet and tools sign transactions with chain ID
2. **Verify chain ID**: Double-check the chain ID in your transaction before signing
3. **Keep protection enabled**: Never disable replay protection in production
4. **Monitor transactions**: Track transaction execution across chains during migrations

## Verification

To verify replay protection status:

```bash theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}}
# Check global parameter
evmd q vm params | grep allow_unprotected_txs

# Check node configuration
grep allow-unprotected-txs $HOME/.evmd/config/config.toml
```

## Related Resources

* [EIP-155 Specification](https://eips.ethereum.org/EIPS/eip-155)
* [Chain ID Configuration](/evm/latest/documentation/concepts/chain-id)
* [Transaction Types](/evm/latest/documentation/concepts/transactions#ethereum-tx-type)
* [EVM Module Parameters](/evm/latest/documentation/cosmos-sdk/modules/vm#parameters)