> ## Documentation Index
> Fetch the complete documentation index at: https://docs.cow.bleu.builders/llms.txt
> Use this file to discover all available pages before exploring further.

# ExtensibleFallbackHandler

> API reference for the Safe fallback handler that enables signature verification multiplexing through domain-specific verifiers.

# ExtensibleFallbackHandler

The `ExtensibleFallbackHandler` is a Safe fallback handler enabling signature verification multiplexing through domain-specific verifiers. It works with `ComposableCoW` to validate programmatic orders via EIP-1271.

## Deployment

Deployed at the same address across multiple networks (Ethereum Mainnet, Gnosis Chain, Arbitrum One, Base, Optimism, and Polygon):

```
0x2f55e8b20D0B9FEFA187AA7d00B6Cbe563605bF5
```

## How It Functions

When CoW Protocol initiates settlement:

1. It calls `isValidSignature()` on the Safe
2. The Safe delegates to `ExtensibleFallbackHandler`
3. The handler extracts the domain separator from the signature
4. It routes to the appropriate domain verifier (typically ComposableCoW)
5. The verifier validates the programmatic order and returns the EIP-1271 magic value if valid

## Essential Setup Steps

Two configuration tasks are required:

1. **Set the Safe's fallback handler** to the `ExtensibleFallbackHandler` address
2. **Configure ComposableCoW as the domain verifier** for CoW Protocol's domain separator using `setDomainVerifier()`

Both operations must execute through Safe transactions, not direct calls.

## Security Notes

<Warning>
  * Only the Safe itself can execute `setDomainVerifier` calls
  * The configured verifier holds substantial authority to approve orders
  * Setting a malicious verifier could enable unauthorized order execution
  * Domain verifiers require careful auditing before deployment
</Warning>