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

• 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