Standards

Trusted Forwarder

A trusted forwarder is the ERC2771 contract a recipient trusts to verify signed requests and relay calls with the original signer appended.

A trusted forwarder is the relayer contract a recipient accepts as honest about who signed the request.

Trusted Forwarder Explained in Detail

A trusted forwarder receives a signed request, checks it, and calls the recipient contract. The recipient trusts that the forwarder appended the real signer address.

If the forwarder is wrong or malicious, the recipient's caller checks can become wrong too.

Smart contract example

function isTrustedForwarder(address forwarder) public view returns (bool);

ERC2771 recipients use this check before treating calldata as forwarded.

Trusted Forwarder in Auditing

The forwarder is a security dependency. If it skips nonce checks, accepts bad signatures, or can be upgraded by a weak admin, users may be impersonated.

Auditors review both the recipient and the forwarder contract.

Red flags in code

  • The recipient trusts any forwarder.

  • The forwarder omits signer, target, calldata, nonce, deadline, or chain from signed data.

  • Forwarder upgrade rights are controlled by a weak owner.

  • Multiple trusted forwarders are configured without clear purpose.

  • Relayed calls bypass normal access control.

How to test or review it

  • Test bad signer, wrong nonce, expired deadline, and wrong target.

  • Try appending a fake signer from a non-forwarder caller.

  • Review EIP-712 domain construction.

  • Check whether the forwarder target must trust the forwarder.

  • Verify admin controls for changing or upgrading the forwarder.

ERC2771

ERC2771 is a meta-transaction standard where a trusted forwarder calls a recipient contract and appends the original signer to calldata.

Nonce

A nonce is a value used once, commonly to prevent replay of signatures, orders, permits, withdrawals, or messages.

Signature Replay

Signature replay happens when a valid signature can be reused more than once or reused in a different context than the signer intended.

Practice this in real audit scenarios

Definitions help, but auditors need reps. SCH turns concepts like Trusted Forwarder into exploit labs, code review habits, and report-writing practice.

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Sources