Signature omits msg.sender, nonce, and expiry, letting any observer relay or front-run a claim

Description

_isValidSignature only checks that the recovered signer equals receiver; msg.sender is not part of the signed SnowmanClaim struct and is never validated, and the struct carries no nonce or expiry.

Any observer of a valid (v, r, s) can submit the claim on the receiver's behalf.

Risk

Likelihood:

A valid signature is visible in the mempool or wherever it is shared. Replaying it requires no privilege.

Impact:

The NFT still mints to receiver, so direct theft is limited. But combined with the live-balance leaf bug, a front-runner can grief the receiver's balance to invalidate the receiver's own transaction, and the absence of a nonce/expiry leaves no replay or time bound. Low severity in isolation.

Proof of Concept

A third party submits the receiver's signature and the claim executes.

Recommended Mitigation

Bind the caller (if relaying is unwanted) or add a per-address nonce and expiry to the signed struct.