what happens if I leave out ensure_signed from extrinsic?

Question

What are the attack vectors of leaving out ensured_signed in one of my extrinsic?

It seems that SignedExtensions already checks if the transactions are signed. When I try to submit an unsigned transaction, it automatically fails.

It seems that this is a convention everyone follows when writing extrinsic, but I am not sure why.

Answer 1

The problem with allowing unsigned extrinsics is that nobody pays for the consumed weight.
So you are basically doing work for free - normally undesirable.

The only use-case where it is a good idea are cleanup operations which remove storage items.
A smaller state means less PoV of storage READ/REMOVE operations and is therefore desirable.
One could say that storage cleanups have an amortized "negative" weight and should therefore be free.

Answer 2

Have you tried submit_unsigned?

I think people could submit unsigned transactions on your chain without paying any fee.

Obviously, this will cause something like DDOS. Use a script to submit the unsigned transactions.

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Why do we need unsigned?

1. Airdrop, like the Polkadot claim module. It's not a good idea to require a user to own some token first and then to claim the airdrop. So they set the claim extrinsic feeless.
2. For some public-welfare calls, like the sync finality proof in the cross-chain bridge pallet. It's a one-time verification/call, after the first successful call it will reject the others.
3. Something like @Oliver described.