> A medallion signature guarantee is a guarantee by the transferring financial institution that the signature is genuine and the financial institution accepts liability for any forgery.
[0] https://en.m.wikipedia.org/wiki/Medallion_signature_guarante...
Maybe I'm forgetting how this works...
...reading the wikipedia page, it sounds like the idea is to deal with forgeries of signatures provided to the guarantor. Not with forgeries of the guarantor's approval.
Then again, maybe you can't really forge a guarantee as long as it can be looked up or invalidated by their database.