  NFTs (Non-Fungible Tokens) have experienced an explosive growth and their
record-breaking prices have been witnessed. Typically, the assets that NFTs
represent are stored off-chain with a pointer, e.g., multi-hop URLs, due to the
costly on-chain storage. Hence, this paper aims to answer the question: Is the
NFT-to-Asset connection fragile? This paper makes a first step towards this end
by characterizing NFT-to-Asset connections of 12,353 Ethereum NFT Contracts
(6,234,141 NFTs in total) from three perspectives, storage, accessibility and
duplication. In order to overcome challenges of affecting the measurement
accuracy, e.g., IPFS instability and the changing availability of both IPFS and
servers' data, we propose to leverage multiple gateways to enlarge the data
coverage and extend a longer measurement period with non-trivial efforts.
Results of our extensive study show that such connection is very fragile in
practice. The loss, unavailability, or duplication of off-chain assets could
render value of NFTs worthless. For instance, we find that assets of 25.24% of
Ethereum NFT contracts are not accessible, and 21.48% of Ethereum NFT contracts
include duplicated assets. Our work sheds light on the fragility along the
NFT-to-Asset connection, which could help the NFT community to better enhance
the trust of off-chain assets.
