If we assume that certain NFTs in a vault over time will have different market demand/price, then the users will try to redeem those specific NFTs. Even if direct redeems are disabled to prevent such a scenario to default to returning randomized NFTs, a user can brute-forced this on-chain randomization (using nonce + blockhash) by repeatedly trying to redeem/swap from a contract, checking the NFT IDs returned from the function and reverting the transaction if those are not the NFT IDs of specific interest.
The impact will be a subversion of the randomization goal to return random NFTs which cannot be specified by the user.
A similar exploit happened recently with Meebit NFTs.
Recommend considering only EOA (external only account) for redeem/swap operations to prevent brute-forcing via contracts. Alternatively, make the user commit to pseudo-random IDs before revealing them.
**- 0xKiwi acknowledged **
