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Currently I have my own full bitcoind node running on a VPS and an LND instance connecting to it and working fine.
However I am thinking, what if I share access to my bitcoind node to some other users willing to connect their own LND instances to it? So, in the end there will be independent LND instances serving for their users but running on top of a single bitcoind instance.
Do I have a risk of exposing data/balances/transactions between these users? Do users have risks of me taking the control over their wallets?
A Lightning node needs a full Bitcoin node to be secure. If the Bitcoin node is operated by someone else, there are some rather trivial attacks that can be used to steal money from you. For example, the Bitcoin node operator might open a channel with you and later close it with an outdated state. Since they control which transactions your Lightning node knows about, it will never learn of the channel close and won’t broadcast a penalty transaction (unless you use an external watchtower).
Sharing a bitcoind instance (which presumably doesn’t run a wallet) doesn’t come with as big of a risk, but Bitcoin Core’s RPC interface is not meant to be exposed to the internet since it doesn’t scale well for multiple users and can be easily overloaded.