Peer synchronization requires too much trust

[xloem]

It looks like the synchronization code picks the peer which gives the highest block height, and then tries to synchronize only from them.

This looks very vulnerable to a misbehaving peer which has a higher block height but will not synchronize to that height. Instead the block tree should be built from all connected peers, and a peer not judged as certainly providing the highest height until it has actually provided valid blocks at that height.

See https://github.com/Qoracoin/Qora/blob/master/Qora/src/controller/Controller.java#L693

[agran]

I too noticed this vulnerability.

[catbref]

There have been changes in v0.26.9 which might address this as misbehaving peers are blacklisted for a while. "misbehaving" includes not sending blocks, or sending out-of-order blocks, or blocks on a radically different fork - which essentially lets a node settle on the right fork/chain by way of network consensus.

Review would be appreciated!