Overview
Long-range attacks are a specific concern for Proof of Stake (PoS) blockchains. Because PoS doesn't require massive physical energy (like mining), an attacker who acquires the private keys of old stakeholders can 're-write' the history of the chain starting from a very early block (the 'long range').
How it Works
The attacker creates a version of the chain that is longer (or has more 'weight') than the current main chain. When a new node joins the network, it might be confused as to which chain is the 'real' one.
Defenses
- Checkpoints: Hardcoding the hashes of recent blocks into the software so nodes won't accept a chain that diverges too far in the past.
- Weak Subjectivity: Requiring new nodes to get a recent 'state' from a trusted source rather than syncing from the genesis block.
- Slashing: Penalizing validators who sign conflicting blocks, though this is harder to enforce for very old blocks.