A sidechain is a blockchain linked to a main chain, such as Bitcoin, so assets can be represented and moved under a separate rule set. Instead of changing the base chain directly, the sidechain maintains its own ledger, validation logic, and operational controls while relying on a peg, reserve, or bridge mechanism to keep value aligned.
In cyber security, sidechains matter because they create a new trust boundary. Security depends not only on cryptography, but also on the integrity of the peg, the redemption path, and any pause or recovery controls. If those components fail, users may be unable to move back to the underlying asset even when balances still exist. Attackers often target these systems through key theft, validator compromise, bridge abuse, or disruption of the control layer. Defenders focus on reserve auditing, access control, monitoring, and safe pause mechanisms so the system can recover without trapping funds.