Cross-Chain Bridges

01 · Concept — what problem does it solve?

Each blockchain is an island; a contract on Ethereum cannot read Solana. A bridge lets value move between them — and in doing so becomes the place where the most money is held with the weakest guarantees. Bridges are consistently the single most exploited category in DeFi: in 2026 they keep accounting for an outsized share of monthly hack losses despite holding a fraction of total value, because the security of moving an asset is only as strong as the messaging layer between two chains that fundamentally cannot verify each other.

02 · Mechanics

• Lock-and-mint: lock the asset in a contract on chain A, mint a wrapped representation on chain B; burn on B to unlock on A. The wrapped token is only worth as much as the lock is secure.
• Liquidity / burn-and-mint pools: pre-funded liquidity on both sides; you withdraw native asset on the destination from a pool rather than minting a wrapper.
• The trust model is everything: who attests that the lock happened? A multisig, an external validator set, an optimistic challenge window, or a light-client/ZK proof — in increasing order of trustlessness and difficulty.
• Message passing: generalized bridges (LayerZero, CCIP, Wormhole) carry arbitrary messages, not just tokens — and inherit the same verification risk.

03 · Formulas

// lock-and-mint invariant (must always hold)
wrapped_supply_on_B == locked_on_A     // break this → infinite mint

// trust spectrum (weakest → strongest)
multisig  <  external validator set  <  optimistic + fraud proof  <  light client / ZK

// the failure that recurs
forge a "lock happened" message on B  →  mint with no real lock  →  drain

04 · Edge cases & risks

• Ronin ($625M, 2022) — five of nine validator keys compromised; the attacker forged withdrawals. The largest DeFi hack ever, and a pure trust-model failure.
• Wormhole ($325M, 2022) — a signature-verification flaw let the attacker mint 120k wETH on Solana with nothing locked behind it.
• Mint-function bugs persist — 2026 bridge exploits keep targeting the same class: forge an off-chain message, mint on the destination with no matching lock. The math is simple; the verification is hard.
• Bridged assets are derivatives — "USDC on chain X" via an unofficial bridge is an IOU on that bridge's solvency; if the bridge is drained, the wrapped token depegs to zero.