BridgeRadarKnowledge base ยท 3 min read
Who you are trusting when you bridge
Every bridge relies on someone: a custodian, a signer group, a validator network or code. Here is how to tell who, and how to judge the risk.
The question behind every bridge
Moving value between chains always involves someone watching one chain and acting on the other. So the useful question is not "is this bridge safe?" but "who could lose or steal my coins, and what stops them?"
That answer is the trust model. It matters more than the brand, the number of chains supported or how nice the website looks.
Most bridge apps describe this somewhere in their documentation, often under security. This site also lists the trust model for each project it covers.
The main kinds of trust
Most routes fall into one of a few groups.
- A single custodian: one company holds the coins. Coinbase for cbBTC, BitGo for WBTC, Circle for USDC. Simple to understand, but you rely on that company staying honest and solvent.
- A multisig: several keys, for example 5 of 9, must sign to move funds. Better than one key, but only if the keys are truly held by different people.
- A validator set with threshold signatures: a larger group shares one key without anyone holding it whole. Chainflip reports 100 of 150 validators must agree. THORChain node operators put up a bond in RUNE that they lose if they cheat.
- A light client: code on one chain checks the other chain's records directly. Closest to trustless, but rare between very different chains.
- An issuer: Circle's CCTP burns and mints USDC with Circle's own attestation.
What past hacks teach
Bridges have been the target of some of the largest thefts in crypto. Three from 2022 show the main ways things go wrong.
Ronin, about $600 million: the bridge used a 5 of 9 multisig. Attackers got 5 keys, and several of them were effectively controlled by one organisation. Lesson: count who really holds the keys, not how many keys there are.
Wormhole, about $320 million: a bug in the contract on Solana let the attacker skip signature checks and create wrapped ETH with nothing behind it. Lesson: code bugs can bypass a strong signer group.
Nomad, about $190 million: a faulty upgrade made almost any withdrawal look valid. Once one person found it, hundreds copied the transaction. Lesson: audits check code at one moment, and later changes can break it.
Time matters as much as design
The risk you take depends on how long you are exposed. A cross-chain swap through Chainflip or an intent bridge like Across holds your funds for minutes. After that, you own a normal coin on the new chain.
A wrapped token is different. You carry its risk every day you hold it. If the vault behind it is hacked a year from now, your token can lose its value.
So for moving money from A to B, a fast route with short exposure is often the safer choice. For holding an asset on another chain, the trust model of the wrapped token is what you are signing up for.
How to judge a route yourself
You do not need to read code. A few questions go a long way.
- Who holds or signs for the coins, and how many of them would need to collude?
- How long will my money be in the bridge's hands?
- How much value has it secured, and for how long, without incidents?
- Are there published audits and a public bug bounty?
- If I hold a wrapped token, how do I redeem it, and is there enough liquidity to sell?
Spread your risk
Do not keep all your cross-chain assets in tokens from one bridge. For large amounts, split the transfer and consider a route with short exposure. And if a deal looks much better than every other quote, ask why before you take it.
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