When evaluating a smaller blockchain connected to a larger one, a key question arises:
If an attacker launches a 51% attack on the smaller chain, how much damage can they cause?
This is a practical and critical consideration. Smaller chains often have a much lower market cap compared to major chains. For an attacker, acquiring 51% of the smaller chain’s tokens—or at least 51% of its staked tokens—can be quite feasible, especially if significant bridged assets are held on that chain.
Here’s how different types of chains respond to such attacks:
- If the small chain is an independent L1, the attacker can steal all assets. They can create a block with an illegally altered state, transfer all tokens to themselves, and withdraw these tokens via a bridge. There is no validation from the larger chain.
- If it is a sidechain, the attacker can also steal everything, for the same reason. However, sidechains benefit from slightly better security than standalone L1s because their block headers are published on the main chain (e.g., Ethereum). This means if the main chain rolls back, the sidechain must roll back too. This protects against theft related to a main chain attack but does not prevent a direct 51% attack on the sidechain.
- If the small chain is a rollup, an attacker can delay transactions, potentially forcing users to pay L1 fees, but they cannot steal any assets. This is due to an on-chain mechanism—such as fraud proofs or data availability proofs—that validates whether events involving the attacker’s withdrawals are legitimate.
Let’s look at a few more specialized cases:
- In a plasma chain, attackers can delay transactions and impose L1 fees on users, but again, they cannot steal assets.
- In a validium, attackers might permanently lock all user assets, but still cannot steal them. Validiums, like StarkWare’s ImmutableX, offer an interesting middle ground. They are less decentralized than rollups since operators could potentially deny access and extort users. Still, they are significantly more secure than sidechains and offer similar scalability.
This is what we mean by "shared security." When you hold or transact assets on a smaller chain, are those assets as secure as they would be on the main chain? The answer depends on the architecture:
- With a rollup or plasma, your assets share the security of the main chain.
- On an independent L1 or sidechain, asset security is considerably lower.
- On a validium, security lies somewhere in between.
It’s also worth noting that these relationships are often symmetrical. For instance, holding ETC in a ZK rollup built on Ethereum Classic is safer than holding wrapped ETC on Ethereum—even if the bridge uses a perfect ZK-SNARK verifier. The goal isn’t always to transact on the largest chain. Instead, safety is strongest when your activities and assets reside within the same shared security zone.
A shared security zone refers to a primary chain (like Ethereum) and all other chains (such as rollups) that ultimately derive their security from it.
How 51% Attacks Impact Different Chain Types
Understanding the security models of various chains helps users and developers gauge risk. Below, we break down how each structure holds up under a 51% attack.
Independent Layer 1 Blockchains
An independent L1 operates with its own consensus mechanism and validators. If attackers gain majority control, they can rewrite transaction history, reverse transactions, and drain all assets held on the chain. Since there's no higher-layer security, users and apps are fully exposed.
Sidechains
Sidechains are semi-independent. They process their own transactions but periodically commit checkpoints or block headers to a parent chain (like Ethereum). This offers limited protection: if the parent chain is compromised, the sidechain may roll back. But a direct 51% attack on the sidechain itself can still lead to total loss of funds.
Rollups
Rollups batch transactions off-chain and submit compressed data and proofs to a main chain. Two types dominate:
- Optimistic Rollups: Use fraud proofs to challenge invalid transactions.
- ZK-Rollups: Use zero-knowledge proofs for immediate validity verification.
In both cases, even with a 51% attack on the rollup’s validators, assets can’t be stolen. Attackers might censor or delay transactions, but cryptographic proofs or community-driven challenges ensure ultimate safety.
Plasma and Validiums
- Plasma chains rely on a root chain for dispute resolution. While funds can’t be stolen, service denial is possible.
- Validiums keep data off-chain but use on-chain proofs. This improves scalability but introduces a small risk of data withholding, which could temporarily lock funds.
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The Role of Bridges in Shared Security
Bridges enable asset and data movement between chains, but they also introduce risks. As Vitalik Buterin highlighted in an EF AMA:
"I am optimistic about multi-chain ecosystems but pessimistic about cross-chain applications due to the fundamental security limitations of bridges."
Why? If a blockchain suffers a 51% attack, native assets usually remain safe. Protocol rules prevent theft even during an attack. But when assets are bridged to another chain, they become vulnerable.
For example:
- If you bridge 100 ETH to Solana and Ethereum gets 51% attacked, an attacker could deposit and then roll back the transaction on Ethereum. Your bridged assets on Solana could become irredeemable or lose value.
Thus, holding native assets within their home security zone is safest. This applies to L2s as well—assets on Arbitrum or Optimism are still within Ethereum’s security zone.
Frequently Asked Questions
What is a shared security zone?
A shared security zone consists of a primary blockchain and any secondary chains (like rollups or plasma) that derive their safety from it. All chains within the zone are ultimately protected by the main chain’s consensus mechanism.
Can a 51% attack steal funds from a rollup?
No. Rollups use cryptographic proofs or fraud challenges to ensure that only valid transactions are confirmed. While attackers might delay transactions, they cannot steal user funds.
Are sidechains safer than independent L1s?
Slightly. Because sidechains anchor their block headers to a parent chain, they benefit from some security inheritance. However, they are still vulnerable to direct 51% attacks.
What makes validiums a trade-off?
Validiums offer high scalability and lower fees by storing data off-chain. But users must trust operators to provide data when needed. While funds can’t be stolen, they could be temporarily locked.
Is it safe to use cross-chain bridges?
Bridges introduce additional risk. If the source chain is 51% attacked, bridged assets on the destination chain may lose value or become stuck. For maximum safety, hold assets natively within their security zone.
Do all Layer 2 solutions offer the same security?
No. Security varies by type:
- Rollups and plasmas inherit full L1 security.
- Validiums offer strong security but with minor trust assumptions.
- Sidechains provide limited security and should be used cautiously.
Real-World Implications and Examples
Consider Ethereum’s ecosystem: Ethereum L1, Optimism, Arbitrum, and ZkSync all exist within the same shared security zone. A 51% attack on Ethereum would affect them all, but their internal consistency would be maintained. In contrast, chains like Avalanche or Ethereum Classic operate in separate zones.
Another example: Bitcoin SV (BSV). It’s a proof-of-work chain with large blocks, making validation difficult for users. A 51% attack could lead to invalid blocks being accepted, putting users at risk.
Shared security isn’t just a technical concept—it’s a practical framework for evaluating risk in a multi-chain world. By understanding how different architectures handle attacks, users can make smarter decisions about where to hold and use assets.