Why Cross-Chain MEV Bridges Threaten Rollup Security

Cross-chain MEV bridges like Across and Stargate are not neutral infrastructure. They function as intent-based order flow auctions, routing user transactions to the sequencer offering the best fee. This outsources the critical sequencer selection process to a third-party economic game.

The security model breaks when the bridge's profit motive overrides the rollup's liveness. A bridge like Across could permanently route all transactions to a single, potentially malicious sequencer if it maximizes extractable value, violating decentralization assumptions baked into optimistic and zk-rollup designs.

This is not theoretical. The $200M Wormhole exploit demonstrated how bridge logic is a high-value attack surface. A cross-chain MEV bridge controlling sequencer access creates a single point of failure more dangerous than a stolen key; it enables silent, persistent control over transaction ordering and censorship.

Cross-chain MEV bridges like Across and Stargate create a new attack surface. Their fast, low-latency finality enables atomic arbitrage between L1 and L2, which directly conflicts with the fraud proof window of optimistic rollups like Arbitrum and Optimism.

The security model breaks when value exits a rollup before its state is final. Attackers can exploit this by executing a profitable cross-chain arbitrage on a fraudulent L2 state, then using a fast bridge to withdraw funds to L1 before the fraud proof can invalidate the transaction.

This is not theoretical. The 2022 Nomad bridge hack demonstrated how a flawed state root commitment enabled the theft of $190M. While not MEV-driven, it validated the core vulnerability: bridges that trust optimistic state roots are exposed to the same liveness assumptions as the rollup's fraud proofs.

The consequence is a security subsidy. Rollup sequencers currently capture most on-chain MEV, which funds their operational costs and security. Cross-chain MEV bridges externalize this value, creating a classic tragedy of the commons where extracted value does not secure the chain it exploits.

Sequencer Centralization Pressure: Cross-chain MEV bridges like Across and Stargate monetize transaction ordering across chains. This creates a centralizing force where the most profitable sequencer is the one with exclusive access to this cross-chain flow, undermining the L2's decentralization premise.

State Corruption Incentive: A sequencer capturing this MEV has a direct financial motive to censor or reorder transactions before finalizing the L2 state. This is a step beyond passive extraction; it's an active attack on state correctness for profit.

The Validator Dilemma: In optimistic rollups, the challenge period is the last line of defense. A malicious sequencer with cross-chain MEV revenue can outbid honest validators in the L1 auction to reorg the rollup's anchor, making fraud proofs economically unviable.

Evidence in Action: The proposer-builder separation (PBS) model on Ethereum exists to mitigate this. Rollups without PBS equivalents, like many Arbitrum or Optimism sequencer sets, are exposed. The value flow from a bridge like LayerZero can exceed the sequencer's honest operating profit.

Security models are misaligned. A bridge like Across or Stargate secures asset transfers, not the integrity of the rollup's state. Their validators have zero visibility into the L2's execution environment, making them incapable of detecting or preventing malicious sequencing.

Economic security is insufficient. A bridge's multi-sig or fraud-proof system is a separate, weaker security perimeter. An attacker who compromises the rollup's sequencer can extract value far exceeding the bridge's bond, making slashing irrelevant.

This creates a systemic backdoor. A compromised rollup sequencer can drain funds via the bridge itself, using it as a sanctioned exit for stolen assets. This turns infrastructure like LayerZero into an attack vector, not a defense.

Evidence: The Wormhole hack exploited a bridge's off-chain validator set, not the underlying chains. A rollup sequencer with similar centralized components presents an identical single point of failure for cross-chain liquidity.

Cross-chain MEV bridges like Across and Stargate introduce a systemic risk by routing value through external, often less secure, settlement layers. This creates a single point of failure where an attack on the bridging chain compromises the security of the bridged rollup's assets.

Economic security is exported. A rollup's security depends on its L1, but a bridge moves finality to its own validators. An attacker who compromises the bridge's consensus can censor or steal funds moving to the rollup, bypassing the rollup's own fraud proofs.

MEV extraction creates perverse incentives. Bridges that batch transactions for MEV, like those used by UniswapX, concentrate liquidity. This makes the bridge a high-value target for validators of chains like BSC or Polygon, whose security budgets are lower than Ethereum's.

Evidence: The Wormhole hack exploited a bridge's signature verification, not Solana's core consensus. This demonstrates that bridge security is the bottleneck, not the underlying rollup or chain it connects.