Rollup Security During Ethereum Congestion

These Optimistic Rollups post all data to Ethereum, guaranteeing full L1 security but inheriting its fee volatility. Their primary mitigation is sequencer-level transaction ordering, which allows them to decouple user experience from L1 gas wars.

• Key Benefit: Censorship resistance is preserved; users can force transactions via L1 if the sequencer is down or malicious.
• Key Risk: During extreme congestion, finality delays and cost spikes are directly passed to users, creating a poor UX.

Validity (ZK) Rollups like StarkNet and zkSync Era also post data to L1 but rely on complex, compute-heavy proofs. Congestion creates a dual-threat: high data costs and proving queue backlogs.

• Key Benefit: Instant finality for users after a proof is generated, with ~1hr L1 state finalization.
• Key Risk: Provers are not permissionless; a congested L1 can stall proof submission, delaying fund withdrawals and creating centralization pressure on the prover network.

Solutions like StarkEx Validium or Arbitrum Nova use off-chain data availability committees (DACs) or Celestia to slash costs. This is the most aggressive congestion mitigation, but it trades off sovereign security.

• Key Benefit: ~10-100x lower fees that are completely decoupled from Ethereum gas prices.
• Key Risk: Users must trust the DAC or alternative DA layer; if it fails, funds can be frozen. This is a fundamental security downgrade from pure rollups.

The long-term fix isn't just better batching—it's removing the centralized sequencer as a single point of failure and rent extraction. Espresso Systems and Astria are building shared sequencer networks that auction block space.

• Key Benefit: Credible neutrality and MEV redistribution via auctions, preventing a single entity from profiting from L1 congestion.
• Key Risk: Early designs add latency and complexity; adoption depends on rollup teams ceding short-term revenue for decentralization.

During L1 congestion, the 7-day challenge window for optimistic rollups becomes a primary attack surface. Malicious actors can cheaply spam the L1 to delay or censor fraud proofs, potentially allowing invalid state roots to finalize. This is a systemic risk for the ~$20B+ TVL secured by Optimism and Arbitrum style rollups.

• Attack Cost: Spamming L1 during a 1-hour gas spike can cost <$50k to delay a proof.
• Mitigation: Sequencers must maintain high-priority L1 wallets and implement proof pre-confirmations.

A rollup's sequencer can become a single point of failure. If the sequencer's L1 transactions are censored during congestion, the entire chain halts—users cannot force inclusions. This violates the liveness guarantee, a core security property. Projects like Arbitrum with centralized sequencers are most exposed, while Espresso Systems and Astria aim to decentralize this role.

• Failure Mode: Chain halts, not theft.
• Solution Path: Decentralized sequencer sets or EigenLayer-style shared security.

zk-Rollups like zkSync and Starknet post validity proofs, but still rely on L1 for data availability (DA). If transaction data is censored or delayed on L1, the rollup state cannot be reconstructed, breaking trust assumptions. This makes EIP-4844 (blobs) and alternative DA layers like Celestia or EigenDA critical for congestion-proof security.

• Core Risk: State cannot be verified without L1 data.
• Metric: ~100-500 KB of data must be posted per block, regardless of L1 gas price.

The user's ultimate safety net—forcing a transaction via the L1 bridge—fails during congestion. With base fees exceeding 500 gwei, a single forced inclusion can cost $500+, making mass exits economically impossible. This renders the security model theoretical for ordinary users and highlights the need for fast withdrawal liquidity pools like those used by Hop Protocol and Across.

• User Impact: Safety net is priced out.
• Design Imperative: Protocols must integrate native fast withdrawal mechanisms.