Sequencer centralization is a feature. It enables low-latency, high-throughput execution by bypassing L1 consensus, which is the core value proposition for users of Arbitrum and Optimism.
security-post-mortems-hacks-and-exploits
Blog
Why Forced Inclusion Failures Could Cripple Your Optimistic Rollup
Forced inclusion is the emergency exit for optimistic rollups. Its failure transforms a temporary sequencer outage into a permanent loss of user funds and protocol sovereignty. This is a first-principles analysis of the critical, under-discussed vulnerability at the heart of L2 security models.
introduction
THE ARCHITECTURAL TRADE-OFF
Introduction: The Sequencer is a Single Point of Failure. That's the Point.
Optimistic rollups centralize transaction ordering for performance, making forced inclusion the critical safety valve when it fails.
Forced inclusion is the kill switch. This mechanism allows users to bypass a censoring or offline sequencer by submitting transactions directly to L1, guaranteeing liveness but at L1 gas costs and latency.
Failure here is catastrophic. If the forced inclusion pathway is blocked—due to contract bugs, high L1 congestion, or economic attacks—the rollup loses its only credible decentralization guarantee, freezing user funds.
Evidence: The 2022 Optimism Bedrock upgrade was primarily architected to harden this forced inclusion mechanism, recognizing it as the systemic risk layer.
key-insights
WHY FORCED INCLUSION FAILURES COULD CRIPPLE YOUR OPTIMISTIC ROLLUP
Executive Summary: Three Uncomfortable Truths
The security of an optimistic rollup depends on a single, often ignored, assumption: that a valid transaction can always be posted to L1. This is a dangerous fallacy.
01
The Problem: The L1 is a Hostile, Unreliable Resource
Your rollup's sequencer competes with every other protocol for L1 block space. During a network surge or targeted spam attack, your forced inclusion transaction can be censored or priced out. This is not a hypothetical; it's a predictable failure mode in a multi-chain, multi-rollup future where L1 blocks are perpetually full.
- Key Risk: A malicious actor can DOS your rollup for the cost of spamming the base layer.
- Key Reality: L1 gas prices are volatile and can spike 10-100x during market events.
10-100x
Gas Spikes
>15s
Censorship Window
02
The Solution: Decentralized Sequencers Are Not Enough
Rotating sequencer duties or using a PoS set (like Arbitrum's planned upgrade) does not solve the L1 inclusion problem. It merely distributes the risk. A decentralized sequencer pool still relies on a single member to successfully post the forced inclusion tx, creating a single point of failure at the L1 gateway.
- Key Limitation: Decentralization at L2 does not guarantee L1 liveness.
- Key Need: The solution must be protocol-level, not just sequencer-level.
1
L1 Gateway
N/A
L2 Redundancy
03
The Uncomfortable Truth: You Need an L1 Resilience Layer
The only robust fix is to treat L1 inclusion as a critical infrastructure layer that requires its own redundancy and economic security. This means mechanisms like multi-channel broadcasting (sending txs via multiple relays/MEV builders), economic insurance pools to subsidize high-gas bids, or integration with systems like Flashbots SUAVE for censorship-resistant inclusion.
- Key Action: Audit your rollup's forced inclusion path as rigorously as your fraud proof system.
- Key Metric: Measure and budget for the worst-case L1 gas cost to guarantee liveness.
>1000 ETH
Insurance Pool
3+
Broadcast Paths
thesis-statement
THE L1-L2 CONTRACT
Core Thesis: Forced Inclusion is the Liveness Guarantee, Not an Optional Feature
Forced inclusion is the non-negotiable mechanism that prevents optimistic rollups from being held hostage by their sequencers.
Forced inclusion is a liveness guarantee. It is the user's right to bypass a malicious or censoring sequencer by submitting a transaction directly to the L1 rollup contract. Without this, the sequencer becomes a centralized point of failure.
The sequencer is a performance feature, not a security primitive. Users trust it for speed and low fees, but the system's liveness depends on the L1. This is the core architectural trade-off of optimistic rollups like Arbitrum and Optimism.
Failure means a frozen rollup. If the forced inclusion mechanism is broken or too expensive, users cannot exit or interact with their assets. This creates systemic risk that protocols like Aave and Uniswap cannot mitigate on L2 alone.
Evidence: The cost of forced inclusion dictates censorship resistance. If an L1 gas spike makes forced txs prohibitively expensive, the rollup's liveness guarantee evaporates. This is a quantifiable failure mode for every optimistic rollup.
deep-dive
THE L1-L2 LIFELINE
Deep Dive: How Forced Inclusion Works (And How It Breaks)
Forced inclusion is the critical mechanism that ensures user withdrawals from an optimistic rollup cannot be censored by its sequencer.
Forced inclusion is a censorship bypass. It allows users to submit transactions directly to the L1 rollup contract if the sequencer ignores them. This guarantees L2 state finality and the ability to withdraw assets, making it the user's ultimate escape hatch.
The mechanism relies on L1 gas economics. Users must post a message with sufficient gas to cover L1 validation and future L2 execution. High L1 congestion, as seen during NFT mints or meme coin frenzies, makes this prohibitively expensive, breaking the guarantee.
Sequencer failure creates a systemic risk. If the sole sequencer (like in early Arbitrum or Optimism) goes offline, thousands of users must spam the L1 contract simultaneously. This triggers a gas auction that prices out ordinary users, creating a de facto withdrawal freeze.
Proof systems like zkSync and StarkNet mitigate this. Their validity proofs allow direct state verification on L1 without a dispute window. Users can prove ownership and withdraw via the L1 contract even if the sequencer is malicious or dead, a stronger guarantee than optimistic models.
FORCED INCLUSION
Failure Mode Analysis: Comparing Rollup Implementations
Comparison of how major rollup stacks handle the critical failure mode where a sequencer censors or goes offline, preventing user withdrawals.
| Failure Mode / Feature | Arbitrum Nitro | Optimism Bedrock | Base (OP Stack) | Polygon zkEVM |
|---|---|---|---|---|
Forced Inclusion Mechanism | AnyTrust L1 escape hatch | L1 > L2 transaction injection | L1 > L2 transaction injection | L1 forced batch via sequenced batch data |
Time to Force Tx (Sequencer Down) | < 24 hours (via AnyTrust) | ~1 week (Dispute Time) | ~1 week (Dispute Time) | < 10 minutes |
User Action Required | Submit Merkle proof to L1 contract | Submit L1 tx to | Submit L1 tx to | Submit L1 tx to |
Cost to User (Est. Gas) | ~500k-800k gas | ~200k-300k gas | ~200k-300k gas | ~300k-500k gas |
Relies on Active Validator Set | ||||
Censorship Resistance (L1 Finality) | High (via fallback committee) | High (direct L1 call) | High (direct L1 call) | High (direct L1 call) |
Primary Failure Vector | AnyTrust committee corruption | L1 gas price spikes | L1 gas price spikes | Sequencer data withholding |
Real-World Tested |
case-study
FORCED INCLUSION FAILURES
Case Studies: When Theory Meets Reality
Theoretical security models break down under real-world economic pressure. Here's where your rollup's liveness guarantee can fail.
01
The Sequencer Censorship Attack
A malicious or compromised sequencer selectively ignores high-value transactions, creating a de-facto chain halt. Users cannot force their tx onto L1, freezing $1B+ in DeFi positions or halting governance votes.
- Attack Vector: Sequencer exploits its monopoly on ordering.
- Real Risk: Centralized sequencer run by a single entity (common today).
- User Impact: Complete loss of withdrawal and interaction capability.
>24h
Downtime Risk
$1B+
TVL at Risk
02
The L1 Gas Auction DoS
During an L1 congestion event (e.g., major NFT mint), the cost to force-include a transaction spikes. Users must outbid others for L1 block space, making censorship economically rational.
- Trigger: Network-wide event like an airdrop or protocol exploit.
- Cost: Force-inclusion fee can exceed 100x the normal rollup fee.
- Result: Only whales can afford liveness, breaking permissionless guarantees.
100x
Fee Spike
~500 Gwei
L1 Gas Threshold
03
The Data Unavailability Time Bomb
If the sequencer withholds transaction data from L1, the 7-day fraud proof window is meaningless. Validators cannot verify state transitions, making the rollup's security model collapse to a multi-signature bridge.
- Core Failure: Breaks the fundamental security assumption of optimistic rollups.
- Precedent: Mirrors the data availability problem faced by validiums.
- Mitigation: Requires robust, incentivized data publication networks.
7 Days
Window Nullified
0%
Security Fallback
04
Arbitrum's Delayed Inbox & The Time-to-Finality Trap
Arbitrum's design queues forced transactions in a 'Delayed Inbox' for ~24 hours before L1 execution. This creates a critical window where a malicious sequencer can front-run or censor the forced tx after seeing it.
- Architectural Quirk: The delay is a feature, not a bug, for sequencing efficiency.
- Exploit: Sequencer can censor within the delay period with impunity.
- Reality Check: Highlights the gap between 'eventual' and 'practical' liveness.
24h
Delay Window
~0 ETH
Sequencer Cost to Censor
05
Optimism's Permissioned Sequencer & The Regulatory Kill-Switch
The Optimism Foundation runs the sole sequencer, retaining technical and legal ability to censor transactions. Forced inclusion is the only escape hatch, making the entire network's liveness dependent on a single, targetable L1 function.
- Centralization Risk: A legal order or internal policy can trigger censorship.
- Systemic Weakness: All $10B+ of OP Mainnet TVL relies on one contract call.
- Trend: A model being replicated by other 'optimistic' chains.
1
Sequencer
$10B+
TVL Dependent
06
The Economic Solution: Decentralized Sequencer Sets
The only robust fix is to eliminate the trusted sequencer. Networks like Espresso Systems and Astria are building shared sequencer layers that use proof-of-stake and MEV redistribution to align incentives.
- Mechanism: Sequencer rights are auctioned or rotated via PoS.
- Benefit: Censorship requires collusion of a decentralized set, not one entity.
- Future State: The endgame for rollup security, merging with EigenLayer-style restaking.
10+
Active Projects
>100
Node Threshold
counter-argument
THE INCENTIVE MISMATCH
Counter-Argument: "Sequencers Are Incentivized to Be Good"
Sequencer incentives are misaligned with user security, creating systemic risk.
Sequencer profit is decoupled from liveness. A sequencer's revenue comes from transaction ordering and MEV. The cost of censorship is a temporary loss of future fees, which is a weak deterrent compared to immediate extraction opportunities.
Forced inclusion is a governance failure. The mechanism requires a user to submit a fraud proof, which is a public good problem. No single user is incentivized to bear the cost of challenging censorship for the network's benefit.
The L2 is only as strong as its weakest sequencer. During an outage or attack, the fallback to L1 is a single, slow, and expensive forced inclusion transaction. This creates a bottleneck for mass exit, crippling the rollup's usability.
Evidence: The 2023 Arbitrum outage demonstrated this. While the sequencer was down, users had no practical recourse via forced inclusion due to high L1 gas costs and complexity, effectively freezing the chain.
risk-analysis
THE FORCED INCLUSION FLAW
Risk Assessment: The Domino Effect of a Failed Exit
Optimistic rollups rely on a single, time-bound escape hatch. If it jams, the entire system freezes.
01
The 7-Day Liquidity Lock
The core vulnerability. User withdrawals require a 1-week challenge period for fraud proofs. If forced inclusion fails, capital is trapped, not just delayed. This transforms a scaling solution into a systemic solvency risk.
- TVL at Risk: The entire bridge value (e.g., $5B+ on Arbitrum) becomes illiquid.
- Cascading Defaults: Protocols relying on cross-chain composability fail.
7 Days
Standard Delay
∞
If Jammed
02
Sequencer Censorship as a Kill Switch
The sequencer is a centralized point of failure. A malicious or coerced operator can selectively censor forced inclusion transactions, blocking all exits. This isn't a hypothetical; it's a protocol-level backdoor.
- Single Point of Control: Most major rollups (Optimism, Arbitrum) use a single sequencer.
- Regulatory Attack Vector: A legal order can freeze billions without touching L1.
1
Active Sequencer
100%
Censorship Power
03
The L1 Gas Auction Death Spiral
When forced inclusion is the only exit, users must bid for L1 block space. This triggers a gas war, making exit costs prohibitive and creating a negative feedback loop.
- Cost Spikes: Exit fees can exceed 1000+ gwei, pricing out normal users.
- MEV Explosion: Block builders extract maximum value from desperate users, worsening congestion.
1000+ Gwei
Exit Cost
>TVL
Potential Loss
04
Mitigation: Escape Hatches with No Single Point of Failure
The solution is architectural: decentralize the exit path. zk-Rollups (like zkSync, Starknet) have instant, cryptographic exits. Optimistic rollups need fallbacks like validium data availability or multi-sequencer forced inclusion.
- zk-Proofs: Validity proofs enable ~10 min trustless exits.
- Alternative DA: Using EigenDA or Celestia reduces reliance on a single sequencer for data.
~10 min
ZK Exit Time
0
Challenge Delay
future-outlook
THE L1 FALLACY
Future Outlook: The Path to Robust Liveness
Optimistic rollup liveness is not inherited from Ethereum; it is a separate, fragile guarantee that depends on sequencer behavior and forced inclusion mechanisms.
Forced inclusion is not censorship resistance. It is a liveness guarantee of last resort that requires a user to manually submit a transaction to L1 and wait a full challenge period. This fails for time-sensitive DeFi operations or withdrawals.
Sequencer failure is a liveness failure. If the sole sequencer (e.g., a centralized operator) goes offline, the entire rollup halts. Users must rely on the slow, expensive forced inclusion path, crippling user experience and protocol functionality.
Decentralized sequencer sets are mandatory. The future is multi-sequencer or shared sequencer networks like Espresso Systems or Astria. This distributes liveness responsibility and prevents a single point of failure, making forced inclusion a rare edge case.
Proof-of-Stake slashing for liveness. Future designs will slash sequencer bonds for downtime, creating a direct economic incentive for high availability. This transforms liveness from a best-effort promise into a cryptoeconomic guarantee.
Evidence: The 2024 dYdX v4 migration to a Cosmos app-chain was driven by the need for sovereign control over liveness and sequencing, highlighting the current inadequacy of rollup liveness models for high-frequency trading.
takeaways
FORCED INCLUSION FAILURES
Key Takeaways: The Architect's Checklist
Forced inclusion is the failsafe mechanism for Optimistic Rollups; its failure is a silent kill switch for user sovereignty.
01
The Sequencer Censorship Problem
A malicious or malfunctioning sequencer can indefinitely censor a user's L2→L1 withdrawal transaction. This is not a delay; it's a permanent denial of exit.\n- Single Point of Failure: Centralizes trust in the sequencer operator.\n- Capital Lockup Risk: Users cannot access funds on the base layer (L1).
1
Single Point of Failure
∞
Delay Risk
02
The Forced Inclusion Escape Hatch
Users can bypass the sequencer by submitting their transaction directly to the L1 rollup contract. This is the core security guarantee of Optimistic Rollups.\n- Sovereign Exit: Users can force L1 to process their transaction after a 7-day challenge window.\n- High Gas Cost: Requires paying expensive L1 gas, often 10-100x the L2 cost.
7 Days
Delay Window
100x
Cost Multiplier
03
The L1 Gas Auction Failure Mode
If L1 base fees spike (e.g., during a mempool flood or NFT mint), the forced inclusion transaction can be priced out indefinitely. The user's escape hatch is bricked.\n- Economic Censorship: An attacker can sustain high L1 fees to block exits.\n- Protocol-Level Risk: This is a systemic vulnerability, not an implementation bug.
$500+
Potential TX Cost
Unbounded
Attack Duration
04
The Mitigation Playbook: Enshrined Proposers & ZK
Solutions exist but require architectural trade-offs. Enshrined proposers (like Arbitrum's AnyTrust) or a move to ZK-Rollups (like zkSync, StarkNet) eliminate this vector.\n- ZK-Rollups: Validity proofs remove the need for a challenge window and forced inclusion.\n- Proposer-Builder Separation: Decouples transaction ordering from execution, reducing centralization.
0 Days
ZK Exit Delay
Architectural
Trade-off Required
ENQUIRY
Get In Touch
today.
Our experts will offer a free quote and a 30min call to discuss your project.
NDA Protected
Confidentiality24h Response
Time to ValueDirectly to Engineering Team
No Sales Fluff10+
Protocols Shipped
$20M+
TVL Overall