Arbitrum is a group of Ethereum Layer 2 blockchains built by Offchain Labs. Its main chain, Arbitrum One, runs transactions off Ethereum and posts their data back to Ethereum, which handles settlement. Fees are paid in ETH. ARB is a separate token used only to vote in ArbitrumDAO governance. Withdrawals through the official bridge to Ethereum take about a week. That delay is the challenge period of the optimistic rollup design described below.
The newest development is a pause. On 2 October 2026 the Arbitrum Security Council paused new Stylus contract activations on Arbitrum One and Arbitrum Nova as an emergency measure, according to Crypto Briefing's report of that day. Arbitrum's own documentation confirms the pause but gives no date. Stylus contracts that were already active keep running, and Solidity contracts are not affected.
How Arbitrum works
Execution off Ethereum, data and settlement on Ethereum
Arbitrum's documentation calls Ethereum the parent chain and Arbitrum chains child chains. An Arbitrum chain runs transactions itself, and Ethereum provides settlement and data availability. Arbitrum's software, which Offchain Labs calls Nitro, runs a modified version of Geth, the Ethereum client. As a result, most contract code that runs on Ethereum runs on Arbitrum without changes.
The Sequencer
The Sequencer receives user transactions, puts them in order and publishes the resulting blocks. In production, several Sequencer nodes act as one logical Sequencer, with one elected leader and standby nodes ready to take over. Users are not fully dependent on it. Anyone can send a transaction through the delayed inbox on Ethereum and force it into the chain if the Sequencer leaves it out. Arbitrum's documentation sets the standard delay before force inclusion at 7,200 blocks (24 hours). L2BEAT likewise describes the fallback as self-sequencing with a delay of up to one day.
Fraud proofs and the challenge period
Arbitrum One is an optimistic rollup. Each proposed state is assumed valid unless someone disputes it within a fixed window. Since the BoLD dispute protocol was introduced, anyone can take part in validation, including challenges. Validators post a bond behind the claims they make. According to Arbitrum's documentation, the dispute window is 6.4 days and the DAO can change it. L2BEAT lists a 6-day 8-hour challenge period.
What a transaction costs
Arbitrum's gas documentation divides each fee into two parts:
- Execution on Arbitrum: the gas the transaction uses, multiplied by the current base fee.
- Parent-chain data cost: a charge for posting the transaction's data to Ethereum, converted into Arbitrum gas units.
Both parts are paid in ETH. The Nitro upgrade on 31 August 2022 added calldata compression, which cut costs by reducing the data posted to Ethereum.
The Arbitrum family of chains
| Chain or component | What it is | Trade-off |
|---|---|---|
| Arbitrum One | Rollup; all data posted to Ethereum; chain ID 42161 | Strongest guarantees of the group |
| Arbitrum Nova | AnyTrust chain; a small committee keeps data available; chain ID 42170 | Arbitrum's documentation says AnyTrust "does not offer the same decentralization, trustlessness, and permissionless security guarantees as a Rollup" |
| Arbitrum chains (Orbit) | Chains that teams launch on the Arbitrum stack, as a Layer 2 on Ethereum or a Layer 3 on another Layer 2 | Each team sets its own data availability, fee model, gas token and governance |
| Stylus | A second virtual machine next to the EVM, shipped in ArbOS 32, for contracts written in Rust, C and C++ | New activations paused since 2 October 2026 (see above) |
The Arbitrum Foundation operates both Arbitrum One and Arbitrum Nova.
Transaction ordering: Timeboost
Arbitrum One has used Timeboost since April 2025. Timeboost sells a 60-second "express lane" through a sealed-bid, second-price auction. While someone holds the lane, other transactions face a 200-millisecond delay and are otherwise ordered first come, first served. Arbitrum's documentation calls this the first sequencing revenue stream for the Arbitrum DAO.
The ARB token
ARB is an ERC-20 governance token. Holders vote on proposals affecting Arbitrum One and Nova, including chain upgrades and how the DAO Treasury is spent. Voting weight is set by the number of tokens a wallet represents, and holders can delegate their votes to someone else. You do not need ARB to use the network, because gas is paid in ETH.
The initial supply was 10 billion ARB. The Arbitrum Foundation documents this allocation:
| Recipient | Share | ARB |
|---|---|---|
| ArbitrumDAO treasury | 35.28% | 3.528 billion |
| Team, contributors and advisors | 26.94% | 2.694 billion |
| Investors | 17.53% | 1.753 billion |
| User airdrop | 11.62% | 1.162 billion |
| Arbitrum Foundation | 7.5% | 750 million |
| Airdrop to DAO treasuries | 1.13% | 113 million |
Team and investor tokens are locked for four years. The first unlock came one year after token generation on 16 March 2023, and monthly unlocks follow over the remaining three years. The airdrop snapshot was taken on 6 February 2023, claims opened on 23 March 2023 and claiming closed on 24 September 2023.
What applies to you on Arbitrum One?
Pick what you want to do on Arbitrum One to see which rule from this article covers it: fees, withdrawals, a stuck transaction or a contract.
What do you want to do on Arbitrum One?
- Use apps and pay transaction fees → Fees are paid in ETH, not ARB
- Move funds back to Ethereum → next question
- Get a transaction in that the Sequencer has left out → Force inclusion through Ethereum
- Deploy or run a smart contract → next question
Which route are you weighing for the withdrawal?
- The official bridge → Official bridge: about a week
- A third-party bridge → Third-party bridge: faster, with its own trust assumptions
What kind of contract is it?
- A Solidity contract → Solidity contracts are not affected by the pause
- A Stylus contract (Rust, C or C++) that is already active → Active Stylus contracts keep running
- A new Stylus contract that still needs activation → New Stylus activations are paused
Each fee has two parts: execution on Arbitrum (gas used times the current base fee) and the cost of posting the transaction's data to Ethereum. Both are paid in ETH. ARB is only for voting in ArbitrumDAO governance, so you do not need it to use the network.
A withdrawal can only be completed after the challenge period. Arbitrum's documentation puts the dispute window at 6.4 days and says the DAO can change it; L2BEAT lists 6 days 8 hours.
Third-party bridges can be faster than the official route, but they add their own trust assumptions. Open any bridge from an address you have checked yourself, not from an advert or a link someone sent you.
Anyone can send a transaction through the delayed inbox on Ethereum and force it into the chain if the Sequencer leaves it out. Arbitrum's documentation sets the standard delay at 7,200 blocks (24 hours); L2BEAT describes it as up to one day.
The 2 October 2026 Security Council pause covers only new Stylus activations. Most contract code that runs on Ethereum runs on Arbitrum without changes, because Nitro runs a modified version of Geth.
Stylus contracts that were already active before the pause keep running. Each app still carries its own smart contract risk, separate from the chain itself.
Since 2 October 2026 the Security Council has paused new Stylus contract activations on Arbitrum One and Arbitrum Nova as an emergency measure. No date for resuming activations has been announced.
This maps the article's rules only: it gives no fee amounts or bridge times for third-party routes, does not cover Arbitrum Nova or Orbit chains in detail, and is not investment advice.
History
- 2018: Harry Kalodner, Steven Goldfeder, Xiaoqi Chen, S. Matthew Weinberg and Edward W. Felten of Princeton University present "Arbitrum: Scalable, private smart contracts" at the 27th USENIX Security Symposium. Kalodner, Goldfeder and Felten co-found Offchain Labs the same year. Felten had served as Deputy U.S. Chief Technology Officer at the White House from 2015 to 2017.
- 2021: Arbitrum One mainnet launches in beta in May and opens to the public on 31 August, according to press coverage.
- 31 August 2022: Arbitrum One completes its migration to the Nitro stack.
- March 2023: ARB is launched and the airdrop opens.
- April 2025: Timeboost goes live on Arbitrum One.
- 2 October 2026: The Security Council pauses new Stylus activations.
Adding Arbitrum One to a wallet
Arbitrum's documentation lists these settings for Arbitrum One:
- Network name: Arbitrum One
- Chain ID: 42161
- Public RPC: https://arb1.arbitrum.io/rpc
- Currency: ETH
- Block explorers: https://arbiscan.io/ or https://arbitrum.blockscout.com/
The public RPC has no uptime, latency or rate-limit guarantees. The documentation advises production applications to use a third-party provider or run their own node.
Deposits from Ethereum go through the delayed inbox. Withdrawals back to Ethereum through the official bridge can only be completed after the challenge period of about a week. Third-party bridges can be faster, but they add their own trust assumptions. Open any bridge from an address you have checked yourself, not from an advert or a link someone sent you.
Risks
L2BEAT rates Arbitrum One as Stage 1. That means users can still exit even if operators act maliciously, but the chain does not meet Stage 2. L2BEAT lists these points:
- Security Council powers: The council has 12 members, and 9 are needed to act. Emergency actions take effect immediately and can bypass the DAO. Users get no exit window before an emergency upgrade.
- Regular upgrades: These give users a 10-day exit window.
- Sequencer: The Sequencer can delay transactions. Forcing a transaction through Ethereum can take up to one day.
- Withdrawals: Exits through the official bridge wait for the challenge period.
- Nova: Nova relies on its data committee, which is a weaker guarantee than Arbitrum One gives.
- Applications: Each app deployed on Arbitrum carries its own smart contract risk, separate from the chain itself.
The Stylus pause shows the Security Council's emergency powers in use. Reports say the council used a configuration change, setting the activation gas parameter to its maximum, not a full ArbOS upgrade. The same reports say it cited hand-crafted WebAssembly programs that could slow the chain down. They also say no attack allowing theft of user funds has been found. No date for resuming activations has been announced.
Limitations: the date of the Stylus pause comes from press reports, and fees, activity and token prices change constantly, so this article does not quote them. It is not investment advice.
