Stylus

The Arbitrum Security Council completed an emergency protocol upgrade on Friday, October 2, 2026, that halts activation of new Stylus contracts on Arbitrum One and Nova while installing a permissionless safeguard for the One-Step Proof settlement system. The moves address growing AI-assisted attacks on hand-coded WebAssembly programs and potential proof vulnerabilities, though the council stated no user funds are currently at risk.

• Stylus activation gas requirement raised to 2^64 – 1, making new contract deployments prohibitively expensive on both chains
• Permissionless guard deployed allowing anyone to pause Arbitrum One settlement if conflicting proofs pass simultaneously
• Already-active Stylus contracts continue to run; existing Solidity deployments unaffected by the emergency action

The Security Council announcement described the upgrade completed at 11:30 EST on Friday, October 2, 2026. The two-part emergency action targets distinct attack vectors that have accelerated with AI-assisted tooling: one class exploits bugs in Arbitrum One’s one-step proof settlement mechanism, and another targets hand-crafted WebAssembly code that bypasses standard compiler protections.

Stylus pause: new deployments blocked indefinitely

New Stylus contracts cannot be activated on either Arbitrum One or Nova during the pause, and neither can upgraded versions of existing applications requiring reactivation. The Security Council achieved this by calling ArbOwner.setWasmActivationGas(2^64 – 1), a configuration change that sets the gas cost for activation to the theoretical maximum, making the operation economically infeasible.

The pause is narrowly targeted: users can continue calling already-active Stylus contracts through August 20, 2027. The ArbOS 61 upgrade in prior months renewed all active contracts, so no running Stylus program faces immediate expiration. Renewal via keepalive mechanisms remains permissionless and is unaffected by the pause. Solidity (EVM) contracts face no restrictions.

Known bugs for Stylus primarily impact chain liveness (e.g., denial of service attacks) and do not put any user funds at risk.

Arbitrum Security Council forum post, October 2, 2026

One-Step Proof guard opens settlement pause to permissionless activation

The second component installs a permissionless guard contract (OspSoundnessGuard) that can pause Arbitrum One’s settlement to Ethereum if it detects a specific class of attacks on the one-step proof mechanism.

Anyone holding two conflicting answers to the same proof step that both pass validation can trigger the guard to pause outbound messages, including user withdrawals, while the Security Council deploys a permanent fix.

The guard does not pause normal chain operation. Arbitrum One continues processing transactions; only cross-chain messages to Ethereum are held. This architecture assumes that the one-step proof vulnerability, if exploited, would produce detectable contradictions in the settlement process rather than covert theft.

What the document does not disclose

The announcement does not name any specific incident, test case or proof-of-concept that triggered the emergency action. It also does not state whether the Security Council discovered these vulnerabilities internally, through external reporting, or through observation of active attacks.

The timeline for re-enabling Stylus activation, described only as subject to future discussion with ArbitrumDAO, remains open.

The document confirms an external audit was conducted for the One-Step Proof Guard contracts but does not name the auditor, publish the audit report, or disclose any findings. For the Stylus deactivation, it states no audit was required because only a configuration parameter was modified, not ArbOS itself.

Practical impact: Stylus builders face indefinite freeze

Developers with Stylus applications in production can continue deploying updates that do not require activation (parameter changes, data updates). Those seeking to launch new WebAssembly contracts or update existing ones to new Stylus compiler versions cannot do so until the Security Council and ArbitrumDAO jointly decide to restore activation and the pause is lifted.

This emergency pause does not reverse prior governance decisions but suspends their implementation at the execution layer. No vote is required to lift the pause; the process is governed by future negotiation between the Foundation and the DAO.

The CCS read. The Arbitrum Security Council treated AI-assisted code generation as an imminent enough threat to halt one of its most-promoted features hours after the decision was made. The move signals that Stylus’ marketing appeal, permissionless WebAssembly execution, now conflicts with the security model required to protect it. The indefinite pause, without a lift timeline, is a material setback for Stylus adoption and may force a redesign of how Arbitrum certifies safe contract code before activation.

The Security Council and ArbitrumDAO must jointly determine the process and timeline for restoring Stylus activation. Watch for a governance proposal or Foundation announcement in the coming weeks on audit plans, compiler hardening, or alternative activation mechanisms. The Ethereum transaction and Arbitrum One transaction implementing the guard are on-chain; no further action is required from users or node operators.