Security Council Emergency Action – 2/10/2026

Security Council
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The Arbitrum Foundation notified the Security Council on the need to perform a potential Emergency action. Following a review of the details and known risks, the Security Council decided to execute the upgrade and it was completed at 11:30 EST on 2nd October 2026.

This report provides an overview of the risks that required the Security Council Action:

  • Add a permissionless guard for the One-Step Proof of BoLD on Arbitrum One

  • Temporarily pause activation of new Stylus contracts on Arbitrum One & Nova

Key point: To date, known bugs for Stylus primarily impact chain liveness (e.g., denial of service attacks) and do not put any user funds at risk.

Why was this action taken

Let’s take this opportunity to explain why action was taken on two separate issues concerning BoLD and Stylus.

Installing the guard for One-Step Proof of BoLD. Given the rise of AI-assisted attacks, we have implemented a precautionary approach that allows anyone to pause settlement of Arbitrum One on Ethereum if there are at least two conflicting answers that pass at the same level in which only one should be valid and pass. This is to halt a class of publicly verifiable and detectable attacks that exploit a bug in the one-step proof. If halted, Arbitrum One will continue to process as normal except that messages from Arbitrum One to Ethereum, such as withdrawals, that are not yet confirmed will have to wait. This provides time for the Security Council to deploy a fix for the bug and resume the rollup.

Preventing activation of new Stylus contracts. AI-assisted tooling has led to an increasing number of sophisticated attacks on hand-crafted WASM programs that do not use the standard Stylus compiler toolchain. These programs are specifically designed to cause unexpected behaviors such as degrading the chain’s performance for all other users. To date, no attacks have been discovered that permits theft of user funds.

New ArbOS releases have patched several reported bugs and known attacks over the past few months, but out of an abundance of caution, we have decided to temporarily pause new activations of Stylus contracts. This was executed as an emergency action to prevent the deployment of new hand-crafted WASM programs that are not yet patched by the current version of ArbOS on the Arbitrum network.

The Arbitrum Foundation has informed the Security Council that they will work with the ArbitrumDAO to discuss next steps on the timeline and manner for enabling activation of new Stylus contracts. The Arbitrum community remains confident in the value of Stylus and want to preserve these use cases while taking preventative measures against Stylus-based attack vectors as AI-assisted attack tooling continues to evolve.

Impact on builders and users

  • No new activations. New Stylus contracts cannot be activated during the pause including new versions of existing applications that require activation.

  • Existing contracts are not paused. Already-active Stylus contracts continue to execute until expiration. This pause does not deactivate them.

    • The ArbOS 61 upgrade renewed all active contracts on Arbitrum One, so no currently active contract expires before August 20, 2027.

    • Renewal via keepalive stays permissionless and is unaffected by the pause.

    • Reactivating an expired contract, or one that needs to be updated after a Stylus version change, is paused.

    • Using/calling any activated Stylus contract stays permissionless for all users and developers.

  • No impact on EVM. Solidity (EVM) contract deployment and execution are unaffected by this Security Council action.

Security Council Actions

There are two actions bundled into a single payload:

  • One-Step Proof Guard. The L1UpgradeExecutor gives a new pause contract (PauseExecutor) the ability to pause and nothing else. The new pause contract grants the guard (OspSoundnessGuard) PAUSER_ROLE to use it. Anyone can show the guard two conflicting answers to the same step of an open challenge. If the one-step proof accepts both, the guard has the pause contract put Arbitrum One’s settlement to Ethereum on hold.

  • Stylus deactivation. The Security Council called ArbOwner.setWasmActivationGas(2^64 - 1) to raise the Stylus activation gas requirement on Arbitrum One and Nova. This makes activation prohibitively expensive and effectively disables new activations. This is a configuration change and does not require an ArbOS upgrade.

The Security Council signed the following transactions to update the configuration:

An external audit was conducted for the OSP Guard contracts. No audit is required for the Stylus deactivation as it is only updating a single parameter in the ArbOS configuration.