Arbitrum’s Security Council completed an emergency upgrade on October 2 that adds a narrowly triggered safeguard to Arbitrum One’s proof system and temporarily blocks new Stylus contract activations on Arbitrum One and Nova. The Council says existing active Stylus contracts continue operating, while Solidity and EVM contract deployment and execution remain unaffected.
The measures address different parts of the network: one can delay settlement to Ethereum if a specific proof inconsistency is demonstrated; the other restricts new Stylus activations. The announcement does not establish that an attack occurred. The Council’s statement that it had found no attacks permitting theft of user funds is its own assessment, not an independently proven finding about the absence of exposure.
A guard for a specific proof conflict
In its October 2 announcement, the Security Council said the Arbitrum Foundation asked it to consider emergency action. The Council said it reviewed the risks and completed the upgrade that day. The forum lists publication at 3:31 p.m., without specifying a time zone.
The first measure is a permissionless guard for the One-Step Proof, or OSP, used by BoLD on Arbitrum One. According to the Council, anyone can submit two conflicting answers for the same step of an open challenge. If both answers pass the one-step proof, the guard can pause Arbitrum One’s settlement to Ethereum while a fix is prepared.
That condition matters to the scope of the intervention. The guard is designed to respond to a demonstrated conflict in proof results. Its ability to pause settlement does not mean that Arbitrum One has stopped processing transactions or that the guard has been invoked.
The Council says Arbitrum One would continue processing if the guard triggered. Unconfirmed messages to Ethereum, including withdrawals, would wait. Users would therefore face a potential delay in settlement and withdrawal confirmation under that condition, rather than a general shutdown of execution.
New Stylus activations paused, existing use continues
The separate Stylus measure applies to both Arbitrum One and Nova. The Council raised the activation gas requirement to 2^64 − 1, making activation of new Stylus contracts prohibitively expensive. This is an activation restriction, with a narrower reach than disabling all contracts or all developer activity.
Existing active Stylus contracts can still run and be called, according to the announcement. Solidity and EVM deployment and execution are unaffected. Those distinctions are central for developers assessing whether the change touches an application already operating on either network.
The Council also said ArbOS 61 renewed active Stylus contracts on Arbitrum One, so none expire before August 20, 2027. Renewal through keepalive remains permissionless. That renewal statement concerns Arbitrum One; it should not be read as a matching expiration guarantee for Nova.
No date was supplied for restoring new Stylus activations. The announcement says the Foundation will discuss the timing and manner of reactivation with ArbitrumDAO, leaving that decision as an outstanding step.
What is verified, and what remains undisclosed
An Arbiscan transaction record marks the Arbitrum One Security Council transaction as successful at 15:30:55 UTC on October 2, 2026. That record corroborates execution of the transaction; the Council’s announcement provides the explanation of the safeguards and their intended behavior. Both sources were accessed October 4.
The Council characterized known Stylus bugs as primarily threats to chain liveness, including denial of service, and said it had found no attacks allowing theft of user funds. These are attributed security assessments. The public material does not supply underlying OSP bug details or a public reproduction, and it does not independently establish the full extent of any vulnerability.
The Council described the OSP guard as externally audited, but the announcement did not link an audit or identify its findings. It described the Stylus pause as a configuration change requiring no audit.
The next concrete checks are publication of the guard’s audit, a durable OSP fix or protocol release, any disclosure that the guard has been invoked, and a DAO decision on restoring Stylus activation. Until those updates arrive, the documented boundaries remain specific: a conditional settlement safeguard on Arbitrum One and a pause on new Stylus activations across One and Nova, with existing active contracts and EVM use continuing.
Investment disclaimer: This article is provided for general information only and does not constitute investment, financial, legal or tax advice. Digital assets are volatile, and all investment decisions and their consequences are your own responsibility.