Optimism publishes required op-challenger security release v1.9.1

What happened

OP Labs published op-challenger v1.9.1 with security and reliability fixes for chains using permissionless fault proofs and designated it a required upgrade for all users.

What changed

A fixed official challenger version became available for a direct dependency of the fault-proof process that secures challenged withdrawals.

What did not change

The release does not establish an exploit, fund loss, invalid withdrawal, bridge-contract upgrade, or adoption by every live challenger operator.

Confirmed

• op-challenger v1.9.1 was released on April 15, 2026.
• The official release calls the update required for all users.
• The release fixes routing that could send large non-Keccak preimages to an incompatible Keccak-only on-chain path.
• The release also introduces strict uint64 configuration parsing and fixes a shutdown race that could make the challenger hang.

Still open

• The candidate does not establish when OP Mainnet challenger operators adopted v1.9.1.
• It is not established whether any live dispute encountered the affected preimage-routing path before upgrade.
• The record does not identify an exploit, failed challenge, or affected withdrawal.

What it means for an advisor

• Review the Optimism canonical-bridge memo's dependence on permissionless challenger availability and software-version monitoring.
• Verify that relevant operators and service providers tracked the required release; publication alone does not prove adoption.
• Continue normal exposure treatment unless separate primary evidence establishes an impaired challenge or withdrawal path.