The cause of the Blue Archive bug that left players bewildered by the sudden appearance of countless 'Koyuki' characters has been identified as a hacking incident.

On September 2, NEXON posted a detailed notice on the official Blue Archive forum regarding the measures taken after the emergency maintenance on August 31. This maintenance was conducted to resolve multiple issues, starting with an anomaly where in-game characters 'Koyuki' and 'Miku' appeared in large numbers within specific content. NEXON identified the situation as a suspected in-game client hack and performed emergency maintenance from 10:22 PM on the 31st to 4:40 AM the following day.

According to NEXON, which analyzed the cause after the maintenance, the incident occurred when an unidentified individual accessed the content delivery network through a specific method and modified certain elements. This led to abnormal banner displays, irregularities within the Cafe content, and the unauthorized display of specific YouTube videos. The mass appearance of Koyuki and Miku, the most prominent symptom of this incident, was also attributed to this cause.

In a notice following the emergency maintenance, NEXON explained, "User accounts, game data, and payment-related information are operated in separate databases and are re-verified on the game server, so they remain unaffected." Furthermore, the company emphasized its full cooperation with investigations, stating, "We have reported the incident to the Korea Internet & Security Agency (KISA) and are currently conducting security measures and investigations into the attacker."

A NEXON official stated, "We identified that abnormal access to a specific network that checks environment settings during client execution caused some content to be displayed arbitrarily. We immediately blocked the path and implemented security enhancements such as dual-control measures. To prevent recurrence, we will fundamentally block unauthorized external access, strengthen post-verification procedures, and conduct regular, high-intensity security audits to reinforce our systems," promising improvements in game operations.

Meanwhile, NEXON plans to provide information regarding unreceived rewards for users who purchased paid content, such as the Monthly Pyroxene Package, through a separate notice in the future.