Why is GMO Internet stock plummeting today?

GMO Internet Group stock plunged 13.6% to ¥3,256 on Monday after a serious cyberattack on the group’s foreign exchange margin trading cloud service — marketed under the "Onamae.com Desktop Cloud" brand — sent investors rushing for the exits.

The breach, which involved unauthorized access to customer server environments and the execution of fraudulent trades in customer accounts, culminated in a ransom demand disclosed on October 24 and a public apology from CEO Masatoshi Kumagai on the evening of October 25, in which he acknowledged that internal engineers had previously identified security vulnerabilities that were not adequately addressed.

The financial fallout from the incident is the core concern driving today’s sell-off. The company announced it would compensate every customer using the affected service with a goodwill payment of 5,000 yen, and separately pledged to cover any trading losses stemming from unauthorized transactions — an open-ended liability that markets are struggling to quantify. The group also formally petitioned the Japanese government to strengthen proactive national cybersecurity defenses.

The broader market environment offered no relief. Japan’s TOPIX fell 0.8% on Monday amid extended declines in Japanese markets.