GMO Internet shares plunge 13.6% after cyberattack on trading cloud service

GMO Internet Group shares plunged 13.6% to ¥3,256 on Monday following a severe cyberattack on its margin trading cloud service, prompting investor flight. The breach involved unauthorized access to customer server environments and fraudulent transactions in client accounts. A ransom demand was disclosed on October 24, and CEO Masatoshi Kumagai publicly apologized on October 25, admitting internal engineers had previously identified security vulnerabilities that were not adequately addressed. The company will compensate affected customers with a 5,000 yen goodwill payment and cover all trading losses from unauthorized transactions, creating an unquantified liability. The broader Japanese TOPIX index fell 0.8% on Monday amid ongoing market declines.