U.S. cybersecurity company F5 fell 12% on Thursday after disclosing a system breach in which a "highly sophisticated nation-state threat actor" gained long-term access to some systems.F5 shares were pacing for the worst day since April 27, 2022, when the stock fell 12.8%.The company disclosed the breach in a Securities and Exchange Commission filing on Wednesday and said the hack affected its BIG-IP product development environment. F5 said the attacker infiltrated files containing some source code and infor ...