An international bank recently discovered a security breach in its transaction processing system. The breach involved a sophisticated malware that not only bypassed the standard antivirus software but also remained undetected by the intrusion detection systems for months. The malware was programmed to intermittently alter transaction values and transfer small amounts to a foreign account, making detection challenging due to the subtlety of its actions. After a thorough investigation, cybersecurity experts identified the nature of this malware. Which of the following best describes the type of malware used in this breach?