A penetration tester discovers malware on a system that disguises itself as legitimate software but...

CEH v13 defines a Trojan as malware that appears as a legitimate, trusted software application while secretly executing malicious actions behind the scenes. Trojans rely on deception rather than replication, often masquerading as tools, utilities, updates, or installers. Once executed, they may install backdoors, steal credentials, exfiltrate data, or modify system settings. The defining characteristic emphasized in CEH is the legitimate-looking façade combined with hidden malicious intent, which matches the scenario perfectly. Spyware (Option B) focuses on monitoring and data collection but does not necessarily disguise itself as legitimate software. Worms (Option C) self-replicate across networks, which is not described here. Rootkits (Option D) hide system compromise but do not necessarily pose as legitimate software. Therefore, the malware described is a Trojan.