The correct next action is tosubmit the file for sandboxing. In professional security operations and threat hunting workflows, sandboxing is the most appropriate step when a file originates from an untrusted source and hash-based reputation checks return anunknownresult. An unknown hash means the file has not yet been classified as benign or malicious by threat intelligence databases, which is common with newly created malware or targeted attacks.

Sandboxing allows the security team to performdynamic analysisby executing the file in an isolated, controlled environment. This process observes runtime behaviors such as process creation, registry modification, network communications, command-and-control callbacks, file system changes, and exploit attempts. These behaviors provide high-fidelity indicators that static analysis or hash lookups cannot reveal.

Option B, reviewing the directory path, is useful for contextual awareness but does not determine whether the file is malicious. Option C, running a full malware scan, is premature; modern malware often evades signature-based scans, especially when the file is previously unknown. Option D, investigating the reputation of the website, is a supporting activity but does not assess the actual behavior or payload of the downloaded file.

From a threat hunting and incident response standpoint, sandboxing bridges the gap betweendetection and confirmation. If the sandbox analysis confirms malicious behavior, the team can escalate to containment actions such as isolating the endpoint, blocking hashes and domains, and performing scope analysis to identify other affected systems. Additionally, sandbox results can be used to create new SIEM detections and EDR behavioral rules, strengthening future defenses.

This approach aligns with professional best practices:unknown file + untrusted source = dynamic analysis first. It ensures accurate classification while minimizing unnecessary disruption to the user or environment.