A JSE file is an encoded JScript script file used by Windows scripting components. The encoding does not make it harmless; it simply stores script content in a form that can be decoded and executed by Windows Script Host when a user opens it or when another process launches it.
In cyber security, JSE files matter because they are a convenient way to deliver code through an attachment that may look less suspicious than an executable. Attackers use them in phishing to trigger native Windows execution, bypassing simple filters that focus on office documents or common malware types. Defenders should treat unexpected JSE attachments as high risk, block or restrict script execution where possible, and monitor for script hosts starting from mail clients, browsers, or download folders. File type awareness is important: a JSE is not a document, but a script that can begin the next stage of an intrusion.