An ISO disk-image file is a container format that Windows can mount as a virtual disk. Instead of exposing one file, it can hold many files and folders inside a single attachment, which makes it useful for software distribution and backups.
In cyber security, ISO files matter because they can change how an attachment is handled by the operating system and email tools. Attackers may hide a launcher, a malicious DLL, and a payload inside the image so the user opens a mounted drive rather than a simple document. That can weaken file-provenance checks, evade basic extension filters, and create a cleaner path for DLL sideloading or other staged execution. Defenders should treat unexpected ISO attachments as suspicious, inspect the mounted contents, and watch for unusual child processes or DLL loads after the image is opened.