Before using any ISO downloaded from a non-Microsoft source, you must verify its integrity using a checksum (SHA-256 or SHA-1). You can do this in Windows PowerShell using the command: Get-FileHash C:\path\to\your\windows8.1.iso Windows 8.1 - Microsoft Lifecycle

Microsoft officially ended extended support for Windows 8.1 on January 10, 2023 .

Sites like Archive.org often host community-uploaded copies of original installation media. While safer than random open directories, these files are still not directly from Microsoft and should be verified against official hashes. Verifying ISO Integrity

An "Index of" page is a server-generated list of files in a directory that lacks a default index file (like index.html ). By searching for intitle:"index of" "Windows 8.1" iso , users can sometimes find direct download links from various web servers. However, downloading from these unverified sources is highly discouraged due to security concerns. Official Status of Windows 8.1 (2026)

If you or your organization has an active Visual Studio Subscription, you can still download verified Windows 8.1 ISOs with official SHA-1 hashes for verification.