Error 0x800f081fSource files not found (.NET / features / DISM)

What it means

0x800f081f is CBS_E_SOURCE_MISSING: the Component-Based Servicing engine needed files to install a feature or repair the component store, and could not find them. It is a source problem, not corruption in the usual sense. Two situations produce it most often:

• Enabling an optional feature such as .NET Framework 3.5 (a "Feature on Demand"), where the payload is not present on disk and must be fetched.
• DISM /RestoreHealth, where the replacement files for a damaged store cannot be obtained from Windows Update.

Supply a source

The reliable fix is to give Windows a source it can read. You need a Windows installation ISO of the exact same version and build as the installed system (check with winver).

Mount the ISO (double-click it; note the drive letter, here E:).

For DISM RestoreHealth:

DISM /Online /Cleanup-Image /RestoreHealth /Source:E:\sources\install.wim:1 /LimitAccess

• :1 is the image index; if your edition is not index 1, list them with DISM /Get-WimInfo /WimFile:E:\sources\install.wim.
• If the media has install.esd, reference that instead.
• /LimitAccess stops DISM from retrying Windows Update.

For .NET Framework 3.5:

DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /Source:E:\sources\sxs /LimitAccess

The sources\sxs folder on the ISO holds the .NET payload.

Other checks

• Match the build exactly. A source from a different Windows version or build produces the same error or introduces new problems.
• Group Policy. In managed environments, the policy "Specify settings for optional component installation and component repair" may point to an unavailable WSUS or network path; the /LimitAccess flag and a local source bypass it.
• After a successful repair, run sfc /scannow to fix any system files that draw on the now-repaired store.

Because feature installation and store repair are handled by Windows Modules Installer, expect some CPU and disk activity while it completes.