VAST error code
401 — Media file not found
What the spec says
The media file URI returned a 404 or was otherwise unreachable.
What actually causes it
- The asset was deleted or moved on the CDN while the tag continued to reference it.
- A CDN path change that was not reflected in the creative.
- Geo-restriction on the asset, so it is reachable from some markets and not others.
- An expired signed URL.
How to fix it
Request the media file URL directly and check the status code. If it 404s, the asset needs re-uploading or the tag needs updating. If it works for you but fails in market, suspect geo-restriction or a signed URL that has expired.
To see whether a tag would hit this before it runs, paste it into the VAST validator. It reads the XML and reports the conditions that map to this code.
Related codes
Code definitions from the IAB Tech Lab VAST 4.3 specification. Causes and fixes are practitioner guidance, last verified July 2026.