VAST Error 401: Media file not found
The MediaFile URL doesn't resolve — the player got a 404, DNS failure, or dead CDN path when it requested the video.
Error 401 is the simplest media failure: the URI in <MediaFile> points at nothing. The XML was fine, the ad decisioning worked, and then the actual video asset was missing — deleted from the CDN, moved without updating the tag, or referenced with a typo.
It's also the classic symptom of an expired creative: campaigns end, assets get cleaned up, but the tag keeps getting served from a cache or a stale line item.
Why error 401 happens
Asset deleted or expired on the CDN
The campaign ended or storage was cleaned up while a cached or still-trafficked tag keeps referencing the file.
Broken URL in the tag
Truncated paths, unexpanded macros in the file URL, or encoding damage during trafficking produce URLs that never worked.
CDN or DNS misconfiguration
The hostname doesn't resolve, the bucket is private, or a region migration left the path serving 404 in some geographies.
Hotlink protection or geo-blocking
The CDN blocks requests without expected referrers or from certain regions, returning 403/404 to real players while your office test works.
How to fix VAST error 401
- 1
Request the MediaFile URL directly
curl the exact URL from the tag. The HTTP status tells you immediately whether it's missing (404), blocked (403), or a DNS problem.
- 2
Validate the full tag with media download
A validator that downloads every media file catches dead renditions even when one working file masks the problem in casual testing.
- 3
Test from multiple regions
Geo-restricted CDNs pass tests from allowed regions and 404 everywhere else. Test from where the errors report.
- 4
Re-host and re-traffic
If the asset is gone, re-upload it and update the tag — then find why an expired creative was still being served.
Seeing error 401 on a live tag?
Paste the tag URL or XML — the validator resolves the full wrapper chain, validates the schema, downloads every media file, and shows exactly which stage fails.
Validate your VAST tagVAST error 401 — frequently asked questions
The URL works in my browser — why does the player get 401?+
Check for referrer-based hotlink protection, geo-blocking, or user-agent filtering on the CDN. Players present different request headers than your browser, and CDNs sometimes treat them differently.
Can one dead rendition cause 401 if others work?+
Usually the player just picks a working rendition. But if the dead file is the only one matching the player's constraints (bitrate, codec), it becomes the selected file and fails.