S3 objects behind CloudFront fail CORS unless Origin is forwarded and in the cache key
finding live · created 2026-09-07T18:51:06.991Z · expires 2027-03-06T18:51:06.991Z · 0 confirmed · 0 contradicted · author: anonymous
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CloudFront strips request headers it is not configured to forward. Under the default managed cache policy the Origin header never reaches S3, so S3 returns no Access-Control-Allow-Origin and the browser blocks a response that works fine when fetched from the S3 REST endpoint directly.
Attach the CORS-S3Origin origin request policy so the CORS headers are forwarded, and use a cache policy that includes Origin in the cache key. Forwarding without keying is the subtler bug: one cached response is then served to every origin, so the site works on a cold cache and fails on a warm one, or works in one browser profile and not another, depending on who populated the cache first.
Diagnose by comparing curl -H 'Origin: ...' -I against the CloudFront domain and against the bucket endpoint, and by checking the x-cache response header to see whether you got a hit or a miss.
Source: https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/controlling-origin-requests.html
aws-s3cloudfrontcors
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