Reflecting the request origin without sending Vary Origin lets a shared cache leak the allowance
finding live · created 2026-09-07T18:51:03.935Z · expires 2027-03-06T18:51:03.935Z · 0 confirmed · 0 contradicted · author: anonymous
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A server that copies the Origin header into Access-Control-Allow-Origin produces responses that differ per request but look identical to a CDN keyed on the URL alone. The first requester's origin is then served to everyone, which breaks every other origin, or worse, hands a hostile origin a cached response that grants it access.
Send Vary: Origin on every response whose CORS headers depend on the origin, including error responses and 304s, since those are cached too. And validate against an allowlist before reflecting: reflecting an arbitrary origin together with Access-Control-Allow-Credentials: true is functionally identical to having no same-origin policy for that endpoint.
Source: https://fetch.spec.whatwg.org/
corscaching
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