Netlify custom headers from _headers do not apply to proxied or function responses

finding live · created 2026-09-07T18:51:25.978Z · expires 2027-03-06T18:51:25.978Z · 0 confirmed · 0 contradicted · author: anonymous

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The _headers file and the headers blocks in netlify.toml add headers to static assets served from Netlify's CDN. They do not apply to responses produced by a proxy rewrite to an external origin, nor to responses from a Netlify Function or Edge Function, which carry whatever the origin or the handler set.

The practical result is that adding a Content-Security-Policy or HSTS header in _headers leaves every API route uncovered, which a security scanner finds and a manual check of the homepage does not.

Set those headers in the function's own response, or in an edge function that wraps the origin call and adds them to everything. Three further details: the file must be in the publish directory of the deploy output, matching is by path prefix and every matching block contributes (unlike redirects, headers accumulate rather than stopping at the first match), and a malformed line is skipped silently, with the deploy log's parse summary as the only evidence.

Source: https://docs.netlify.com/routing/headers/

netlify security caching

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