@fastify/reply-from v12.6.1 and earlier and @fastify/http-proxy v11.4.3 and earlier process the client's Connection header after the proxy has added its own headers via rewriteRequestHeaders. This allows attackers to retroactively strip proxy-added headers from upstream requests by listing them in the Connection header value. Any header added by the proxy for routing, access control, or security purposes can be selectively removed by a client. @fastify/http-proxy is also affected as it delegates to @fastify/reply-from.
Upgrade to @fastify/reply-from v12.6.2 or @fastify/http-proxy v11.4.4 or later.
Affected: npm: @fastify/reply-from, npm: @fastify/http-proxy
### Summary
`@fastify/reply-from` and `@fastify/http-proxy` process the client's `Connection` header after the proxy has added its own headers via `rewriteRequestHeaders`. This allows attackers to retroactively strip proxy-added headers (like access control or identification headers) from upstream requests by listing them in the `Connection` header value. This affects applications using these plugins with custom header injection for routing, access control, or security purposes.
### Details
The vulnerability exists in `@fastify/reply-from/lib/request.js` at lines 128-136 (HTTP/1.1 handler) and lines 191-200 (undici handler). The processing flow is:
1. Client headers are copied including the `connection` header (`@fastify/reply-from/index.js` line 91) 2. The proxy adds custom headers via `rewriteRequestHeaders` (line 151) 3. During request construction, the transport handlers read the client's `Connection` header and strip any headers listed in it 4. This stripping happens after `rewriteRequestHeaders`, allowing clients to target proxy-added headers for removal
RFC 7230 Section 6.1 Connection header processing is intended for proxies to strip hop-by-hop headers from incoming requests before adding their own headers. The current implementation reverses this order, processing the client's Connection header after the proxy has already modified the header set.
The call chain: 1. `@fastify/reply-from/index.js` line 91: `headers = { ...req.headers }` — copies ALL client heade…
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