Caddy is an extensible server platform that uses TLS by default. Prior to 2.11.5, the reverse proxy's finalizeResponse method in modules/caddyhttp/reverseproxy/reverseproxy.go handles an upstream 101 Switching Protocols response before running removeConnectionHeaders and the hopHeaders deletion loop used for ordinary responses. The early return invokes handleUpgradeResponse, which copies the upstream headers to the client and normalizes only WebSocket-specific header names. An attacker who controls or compromises a proxied WebSocket upstream can therefore cause hop-by-hop headers such as Alt-Svc, Keep-Alive, and Proxy-Authenticate, as well as headers named by Connection, to cross the proxy boundary. For HTTP/2 or HTTP/3 extended CONNECT, the upstream 101 response may be represented to the client as an ordinary successful response, increasing the chance that leaked headers are processed. This issue is fixed in version 2.11.5.

Affected: Go: github.com/caddyserver/caddy/v2

### AI Disclosure This vulnerability was discovered via deepseek-v4-pro[1m] for finding potential vulnerabilities / inconsistencies in the code base. I made the AI check if Caddy is compliant with various RFCs, incl. 7230. I also used it to help write the PoC, since it was honestly much faster than writing it by hand. I also let it write parts of this advisory text for brevity's sake, but had to rewrite half of it by hand.

### Summary Caddy's reverse proxy fails to strip RFC 7230 hop-by-hop headers from upstream responses when handling WebSocket upgrades (HTTP 101 Switching Protocols). Headers including `Alt-Svc`, `Keep-Alive`, `Proxy-Authenticate`, `Transfer-Encoding`, or `Trailer`, which Caddy actually correctly strips for normal HTTP responses, pass through unfiltered to the client during protocol upgrades. An upstream that sets these headers (intentionally or otherwise) can inject them into client responses, enabling connection hijacking via `Alt-Svc`, cookie injection via `Set-Cookie`, and proxy authentication phishing.

The normal response path in `finalizeResponse` correctly strips all hop-by-hop headers before forwarding, but the 101 code path returns early, skipping the stripping entirely.

### Details

In `modules/caddyhttp/reverseproxy/reverseproxy.go`, the `finalizeResponse` method (line 1178) handles upstream responses. For all normal responses, it strips hop-by-hop headers before copying to the client:

```go // reverseproxy.go:1194-1198 removeConnectionHead…

Affected: Ubuntu:Pro:24.04:LTS: caddy, Ubuntu:26.10: caddy, Ubuntu:Pro:26.04:LTS: caddy

Caddy is an extensible server platform that uses TLS by default. Prior to 2.11.5, the reverse proxy's finalizeResponse method in modules/caddyhttp/reverseproxy/reverseproxy.go handles an upstream 101 Switching Protocols response before running removeConnectionHeaders and the hopHeaders deletion loop used for ordinary responses. The early return invokes handleUpgradeResponse, which copies the upstream headers to the client and normalizes only WebSocket-specific header names. An attacker who controls or compromises a proxied WebSocket upstream can therefore cause hop-by-hop headers such as Alt-Svc, Keep-Alive, and Proxy-Authenticate, as well as headers named by Connection, to cross the proxy boundary. For HTTP/2 or HTTP/3 extended CONNECT, the upstream 101 response may be represented to the client as an ordinary successful response, increasing the chance that leaked headers are processed. This issue is fixed in version 2.11.5.