CVE-2026-69205: Http4s Ember Transfer-Encoding value parsing (TE.CL / TE.0 request smuggling)

## Summary Ember's HTTP/1.1 header parser matches the `Transfer-Encoding` header value with a case-sensitive substring test (`hValue.contains("chunked")`). RFC 9112 §7 requires transfer-coding names to be compared case-insensitively. A request carrying `Transfer-Encoding: Chunked` (capital C) is therefore not recognised as chunked, and Ember falls back to framing by `Content-Length` (or zero if absent) while a compliant intermediary frames the same bytes by chunked encoding. The two parsers then disagree on where the request body ends, enabling HTTP request smuggling (TE.CL / TE.0). The same line of code admits two further variants: - The substring test misfires on `Transfer-Encoding: notchunked` (an inverse desync — Ember treats it as chunked while a compliant intermediary rejects the unknown coding). - Header field bytes are decoded with the platform-default charset. Under UTF-8 the wire bytes `E2 84 AA` decode to U+212A KELVIN SIGN, which `String.equalsIgnoreCase` Unicode-case-folds to `k`, so `Transfer-Encoding: chun<U+212A>ed` matches `chunked` once the comparison is made case-insensitive without also pinning the decode to ISO-8859-1. ## Impact ### Server Request smuggling when ember-server is an origin behind an intermediary that honours `Transfer-Encoding` case-insensitively per RFC, forwards the header value verbatim, and reuses keep-alive connections to the backend: - Front-end security bypass: the smuggled request reaches paths the intermediary's ACL/auth layer would have blocked, with attacker-chosen method and headers. - Cross-user request hijack: a partial smuggled prefix left in Ember's connection buffer is concatenated with the next victim's request on the same pooled backend connection, exposing its headers (e.g. `Cookie`, `Authorization`) to the attacker. - Cache poisoning: the smuggled response is associated with the next request key in a caching proxy. ### Client ember-client shares the same `HeaderP.parse` on the response path, enabling response smuggling when http4s is used as a gateway. This is less severe: it requires a malicious or compromised upstream rather than an anonymous remote client. ## Preconditions - Unauthenticated remote attacker (server) - ember-server as origin behind a keep-alive intermediary - Intermediary treats `Transfer-Encoding` case-insensitively (per RFC) and forwards the header value without lowercasing it - Malicious or compromised upstream (client) ## Workarounds - Intermediary fully buffers and re-encodes request bodies (e.g. nginx with default `proxy_request_buffering on`) - Intermediary normalises the `Transfer-Encoding` value (lowercases the token) before forwarding - Disable backend keep-alive between the intermediary and Ember