GHSA-pf96-p4fj-6566

### Summary

HTTPX2 can automatically add a `Content-Length` header to a request that already contains a caller-supplied `Transfer-Encoding` header. The resulting HTTP/1.1 request contains both framing headers, which can create an ambiguous message boundary and enable request smuggling or connection desynchronization when processed by intermediaries that disagree about which header takes precedence.

### Details

When a request body has a known size, HTTPX2's content encoder returns a default `Content-Length`. `Request._prepare()` applies each default header with `setdefault()`, which only checks whether that same header is already present. It does not check whether the mutually exclusive `Transfer-Encoding` header is present.

For example:

```python import httpx2

request = httpx2.Request( "POST", "http://example.com/", headers={"Transfer-Encoding": "chunked"}, content=b"test 123", )

print(request.headers) ```

The request contains both:

```text Transfer-Encoding: chunked Content-Length: 8 ```

On an HTTP/1.1 connection, the body is serialized using chunked transfer coding while both headers are sent on the wire. This violates HTTP message-framing requirements. Fixed-size byte, JSON, form, and known-length multipart bodies can reach the affected path.

Streaming bodies with an explicit `Content-Length` are not affected in current HTTPX2 releases because the automatically generated `Transfer-Encoding` is already suppressed in that direction.

### Impact

An attacker may be able to use the conflicting framing headers as a request-smuggling or desynchronization primitive. Exploitation requires an application to pass attacker-controlled request framing headers and associated body data to HTTPX2, use HTTP/1.1, and communicate through a proxy or origin that accepts conflicting headers and interprets them differently from another hop.

Depending on the downstream infrastructure, successful exploitation could interfere with requests sharing a persistent connection, bypass front-end routing or authorization decisions, or poison responses or caches. Applications that do not forward attacker-controlled `Transfer-Encoding` headers are not directly exposed.

### Mitigation

Upgrade to HTTPX2 `2.11.0` or later. Patched versions treat `Content-Length` and `Transfer-Encoding` as mutually exclusive when applying automatically generated request headers.

If upgrading is not immediately possible, remove `Transfer-Encoding` and other hop-by-hop framing headers from untrusted input before constructing outbound requests. Applications acting as proxies should derive outbound framing from the body rather than forwarding inbound `Content-Length` or `Transfer-Encoding` headers.