Vulnerability Instance

Possible request smuggling in HTTP/2 due missing validation
### Impact
If a Content-Length header is present in the original HTTP/2 request, the field is not validated by `Http2MultiplexHandler` as it is propagated up.  This is fine as long as the request is not proxied through as HTTP/1.1.
If the request comes in as an HTTP/2 stream, gets converted into the HTTP/1.1 domain objects (`HttpRequest`, `HttpContent`, etc.) via `Http2StreamFrameToHttpObjectCodec `and then sent up to the child channel's pipeline and proxied through a remote peer as HTTP/1.1 this may result in request smuggling.  

In a proxy case, users  may assume the content-length is validated somehow, which is not the case.  If the request is forwarded to a backend channel that is a HTTP/1.1 connection, the Content-Length now has meaning and needs to be checked.

An attacker can smuggle requests inside the body as it gets downgraded from HTTP/2 to HTTP/1.1.   A sample attack request looks like:

```
POST / HTTP/2
:authority:: externaldomain.com
Content-Length: 4

asdfGET /evilRedirect HTTP/1.1
Host: internaldomain.com
```

Users are only affected if all of this is `true`:
 * `HTTP2MultiplexCodec` or `Http2FrameCodec` is used
 * `Http2StreamFrameToHttpObjectCodec` is used to convert to HTTP/1.1 objects
 * These  HTTP/1.1 objects are forwarded to another remote peer.
 

### Patches
This has been patched in 4.1.60.Final

### Workarounds
The user can do the validation by themselves by implementing a custom `ChannelInboundHandler` that is put in the `ChannelPipeline` behind `Http2StreamFrameToHttpObjectCodec`.

### References
Related change to workaround the problem: https://github.com/Netflix/zuul/pull/980