CVE-2021-21295

Netty is an open-source, asynchronous event-driven network application framework
for rapid development of maintainable high performance protocol servers &
clients. In Netty (io.netty:netty-codec-http2) before version 4.1.60.Final there
is a vulnerability that enables request smuggling. 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. For an example attack
refer to the linked GitHub Advisory. 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, and
these HTTP/1.1 objects are forwarded to another remote peer. This has been
patched in 4.1.60.Final As a workaround, the user can do the validation by
themselves by implementing a custom `ChannelInboundHandler` that is put in the
`ChannelPipeline` behind `Http2StreamFrameToHttpObjectCodec`.

References:
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2021-21295
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-21295
https://github.com/Netflix/zuul/pull/980
https://github.com/netty/netty/commit/89c241e3b1795ff257af4ad6eadc616cb2fb3dc4
https://github.com/netty/netty/security/advisories/GHSA-wm47-8v5p-wjpj