CVE-2021-21295

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