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Possible request smuggling in HTTP/2 due missing validation

Moderate
normanmaurer published GHSA-wm47-8v5p-wjpj Mar 9, 2021

Package

maven io.netty:netty-codec-http2 (maven)

Affected versions

< 4.1.60.Final

Patched versions

4.1.60.Final

Description

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: Netflix/zuul#980

CVE ID

CVE-2021-21295

Credits