Header normalization allows for client to clobber proxy set headers in Puma

Vulnerability Description

Puma is a Ruby/Rack web server built for parallelism. In affected versions clients could clobber values set by intermediate proxies (such as X-Forwarded-For) by providing a underscore version of the same header (X-Forwarded_For). Any users relying on proxy set variables is affected. v6.4.3/v5.6.9 now discards any headers using underscores if the non-underscore version also exists. Effectively, allowing the proxy defined headers to always win. Users are advised to upgrade. Nginx has a underscores_in_headers configuration variable to discard these headers at the proxy level as a mitigation. Any users that are implicitly trusting the proxy defined headers for security should immediately cease doing so until upgraded to the fixed versions.

References

• https://github.com/puma/puma/security/advisories/GHSA-9hf4-67fc-4vf4
• https://nginx.org/en/docs/http/ngx_http_core_module.html#underscores_in_headers