id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	uname	nginx_version
1724	Nginx doesn't sanitize and is inconsistent with multiple, repeated input headers	capile@…		"Ola,

I understand this is not supported by HTTP specification, but nginx behavior on multiple, repeated headers is inconsistent, possibly leading to errors and attack vectors.

If a client sends a repeated HTTP header, internally nginx uses the first incoming header, but forwards the last one to the backend. For example, if a client sends a request like this:

{{{
GET / HTTP/1.1
Host: myserver.com
Host: notmyserver.com
}}}

The myserver.com will be accepted and parsed as the $server_name, but backend servers (tested with php-fpm on fgci_pass) actually receive notmyserver.com.

Still, if I manually set it:
{{{
fastcgi_param HOST_NAME $host;
# or
fastcgi_param HOST_NAME $http_host;
}}}

The correct host (first one) will be sent to backend.

The same odd behavior is found on other repeated http headers (like content-length), possibly leading to other attack vectors.

Tested on docker nginx-alpine:latest


Cheers,

Guilherme Capilé
"	enhancement	closed	minor		nginx-core	1.15.x	fixed	http header		Linux 186b85fdedd6 4.20.6-100.fc28.x86_64 #1 SMP Thu Jan 31 15:51:26 UTC 2019 x86_64 Linux	"nginx version: nginx/1.15.8
built by gcc 8.2.0 (Alpine 8.2.0) 
built with OpenSSL 1.1.1a  20 Nov 2018
TLS SNI support enabled
configure arguments: --prefix=/etc/nginx --sbin-path=/usr/sbin/nginx --modules-path=/usr/lib/nginx/modules --conf-path=/etc/nginx/nginx.conf --error-log-path=/var/log/nginx/error.log --http-log-path=/var/log/nginx/access.log --pid-path=/var/run/nginx.pid --lock-path=/var/run/nginx.lock --http-client-body-temp-path=/var/cache/nginx/client_temp --http-proxy-temp-path=/var/cache/nginx/proxy_temp --http-fastcgi-temp-path=/var/cache/nginx/fastcgi_temp --http-uwsgi-temp-path=/var/cache/nginx/uwsgi_temp --http-scgi-temp-path=/var/cache/nginx/scgi_temp --user=nginx --group=nginx --with-http_ssl_module --with-http_realip_module --with-http_addition_module --with-http_sub_module --with-http_dav_module --with-http_flv_module --with-http_mp4_module --with-http_gunzip_module --with-http_gzip_static_module --with-http_random_index_module --with-http_secure_link_module --with-http_stub_status_module --with-http_auth_request_module --with-http_xslt_module=dynamic --with-http_image_filter_module=dynamic --with-http_geoip_module=dynamic --with-threads --with-stream --with-stream_ssl_module --with-stream_ssl_preread_module --with-stream_realip_module --with-stream_geoip_module=dynamic --with-http_slice_module --with-mail --with-mail_ssl_module --with-compat --with-file-aio --with-http_v2_module"