Added comments about httpoxy

README.md

@@ -205,11 +205,15 @@ proxy_set_header Connection $proxy_connection;
 proxy_set_header X-Real-IP $remote_addr;
 proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
 proxy_set_header X-Forwarded-Proto $proxy_x_forwarded_proto;
+
+# Mitigate httpoxy attack (see README for details)
 proxy_set_header Proxy "";
 ```
 
 ***NOTE***: If you provide this file it will replace the defaults; you may want to check the .tmpl file to make sure you have all of the needed options.
 
+***NOTE***: The default configuration blocks the `Proxy` HTTP request header from being sent to downstream servers.  This prevents attackers from using the so-called [httpoxy attack](http://httpoxy.org).  There is no legitimate reason for a client to send this header, and there are many vulnerable languages / platforms (`CVE-2016-5385`, `CVE-2016-5386`, `CVE-2016-5387`, `CVE-2016-5388`, `CVE-2016-1000109`, `CVE-2016-1000110`, `CERT-VU#797896`).
+
 #### Proxy-wide
 
 To add settings on a proxy-wide basis, add your configuration file under `/etc/nginx/conf.d` using a name ending in `.conf`.

nginx.tmpl

@@ -51,6 +51,8 @@ proxy_set_header Connection $proxy_connection;
 proxy_set_header X-Real-IP $remote_addr;
 proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
 proxy_set_header X-Forwarded-Proto $proxy_x_forwarded_proto;
+
+# Mitigate httpoxy attack (see README for details)
 proxy_set_header Proxy "";
 {{ end }}