H4: Disable userdir for 'nobody' to reduce offensive attack vector

exercises/h4.md

@@ -12,6 +12,8 @@ This exercise is a part of [Linux Server Administration (ICT4TN021, spring 2018)
 - [a) **Websites on the server** Make it possible to create home pages with normal user privileges in your virtual server environment.](https://github.com/Fincer/linux-server-setup/blob/master/exercises/h4.md#a-make-it-possible-to-create-home-pages-with-normal-user-privileges-in-your-virtual-server-environment)
 
     - [EXTRA: Deleting Server field from HTTP header by updating Apache source code on Debian-based Linux distributions](https://github.com/Fincer/linux-server-setup/blob/master/exercises/h4.md#extra-deleting-server-field-from-http-header-by-updating-apache-source-code-on-debian-based-linux-distributions)
+    
+    - [EXTRA: Disable userdir module for user nobody to avoid server detection]()
 
 - [b) **Default website** Set user default website to be the default website for Apache in your virtual server environment.](https://github.com/Fincer/linux-server-setup/blob/master/exercises/h4.md#b-set-user-default-website-to-be-the-default-website-for-apache-in-your-virtual-server-environment)
 
@@ -415,6 +417,28 @@ Header unset ETag
 
 More about HTTP header syntax in [Wikipedia](https://en.wikipedia.org/wiki/List_of_HTTP_header_fields). More articles in [ETag](https://en.wikipedia.org/wiki/HTTP_ETag), [Vary: Accept-Encoding](https://blog.stackpath.com/accept-encoding-vary-important), etc.
 
+### EXTRA: Disable userdir module for user nobody to avoid server detection
+
+It is recommended to set `UserDir disabled nobody` in `/etc/apache2/mods-enabled/userdir.conf` file as Metasploit offensive scanning method `scanner/http/dir_scanner` can detect existence of URL/folder path `<myserver:80>/~nobody`. Minimize attack vector, and just disable the userdir module for user `nobody` on the server as follows:
+
+```
+<IfModule mod_userdir.c>
+        UserDir public_html
+        UserDir disabled root
+        UserDir disabled nobody
+...
+```
+
+The following demonstration is the view of Metasploit Framework console, using HTTP `dir_scanner` against a server:
+
+```
+msf auxiliary(scanner/http/dir_scanner) > run
+
+[*] Detecting error code
+[*] Using code '404' as not found for AAA.BBB.XXX.CCC
+[+] Found http://AAA.BBB.XXX.CCC:80/~nobody/ 403 (AAA.BBB.XXX.CCC)
+```
+
 **b)** Set user default website to be the default website for Apache in your virtual server environment.
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