Is the server still vulnerable to attack if we don't turn off cgi.fix_pathinfo?

Question

The /etc/nginx/sites-available/default file that comes with nginx installation has the following content:

# You may add here your
# server {
#   ...
# }
# statements for each of your virtual hosts to this file

##
# You should look at the following URL's in order to grasp a solid understanding
# of Nginx configuration files in order to fully unleash the power of Nginx.
# http://wiki.nginx.org/Pitfalls
# http://wiki.nginx.org/QuickStart
# http://wiki.nginx.org/Configuration
#
# Generally, you will want to move this file somewhere, and start with a clean
# file but keep this around for reference. Or just disable in sites-enabled.
#
# Please see /usr/share/doc/nginx-doc/examples/ for more detailed examples.
##

server {
    listen 80 default_server;
    listen [::]:80 default_server ipv6only=on;

    root /usr/share/nginx/html;
    index index.html index.htm;

    # Make site accessible from http://localhost/
    server_name localhost;

    location / {
        # First attempt to serve request as file, then
        # as directory, then fall back to displaying a 404.
        try_files $uri $uri/ =404;
        # Uncomment to enable naxsi on this location
        # include /etc/nginx/naxsi.rules
    }

    # Only for nginx-naxsi used with nginx-naxsi-ui : process denied requests
    #location /RequestDenied {
    #   proxy_pass http://127.0.0.1:8080;    
    #}

    #error_page 404 /404.html;

    # redirect server error pages to the static page /50x.html
    #
    #error_page 500 502 503 504 /50x.html;
    #location = /50x.html {
    #   root /usr/share/nginx/html;
    #}

    # pass the PHP scripts to FastCGI server listening on 127.0.0.1:9000
    #
    #location ~ \.php$ {
    #   fastcgi_split_path_info ^(.+\.php)(/.+)$;
    #   # NOTE: You should have "cgi.fix_pathinfo = 0;" in php.ini
    #
    #   # With php5-cgi alone:
    #   fastcgi_pass 127.0.0.1:9000;
    #   # With php5-fpm:
    #   fastcgi_pass unix:/var/run/php5-fpm.sock;
    #   fastcgi_index index.php;
    #   include fastcgi_params;
    #}

    # deny access to .htaccess files, if Apache's document root
    # concurs with nginx's one
    #
    #location ~ /\.ht {
    #   deny all;
    #}
}


# another virtual host using mix of IP-, name-, and port-based configuration
#
#server {
#   listen 8000;
#   listen somename:8080;
#   server_name somename alias another.alias;
#   root html;
#   index index.html index.htm;
#
#   location / {
#       try_files $uri $uri/ =404;
#   }
#}


# HTTPS server
#
#server {
#   listen 443;
#   server_name localhost;
#
#   root html;
#   index index.html index.htm;
#
#   ssl on;
#   ssl_certificate cert.pem;
#   ssl_certificate_key cert.key;
#
#   ssl_session_timeout 5m;
#
#   ssl_protocols SSLv3 TLSv1 TLSv1.1 TLSv1.2;
#   ssl_ciphers "HIGH:!aNULL:!MD5 or HIGH:!aNULL:!MD5:!3DES";
#   ssl_prefer_server_ciphers on;
#
#   location / {
#       try_files $uri $uri/ =404;
#   }
#}

You can see there is a note "You should have "cgi.fix_pathinfo = 0; in php.ini". Above it, we also have this line:

fastcgi_split_path_info ^(.+\.php)(/.+)$;

According to the post Security issue on Nginx, PHP & fastcgi_split_path_info, we can mitigate the path info vulnerability by turning off cgi.fix_pathinfo or making use of the fastcgi_split_path_info.

Since it already makes use of the fastcgi_split_path_info directive to counter such vulnerability, why does it still say we should turn off the cgi.fix_pathinfo? If we don't, will our server be vulnerable to attack?

Answer 1

The Nginx wiki page on PHP-FPM configuration recommends turning cgi.fix_pathinfo on. Pay attention to the regex used:

fastcgi_split_path_info ^(.+?\.php)(/.*)$;

The page also provides a test where you access different URLs as listed below and check for the correct values of REQUEST_URI, SCRIPT_NAME, PATH_INFO and PHP_SELF.

/test.php
/test.php/
/test.php/foo
/test.php/foo/bar.php
/test.php/foo/bar.php?v=1 

test.php contains just <?php var_export($_SERVER)?>. This helps determine whether your setup is safe. On all URLs, your SCRIPT_NAME should be test.php. In any case, you should not see bar.php there.