How to secure your web applications with NGINX

Securing Web Apps with NGINX
http://wallarm.com
Stephan Ilyin, si@wallarm.com

How many of you have
your websites hacked?

Each application
probably has
vulnerabilities

… and someday it
can be hacked

How to harder/secure
your application?

How deal with attacks
to your application?
Chapter 1.

Tip #1. mod_security
can be a good choice

Mod_security rocks!
• Open-source. Finally available for NGINX
• It works! It can be quite efficient in detecting
attacks
• Supports virtual patching
• It is incredible customisable

server {
listen 80;
server_name localhost;
location / {
ModSecurityEnabled on;
ModSecurityConfig modsecurity.conf;
ModSecurityPass @backend;
}
location @backend {
proxy_pass http://localhost:8011;
proxy_read_timeout 180s;
}
}

but mod_security
is not so good!
• Relies on regex
• It is expensive in performance prospective
• If you use default rulesets, you will get a huge number
of false-positives
• Rules tuning is a hard job (difficult to maintain)
• Signatures never covers all the attacks
• REGEXs can be bypassed

What rules look like
# ShellShock virtual patch (Bash attack)
SecRule REQUEST_HEADERS
"^(s*)s+{" "phase:1,deny,id:
1000000,t:urlDecode,status:
400,log,msg:'CVE-2014-6271 - Bash
Attack'"

Good practice (imho)
• Use public ruleset — for monitoring mode
• Craft rules from scratch specifically for your
application — for blocking mode

Using phases is good idea
1. Request headers (REQUEST_HEADERS)
2. Request body (REQUEST_BODY)
3. Response headers (RESPONSE_HEADERS)
4. Response body (RESPONSE_BODY)
5. Logging (LOGGING)

SecRule phase 2
SecRule REQUEST_BODY "/+etc/+passwd"
"t:none,ctl:ResponseBodyAccess=On,msg:'-
IN- PASSWD path detected', phase:
2,pass,log,auditlog,id:'10001',t:urlDeco
de,t:lowercase,severity:1"

SecRule phase 4
SecRule RESPONSE_BODY "root:x:0:0"
"id:'20001',ctl:auditLogParts=+E, msg:'-
OUT- Content of PASSWD detected!',phase:
4,allow,log,auditlog,t:lowercase,severit
y:0"

Handbook by Ivan Ristic. Must read!

Tip #2. Give a chance to
naxsi (another WAF for
NGINX)

Why naxsi?
• NAXSI means Nginx Anti Xss & Sql Injection
(but do more)
• Naxsi doesn't rely on a signature base (regex)!
https://github.com/nbs-system/naxsi

naxsi rules
• Reads a small subset of simple scoring rules
(naxsi_core.rules) containing 99% of known
patterns involved in websites vulnerabilities.
• For example, '<', '|' or 'drop' are not supposed to
be part of a URI.

naxsi setup
http {
include /etc/nginx/naxsi_core.rules;
include /etc/nginx/mime.types;
[...]
}

But! Ruleset is not enough!
• Those patterns may match legitimate queries!
• Therefore, naxsi relies on whitelists to avoid false
positives
• Nxutil tool helps the administrator to create the
appropriate whitelist
• there are pre-generated whitelists for some CMS
(e.g. WordPress)

LearningMode; #Enables learning mode
SecRulesEnabled;
#SecRulesDisabled;
DeniedUrl "/RequestDenied";
## check rules
CheckRule "$SQL >= 8" BLOCK;
CheckRule "$RFI >= 8" BLOCK;
CheckRule "$TRAVERSAL >= 4" BLOCK;
CheckRule "$EVADE >= 4" BLOCK;
CheckRule "$XSS >= 8" BLOCK;

Naxsi pros and cons
Pros:
• Pretty fast!
• Update independent
• Resistant to many waf-bypass techniques
Cons:
• You need to use LearningMode with each
significant code deployment

Tip #3.
Try repsheet
(behaviour based security)

Watch Aaron Bedra’s talk
http://getrepsheet.com/

Tip #4.
And there is also
Wallarm WAF based on NGINX

How deal with DDoS?
Chapter 2.

How to deal with DDoS?
• The traditional technique for self-defense is to read
the HTTP server’s log file, write a pattern for grep
(to catch bot requests), and ban anyone who falls
under it.
• That’s not easy!
• The following are tips on where to place pillows in
advance so it won’t hurt so much when you fall.

Tip #5.
Use test_cookie module

• Usually HTTP-flooding bots are pretty stupid
• Lack HTTP cookie and redirect mechanisms
• Testcookie-nginx works as a quick filter between
the bots and the backend during L7 DDoS attacks,
allowing you to screen out junk requests

Straightforward checks:
• Whether the client can perform HTTP Redirect
• Whether it supports JavaScript
• Whether it supports Flash

In addition to its merits, test_cookies also has its
drawbacks:
• Cuts out all bots (including Googlebot)
• Creates problems for users with Links and w3m browsers
• Does not protect against bots with full-browser-stack
https://github.com/kyprizel/testcookie-nginx-module

Code 444
• The goal of DDoSers is often the most resource-intensive
part of the site.
• A typical example is a search engine. Naturally, it
can be exploited by charging tens of thousands of
queries
• So what can we do?

Code 444
• Temporarily disable this search function
• Nginx supports custom code 444, which allows you
to simply close the connection and give nothing in
response

Code 444
location /search {
return 444;
}

Ban bots’ IPs with ipset
• If you’re sure that location/search requests are
coming only from bots
• Ban bots (getting 444) with a simple shell script
ipset -N ban iphash
tail -f access.log | while read LINE; do
echo “$LINE” | cut -d’”’ -f3 | cut -d’ ‘
-f2 | grep -q 444 && ipset -A ban “${L%%
*}”; done

Tip #8. Banning based
on geographic indicators

Tip #8. Banning based on
geographic indicators
• You can strictly limit certain countries that make
you feel uneasy
• But. It is a bad practice! GeoIP data isn’t
completely accurate!

Tip #8. Banning based on
geographic indicators
• Connect to the nginx GeoIP module
• Display the geographic indicator information on the
access log
• grep the nginx access log and add clients by
geographic indicators to the ban list.

Tip #9. You can use
neural network!

Tip #9. You can use neural
network
• Bad request:
0.0.0.0 - - [20/Dec/2011:20:00:08 +0400] "POST
/forum/index.php HTTP/1.1" 503 107 "http://
www.mozilla-europe.org/" “-"
• Good request:
0.0.0.0 - - [20/Dec/2011:15:00:03 +0400]
"GET /forum/rss.php?topic=347425 HTTP/1.0" 200
1685 "-" "Mozilla/5.0 (Windows; U; Windows NT
5.1; pl; rv:1.9) Gecko/2008052906 Firefox/3.0"

network
Use Machine Learning (ML) to detect bots:
• use neural network (e.g. PyBrain)
• stuffed logs inside
• analyse the requests for classification
between "bad" and "good" clients under DDoS
A good proof-of-concept:
https://github.com/SaveTheRbtz/junk/tree/master/
neural_networks_vs_ddos

network
• Useful to have the access.log before a DDoS
attack, because it lists virtually 100% of your
legitimate clients
• It is an excellent dataset for neural network training

Tip #10.
Keep track of the number
of requests per second

Tip #10. Keep track of the
number of requests per second
• You can estimate this value with the following shell
command
echo $(($(fgrep -c "$(env LC_ALL=C date
--date=@$(($(date +%s)-60)) +%d/%b/%Y:
%H:%M)" “$ACCESS_LOG”)/60))

Tuning the web server
• Of course, you put nginx on silent and hope that
everything will be OK.
• However, things are not always OK.
• So the administrator of any server should devote a
lot of time to tweaking and tuning nginx.

Tip #11.
Limit buffer sizes and
timeouts in NGINX

Every resource has a limit
• Every resource has a limit. In particular, this applies
to memory.
• the size of the header and all buffers need to be
limited to adequate values on the client and on the
server as a whole

Limit buffers
• client_header_buffer_size
• large_client_header_buffers
• client_body_buffer_size
• client_max_body_size

And time_outs
• reset_timeout_connection
• client_header_timeout
• client_body_timeout
• keepalive_timeout
• send_timeout

Question: what are the
correct parameters for the
buffers and timeouts?

• There’s no universal recipe here
• But there is a proven approach you can try

How to limit
buffers and timeout?
1. Mathematically arrange the
minimum parameter value.
2. Launch site test runs.
3. If the site’s full functionality works without a
problem, the parameter is set.
4. If not, increase the parameter value and
go to step 2.

Tip #12.
Limit connections in NGINX
(limit_conn and limit_req)

Ideally you need to test application to
see how many requests it can handle
and set that value in the NGINX
configuration

http {
limit_conn_zone $binary_remote_addr zone=download_c:10m;
limit_req_zone $binary_remote_addr zone=search_r:10m
rate=1r/s;
server {
location /download/ {
limit_conn download_c 1;
..
}
location /search/ {
limit_req zone=search_r burst=5;
..
}
}
}

What to limit?
• It makes sense to set limits for limit_conn and
limit_req for locations where it’s costly to implement
scripts
• You can also fail2ban utility here:
http://www.fail2ban.org

Bad practices /
How not to configure NGINX
Chapter 3.

Bad practices
• NGINX has secure-enough defaults
• Sometimes administrators can make mistakes
cooking it

Tip #13.
Be careful with
rewrite with $uri

rewrite with $uri
• Everyone knows $uri /
(“normalized" URI of the request)
• normalization is decoding the text encoded in the
'%XX' form, resolving references to the relative path
components '.' and '..', and possible compression
of two or more adjacent slashes into a single slash

rewrite with $uri
Typical HTTP -> HTTPS redirect snippet:
location / {
rewrite ^ https://$host/$uri;
}
location / {
return 302 https://$host$uri;
}
What can go wrong? CRLF (%0d%0a) comes to play

rewrite with $uri
• Request
GET /test%0d%0aSet-Cookie:%20malicious%3d1 HTTP/1.0
Host: yourserver.com
• Respond
HTTP/1.1 302 Moved Temporarily
Server: nginx
Date: Mon, 02 Jun 2014 13:08:09 GMT
Content-Type: text/html
Content-Length: 154
Connection: close
Location: https://yourserver.com/test
Set-Cookie: malicious=1

Use $request_uri
instead of $uri

Tip #14. Pay attention
to try_files

try_files
• try_files checks the existence of files in the
specified order and uses the first found file for
request processing
• if none of the files were found, an internal redirect
to the URI specified in the last parameter is made

try_files
There is a Django project
$ tree /your/django/project/root
+-- media
+---- some_static.css
+-- djangoproject
+---- __init__.py
+---- settings.py
+---- urls.py
+---- wsgi.py
+-- manage.py

try_files
Administrators decide to serve static files with nginx and use
this configuration
root /your/django/project/root;
location / {
try_files $uri @django;
}
location @django {
proxy_pass http://django_backend;
}

try_files: what’s wrong?
• NGINX will first try to serve static file from root, and
only if it does not exists pass the request to
@django location
• Therefore, anyone can access manage.py and all
of the project sources (including djangoproject/
settings.py)

Tip #15. Use
disable_symlinks
if_not_owner

Hosters usually do this
location /static/ {
root /home/someuser/www_root/static;
}

What’s the problem?
User can create symlink to any file available to nginx
worker (including files of another users)!
[root@server4 www]# ls -alh
total 144K
drwxr-x--- 6 usertest nobody 4.0K Apr 10 20:09 .
drwx--x--x 13 usertest usertest 4.0K Apr 7 02:16 ..
-rw-r--r-- 1 usertest usertest 184 Apr 6 21:29 .htaccess
lrwxrwxrwx 1 usertest usertest 38 Apr 6 22:48 im1.txt -> /home/
another_user/public_html/config.php
-rw-r--r-- 1 usertest usertest 3 May 3 2011 index.html

What you can do
1. Turn off symlinks (and users will suffer)
2. Use option disable_symlinks if_not_owner
(best choice)

Slides:
bit.ly/nginx_secure_webapps
http://wallarm.com
Stephan Ilyin, si@wallarm.com

How to secure your web applications with NGINX

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